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Ecological Processes. 2019 Jul 23;8(1):33
The Author(s).
Beese, William J
Deal, John
Dunsworth, B. G
Mitchell, Stephen J
Philpott, Timothy J
Stand-level retention is an important component of sustainable forest management which aims to balance ecological, social and economic objectives. Long-term retention of mature forest structures at the time of harvesting (variable retention) is intended to produce future forest stands that more closely resemble conditions that develop after natural disturbances, thereby maintaining greater diversity of habitats for a variety of organisms. Structure includes features such as live and dead trees representing multiple canopy layers, undisturbed understory vegetation and coarse woody debris. Over the past two decades, variable retention has become common on forest lands in the temperate rainforests of coastal British Columbia (BC) and has been applied to a lesser extent in inland forest types. Our review of studies in BC and in similar forest types in our region indicates that both aggregated and dispersed retention can contribute to biodiversity conservation by providing short-term ‘life-boating’ habitat for some species and by enhancing the structural characteristics of future stands. For example, greater abundance of species present in the pre-harvest forest have been documented for vegetation, birds, carabid beetles, gastropods, ectomycorrhizal fungi and soil fauna in retention cutblocks compared to clearcuts. There are, however, some negative consequences for timber production such as wind damage to retained trees and reduced growth rates of tree regeneration compared to clearcuts. The authors suggest an adaptive management approach for balancing competing objectives when faced with uncertainty. This includes monitoring the implementation and effectiveness of various strategies for achieving goals. Over two decades of experience applying variable retention harvesting to industrial-scale management of forest lands in BC suggests that it is possible to balance production of wood with biodiversity conservation.
REVIEWriciretention, British Columbiarest manage-017) but thised in a com-8). Palik andnderstandingBeese et al. Ecological Processes (2019) 8:33 https://doi.org/10.1186/s13717-019-0181-9various approaches to ecologically based forestry have1Vancouver Island University, 900 Fifth Street, Nanaimo, BC V9R 5S5, CanadaFull list of author information is available at the end of the articletivity, in part, by using natural disturbance patterns andprocesses to guide stand- and forest-level interventionsand resulting vegetation structures (e.g. Hunter 1990).the distinction among terms for these approaches thatforesters may use carelessly. As a hierarchy, ecologicalforestry is a framework of ecological principles andgoals, natural disturbance emulation is a strategy andretention harvesting is a practice. In the past 30 years,* Correspondence: bill.beese@viu.caForest ecosystems and associated species have evolved inresponse to climate, other biophysical attributes and arange of natural disturbances at various temporal andspatial scales. Ecologically based forest managementaims to conserve biological diversity and forest produc-system based, ecologically based, etc.) foment have several origins (D’Amato et al. 2approach has been most recently articulatprehensive manner by Franklin et al. (201D’Amato (2017) discuss the importance of uBackground The foundations and principles of ecological (i.e. eco-AbstractStand-level retention is an important component of sustainable forest management which aims to balanceecological, social and economic objectives. Long-term retention of mature forest structures at the time ofharvesting (variable retention) is intended to produce future forest stands that more closely resemble conditionsthat develop after natural disturbances, thereby maintaining greater diversity of habitats for a variety of organisms.Structure includes features such as live and dead trees representing multiple canopy layers, undisturbed understoryvegetation and coarse woody debris. Over the past two decades, variable retention has become common on forestlands in the temperate rainforests of coastal British Columbia (BC) and has been applied to a lesser extent in inlandforest types. Our review of studies in BC and in similar forest types in our region indicates that both aggregatedand dispersed retention can contribute to biodiversity conservation by providing short-term ‘life-boating’ habitat forsome species and by enhancing the structural characteristics of future stands. For example, greater abundance ofspecies present in the pre-harvest forest have been documented for vegetation, birds, carabid beetles, gastropods,ectomycorrhizal fungi and soil fauna in retention cutblocks compared to clearcuts. There are, however, somenegative consequences for timber production such as wind damage to retained trees and reduced growth rates oftree regeneration compared to clearcuts. The authors suggest an adaptive management approach for balancingcompeting objectives when faced with uncertainty. This includes monitoring the implementation and effectivenessof various strategies for achieving goals. Over two decades of experience applying variable retention harvesting toindustrial-scale management of forest lands in BC suggests that it is possible to balance production of wood withbiodiversity conservation.Keywords: Alternative silvicultural systems, Clearcutting, Biological legacies, Forest biodiversity, Old-growth, VariableWilliam J. Beese , John Deal , B. Glen Dunsworth , Stephen J. Mitchell and Timothy J. PhilpottTwo decades of variablein British Columbia: a revimplementation and effefor biodiversity conservat1* 2 3© The Author(s). 2019 Open Access This articleInternational License (http://creativecommons.oreproduction in any medium, provided you givthe Creative Commons license, and indicate ifOpen Accessetentionew of itstivenesson4 5is distributed under the terms of the Creative Commons Attribution 4.0rg/licenses/by/4.0/), which permits unrestricted use, distribution, ande appropriate credit to the original author(s) and the source, provide a link tochanges were made.Beese et al. Ecological Processes (2019) 8:33 Page 2 of 22become common in the province of British Columbia(BC), Canada as part of sustainable forest managementsystems that aim to integrate environment, social andeconomic goals, often guided by forest certificationschemes (van Kooten et al. 2005).Compared to other parts of the world, commercial useof BC’s forests progressed from exploitation to a highlyregulated and ecologically based framework in a relativelyshort period of time, notwithstanding millennia of forestuse by indigenous peoples using traditional ecologicalknowledge (Chisholm 2013). Here, we provide a very briefaccount of this transition. Industrial-scale logging in-creased rapidly in BC during the 1950s through 1970swith the government’s expansion of volume-based licensesto forest companies in public sustained yield units (nowtimber supply areas) and area-based tenures such as TreeFarm Licences (BC Ministry of Forests and Range 2012).Harvest levels peaked in the middle 1980s, except forspikes in more recent years due to salvage harvesting frommountain pine beetle mortality (Environmental ReportingBC 2018). Eventually, the desire for other values from for-ests besides timber led to a clash with large-scale clearcut-ting in the late 1980s and early 1990s. Alternatives toclearcutting became a focus of research in the early 1990sthroughout BC (Arnott and Beese 1997; Vyse et al. 2005;Wiensczyk 2012). In the summer of 1993, protests againstlogging in Clayoquot Sound (a 350,000 ha region on thewest coast of Vancouver Island) under a BC governmentland use decision became the largest act of civil dis-obedience in Canadian history with over 850 peoplearrested (Tindall and Robinson 2017). Part of the BCgovernment response was to set up the Scientific Panelfor Sustainable Forest Practices in Clayoquot Sound(aka Clayoquot Scientific Panel or CSP), whose mandatewas to ‘develop world-class standards for sustainable for-est management by combining traditional and scientificknowledge’ (CSP 1994). The panel included academicspecialists as well as four indigenous members from theNuu-chah-nulth First Nations. One of the panel’srecommendations was to leave a portion of the originalforest stand behind during harvesting using a more eco-logically based approach to harvesting and silviculturalsystems it called variable retention1 (CSP 1995). The BCgovernment introduced the Forest Practices Code in 1994,which set stand- and landscape-level retention targets forBC public forests (Province of BC 1995).In 1998, MacMillan Bloedel, one of the region’s largestforest companies at the time, adopted a strategy to replaceconventional clearcutting with variable retention and toincrease conservation of old-growth forests. Other com-panies in BC followed, adopting variable retention forvarying proportions of their harvesting, especially incoastal old-growth forests. BC’s publicly owned forestsremain a significant source of industrial wood throughlong-term forest management licences with forest com-panies. Forest stewardship plans approved by governmentmust manage resources for multiple economic, social andenvironmental values. While stand-level retention hasbecome an important strategy for helping to achievebiodiversity conservation goals, such as ‘life-boating’ ofmature-forest species and structural enrichment of futurestands, there are potential conflicts with the traditionalgoals of silviculture and timber management. The authorsbelieve that it is timely to assess the outcomes of variableretention in BC now that many initial effects areknown. Although our focus is regional rather than glo-bal, BC provides a useful case-study for other jurisdic-tions because of the large number of hectares harvestedusing variable retention.The objectives of this paper are to (1) summarize theapplication of variable retention in British Columbia overthe past 20 years, (2) describe adaptive management andmonitoring initiatives that were established to evaluate theoutcomes of the approach and (3) review the findingsfrom monitoring and research to assess whether or notthe goals of variable retention are being achieved.Origins and goals of variable retentionVariable retention is based on the pioneering work with‘green tree retention’ in the Pacific Northwest region ofthe USA that was described by Franklin et al. (1997). Thevariable retention approach maintains structural elementsof the pre-harvest stand throughout a harvested area toachieve specific objectives. Variable retention follows anatural disturbance-based management paradigm—recog-nizing the importance of structural complexity to forestecosystem function and biological diversity. Live and deadtrees of varying sizes and canopy layers, and coarse woodydebris are retained as habitat for a host of organisms. Ofparticular focus is the maintenance of biological legacieswhen harvesting older forests (Lindenmayer and Franklin2002; Beese et al. 2003; Franklin et al. 2018).Naturalistic silvicultural systems that attempt to main-tain natural forest structure and dynamics are not new toforestry and have long been recognized as a means for re-ducing economic and ecological uncertainty (e.g. Bruenigand Klemp 2000). For example, the single tree selectionsystem has been used to regulate yield and maintain con-tinuous cover in uneven-aged forests for over 100 years.In the classical silvicultural systems (i.e. patch, clearcut,seed-tree, shelterwood and selection), maintaining orimproving yield by enhancing conditions for regenerationand improving the quality of the growing stock are para-mount. In variable retention stands, the retained treesmay be very old, of poor timber quality and low vigor, withfeatures that increase their value for wildlife habitat. Over-story influence is retained over the majority of the openingthroughout the rotation, even at the expense of theBeese et al. Ecological Processes (2019) 8:33 Page 3 of 22growth and vigor of regeneration and sapling layers(Mitchell and Beese 2002). Multiple age classes may beretained, not to balance age class distribution, but toemulate age class distributions typical of naturaldisturbances.Global application of retention forestryRetention has been applied in many forest types aroundthe world (Gustafsson et al. 2012). The most widespreaduse has been in western Canada and the USA, Australia(mostly Tasmania), Argentina and Europe (particularlythe Nordic countries) with documented benefits forconservation of biological diversity in forest stands du-ring the initial years following harvesting (e.g. Rosenvaldand Lõhmus 2008, Aubry et al. 2009, Craig and Macdonald2009, Baker and Read 2011, Fedrowitz et al. 2014, Soleret al. 2016). Lindenmayer et al. (2012) present a case forwidespread adoption of the retention approach to meetsustainability goals for the vast majority of the world’sforests that are outside of formal protected areas and arenot intensive forest plantations. They argue that an inter-mediate approach between protection and plantations,which integrates multiple values, is broadly applicable todiverse forest types and is an essential supplement to largeprotected areas.Retention silvicultural system in BCA variety of terms and uses appear in the literature onstand-level retention. In BC, we make a distinction be-tween the terms variable retention (VR) and the retentionsystem. We use variable retention to describe the overallapproach to forest harvesting that requires long-termretention of trees and associated habitat. VR can utilizetraditional silvicultural systems, such as shelterwood orselection, to meet forest regeneration objectives. We usethe term retention system to refer to a specific silviculturalsystem designed to meet the goals of VR, which wasoriginally defined in the BC Operational Planning Regula-tions (FPC 1999). This silvicultural system has threerequirements: (1) retained trees are distributed over thearea of the cutblock, (2) standing trees are left for the longterm (at least one rotation) and (3) distribution of leavetrees achieves > 50% ‘forest influence’ (i.e. the area sur-rounding a tree or forest edge with a radius equal to thetree height). This new silvicultural system acknowledgesthat retention of trees and other structural attributes offorests for purposes other than tree regeneration and tim-ber production are legitimate goals of forest management(Mitchell and Beese 2002). The two main variants of theretention system are group (or aggregated) and dispersed.For safety, economic and ecological reasons, aggregatedretention is often preferred; however, both variants andmixed approaches may be used to meet specific objectives(Franklin et al. 1997, 2018). The retention systemnormally uses a one-pass harvesting approach, but mayalso be prescribed with several harvesting entries using awide range of logging systems, including ground-based,cable and helicopter (Beese et al. 2003).We also use the term clearcut with reserves—a modifica-tion of traditional clearcutting where trees are retainedwithin or adjacent to the cutblock (Helms 1998). This ap-proach differs from the retention system in two ways: (1)there is no spatial distribution requirement for the re-serves, so they generally do not meet the > 50% forest in-fluence rule; and (2) the reserves need not be left for thelong term. Some view the term ‘clearcut with reserves’ asan oxymoron which does not clearly convey the manage-ment intent. In practice, because clearcut with reservesusually does result in long-term retention, it can be an ef-fective tool within a landscape-level biodiversity strategy.Where at least some long-term reserves are distributedwithin the cutblock, the approach can be quite similar tothe retention system in both appearance and function.Overview of variable retention implementationin BCForests and forest policy contextBritish Columbia has diverse biomes, including coastaland inland temperate rainforests, dry intermountainplateaus, subalpine and alpine areas and sub-boreal andboreal forests. The province’s ecosystems are classifiedinto 16 zones and numerous subzones and variantsusing the biogeoclimatic ecosystem classification system(Klinka et al. 1991). Forests cover 58% of the total landarea of BC (55 of 95 million ha). The forests are 83%coniferous, dominated by lodgepole pine (Pinus contortaDouglas ex Loudon), spruce (Picea spp.), Douglas-fir(Pseudotsuga menziesii (Mirb.) Franco) and true fir(Abies spp.) in the drier inland (interior) forests, andDouglas-fir, western hemlock (Tsuga heterophylla (Raf.)Sarg.), true fir, western redcedar (Thuja plicata Donn exD. Don) and Sitka spruce (Picea sitchensis (Bong.) Carr.in the rainforests (BC Ministry of Forests, Mines andLands, BCMFL 2010). The diverse forest and alpine tun-dra ecosystems of BC are characterized by five naturaldisturbance types, which use the frequency of stand-initiating or stand-maintaining events to classify forests(Province of BC 1995). Natural forests on the BC coastare typically influenced by infrequent or rare stand-initiating events, while forests in the BC interior have asignificant proportion of forests with frequent stand-initiating events or frequent stand-maintaining fires.Under Canada’s political system, most forest classifiedas productive and available for timber harvesting in BC(24 million ha) is administered by the Provincial govern-ment (92%) with very little Federal land. There are agrowing number of areas managed by First Nations(indigenous peoples). About 8% of BCs productive forestsBeese et al. Ecological Processes (2019) 8:33 Page 4 of 22are privately owned. The ten largest forest companies inBC have 42% of the harvesting rights on public land(BCMFML 2010). About 14% of BC’s forests (7.6 millionha) are in formal Parks and protected areas and 59% of theprotected forest area is forest > 140 years old. About 23%of all forests over 250 years old are protected, and an esti-mated 50% of all forest > 140 years old will not be har-vested (BCMFML 2010).The BC Forest and Range Practices Act (BC-FRPA2002) and associated regulations governing public landrequire average stand-level wildlife tree retention of atleast 7% of the area of cutblocks harvested over a 1-yearperiod, with a minimum of 3.5% in any single cutblock;minimum coarse woody debris levels are also required(FPPR 2004). Some regional land-use plans require 15%stand-level retention (e.g. Clayoquot Sound, Great BearRainforest). In addition, Old Growth ManagementAreas, riparian reserves and numerous reserves for wild-life and other resource values are required at the land-scape level. Old-growth reserve requirements vary from7 to 28% based on natural disturbance type and bio-diversity emphasis zoning (Province of BC 1995). Indi-vidual companies may set voluntary targets for higherlevels of retention under Forest Stewardship Plans, tomeet or exceed certification requirements, or applyscientific results to maintain biodiversity. Over 50 millionha of BC’s forest land is certified under independentsustainable forest management systems (Forest ProductsAssociation of Canada 2019). The three major systems arethe Canadian Standards Association (CSA), SustainableForestry Initiative (SFI) and Forest Stewardship Council(FSC). Retention is a key component of the criteria andindicators for harvesting practices. Certification andstreamside tree retention requirements are the primary in-fluences on retention practices of private forest owners,governed by the Private Managed Forest Land Act (2003).The variable retention approach is primarily imple-mented using the ‘retention silvicultural system’ (FPC1999) and ‘clearcut with reserves’ as defined above.Aggregated (group) retention is the most common appli-cation of the approach, although single dispersed treesare often left in combination with aggregates. Aggregatesize is typically 0.25–1 ha, anchored on riparian areas(required buffers) or other important ecological features.Retention must remain for at least one rotation, but theintention is that most will never be cut. Typical reten-tion levels average 20% in coastal rainforests, and less ininterior forests, not including landscape-scale reserves.History of VR adoption and spread in BCThe announcement in June 1998 by MacMillan Bloedel(MB) that it would use VR for virtually all forest harvest-ing—at the time the largest forest company operating onthe BC coast—shocked the forest industry (Chipello 1998).Prior to this time, partial-cutting silvicultural systems hadbeen rarely employed in either the second-growth or old-growth forests of the BC coast by large companies; theywere largely regarded as experimental or used by smallwoodlot owners (Arnott and Beese 1997; Macy 2011). Fol-lowing MB’s lead, other forest companies in coastal BCadopted VR for varying proportions of their harvesting.Weyerhaeuser purchased MB in 1999 and continuedphase-in of the VR approach throughout its BC coastaloperations over a 5-year period, completing over 90% ofharvesting using VR in 2003. After several changes in own-ership and a merger of three companies, use of VR hascontinued under Western Forest Products Inc. (Western)as part of the ‘Western Forest Strategy’ for managing over1.6 million ha of forest land on the BC coast (Western2018). Through 2018, more than 96,600 ha have beenharvested using the retention system on Western’s andpredecessor companies’ private land and public tenures(Fig. 1). The drop in total harvested area and the percent-age of the retention system used from 2007 to 2009 reflectsthe change in landbase resulting from company mergers,reduced overall harvesting due to poor market conditionsand a revised strategy. Implementation and monitoringshowed that the retention system was not suitable for allsites (e.g. extensive damage to retention in windward areas,and high costs on cable harvested sites), so the 100% reten-tion system (i.e. no clearcutting) goal was replaced withregional and ecosystem-based targets. The retentionsystem continues to be used for about 60% of Western’sharvesting—more than double the coastal average.The use of stand-level retention has risen steadily overthe past 20 years in BC (Fig. 2); however, most has beendone using clearcut with reserves rather than the reten-tion system. For all of BC (1998–2017), 3.4% of harvestingon public lands used the retention system, 4.4% used otherforms of partial cutting (i.e. seed tree, shelterwood, selec-tion, thinning), 62% clearcut with reserves and 30% clear-cut (Province of BC 2019). About 95% of the areaharvested using the retention silvicultural system has beenon the coast, which is roughly one-third of BC’s forestarea. For the BC coast, 29% (64,828 ha) of harvesting onpublic lands used the retention system over the 12-yearperiod 2006 through 2017. The use of conventional clear-cutting has gone from 88% of the BC public land harvestin 1998 to 14% in 2017. The difference in adoption of va-riable retention and retention system terminology betweenthe BC coast and interior can be attributed to less publicpressure on clearcutting and old-growth issues, and larger,more frequent natural disturbances (e.g. fire, insects)within interior regions. This is illustrated by the differentmaximum clearcut sizes allowed under BC regulations(40 ha, coast; 60 ha, interior; FPPR 2004). Actual averagecutblock sizes in BC are considerably smaller than themaxima (16 ha, coast; 23 ha, all of BC; Province of BCFig. 1 Percent of total harvested area using variable retention (line) and area harvested (bars) using different variable retention types over 20 years byMacMillan Bloedel/Weyerhaeuser/Cascadia (1999–2006) and Western Forest Products (2007–2018) in BC Coastal forest operations. Total area through2004 includes private lands (now Island Timberlands). Group = aggregated retention with groups predominantly 0.25 ha or larger; Dispersed = singletrees or small clusters of trees spread across a cutblock; Mixed = both group and dispersed retention in the same cutblock; Other = patch cut,shelterwood or selection systems (with long-term reserves). Retention type was not tracked after 2008 because nearly all VR was aggregated retentionBeese et al. Ecological Processes (2019) 8:33 Page 5 of 222019). Also, not all foresters in Canada have embraced thenew terminology (Groot et al. 2005).Application of variable retention under special land use plansStandards for application of VR vary from BC provincialdefaults where there is a ‘higher level’ regional land useplan, such as in Clayoquot Sound. The ClayoquotFig. 2 Area harvested by silvicultural system over 20 years (1998–2017) onshelterwood and selection systems, and intermediate cuts (e.g. thinning). Dbc.ca/gov/content/industry/forestry/managing-our-forest-resources/silvicultScientific Panel prescribed minimum retention levelsfor areas without significant values for resources otherthan timber (15%) and high levels (70%) for areas withsignificant non-timber values or sensitive features (CSP1995). The principles and practices defined by the CSPand in several Clayoquot Sound Watershed Planscompleted 2003–2006 guide landscape- and stand-levelBC public lands. ‘Partial cutting’ includes seed tree, patch cut,ata from the Province of BC, Silviculture Statistics (https://www2.gov.ure/silviculture-statistics) (accessed 15 Mar 2019)Beese et al. Ecological Processes (2019) 8:33 Page 6 of 22practices (Province of BC 2008). In the years following theCSP report, implementation of harvesting was slowbecause of new planning requirements. The CSP re-commendations and subsequent adjustments reducedthe allowable annual cut (AAC) from 900,000 m3 to290,000 m3, largely due to the 61% of the forested land-base reserved from harvesting. About 88,000 m3 wereharvested annually (roughly 130 ha/year) between 1996and 2008 in Clayoquot under the variable retention guide-lines, significantly below the AAC (Mychajlowycz 2010).On the central and north coast of BC (aka Great BearRainforest), an ecosystem-based management (EBM)approach was adopted for land use plans covering a7.5 million ha area (Coast Information Team 2004).Under EBM, at least 15% stand-level retention mustbe left at the time of harvest within the context of a highproportion of landscape reserves and protected areas (i.e.about 70% of the natural range of old-growth forest willbe reserved). Higher stand-level retention percentagesapply to certain riparian and wetland ecosystems(Province of BC 2016). The Great Bear Rainforest isdivided into two timber supply areas (as of January 2017),each with an AAC of about 800,000 m3 apportioned amongroughly ten licencees (BC Ministry of Forests, Lands andNatural Resource Operations, BCMFLNRO 2016).In Haida Gwaii (aka Queen Charlotte Islands), a forestcompany managed by the Council of the Haida Nation(Taan Forest) manages 193,000 ha under a special landuse order with similar provisions to EBM in the GreatBear Rainforest (Province of BC 2014). The Tree FarmLicence managed by Taan Forest is certified by theForest Stewardship Council. The FSC certification is inpartnership with BC Timber Sales and includes adaptivemanagement strategies with monitoring of stand-levelbiodiversity indicators. Between 2011 and 2017, 3092 hawere harvested using VR (Taan Forest 2018).Adaptive management and monitoringIn this section, we review the major adaptive management(AM) and monitoring initiatives underway in BC to eva-luate the outcomes of VR and other practices associatedwith ecologically based management. Many of the BCstudies reviewed in this paper section four arise fromthese programs.The Forest Project/Coast Forest StrategyThe ‘Forest Project’ was a multi-faceted initiative byMacMillan Bloedel, created in response to the issues ofclearcutting and old-growth conservation (Coady 2000).The project included ecological, economic and socialcomponents designed to create a ‘social license’ to operatewith a largely old-growth timber supply. The ecologicalcomponent addressed public concerns about clearcutting,old-growth and biodiversity conservation by adoptingvariable retention, reserving more old-growth using alandscape zoning scheme and designing an adaptivemanagement program (Bunnell and Dunsworth 2009). Be-fore 1998, the variable retention approach had never beenattempted in BC on an industrial scale and as such was agrand experiment. The adoption of VR within a 5-yearphase-in period required rapid adjustments in planning,harvest operations and silviculture practices. A primaryobjective was to sustain biological diversity throughoutthe company’s private and public lands. An estimated 175native terrestrial vertebrates potentially breed on theselands, plus an unknown number of other organisms(Bunnell et al. 1998). Retention of these organisms, whileharvesting the wide range of tree species, is the ultimatetest of the goal to sustain biodiversity. The Forest Projectteam recognized that the novel approach required AM,including a cost-effective monitoring program. AM is astructured approach to learning from operational prac-tices, monitoring and experiments to provide feedback tomanagement and continual improvement (Holling 1978).The strategy for maintaining biological diversity in-cluded two key elements: variable retention and landscapezoning. Stewardship zones varied in harvesting intensityand landscape-level reserves. The AM framework wasstructured with a criterion and indicators (Bunnell andDunsworth 2009). The criterion was ‘biodiversity issustained within the 1.1 million ha coastal forest tenure’.The indicators of success were:1. Representation. Ecologically distinct ecosystemtypes are represented in the non-harvestable landbase of the tenure to maintain lesser known speciesand ecological functions.2. Structure. The amount, distribution andheterogeneity of stand and forest structuresimportant to sustaining native species richness aremaintained over time.3. Species. The abundance, distribution andreproductive success of native species are notsubstantially reduced by forest practices.The focus of the AM program was structured learn-ing—answering key strategic and tactical questions. Learn-ing was primarily through operational and experimentalcomparisons that informed the key questions, as describedin detail by Bunnell and Dunsworth (2009). Although theAM program included monitoring of operational sites,experimental sites were established to facilitate quickerlearning and to provide a series of comparisons withgreater control of confounding variables. The experimentsare known as the Variable Retention Adaptive Manage-ment (VRAM) sites (Beese et al. 2005).The experimental design for the VRAM areas consistsof three replicates for each of five comparisons(Additional file 1: Figure A1). Each VRAM site consistsof four or five treatments, each applied to a 20 ha area:two or three VR options, plus clearcut and uncut areas.Each VRAM site requires an area of 80 to 100 ha that isas uniform as possible in timber type, plant associationsand topographic features. Treatments were randomlyallocated to blocks before field layout. Nine of the 15planned VRAM sites were completed by 2005; threecompare group (aggregate) retention percentage (GoatIsland, Hoodoo, Tsitika), two compare group size(Cluxewe, Klanawa), two compare riparian retention(Lewis Lake, Moakwa), one compares dispersed retentionpercentage (Horseshoe Lake, Fig. 3) and one compares thesize and timing of group removal (Memekay); seeAdditional file 1: Table A1. Due to company ownershipchanges and economic conditions, no further VRAM siteshave been established. Company biologists collaboratedwith academic and government scientists to supportresearch, and a group of international scientists providedadvice on the AM program implementation. Forestry andacross the zones and (2) to use ecosystem characteristicsto guide objectives and practices. The zones use broadecological units (ecosections), landscape objectives underthe Vancouver Island Land Use Plan (BC Integrated LandManagement Bureau 2000) and biogeoclimatic subzones(Klinka et al. 1991) as a basis for assigning different pro-portions of the retention system and minimum levels ofretention by zone. Landscapes with high windthrowhazard and a timber production emphasis have a target of10% stand-level retention with at least 30% of harvestingmeeting the retention system definition. In contrast, land-scapes with low windthrow hazard, low levels of landscapereserves due to a long history of development and a highbiodiversity value have targets of 20% stand-level retentionand at least 70% use of the retention system. Western’sForest Stewardship Zones combine elements of differentapproaches implemented by predecessor companies (Deal2005; Bunnell and Dunsworth 2009). They are also similarin concept to the ‘TRIAD’ zoning approach (Hunter andCalhoun 1996), whereby a forest area is divided into threeBeese et al. Ecological Processes (2019) 8:33 Page 7 of 22Biodiversity (Bunnell and Dunsworth 2009) provides acomprehensive description of this case study.The Forest Strategy—ongoing implementation byWestern Forest Products Inc.The original Forest Project/Coast Forest Strategy zoningscheme was modified after company mergers substan-tially changed the land base, now managed by WesternForest Products Inc. (Western). The basic principlesapplied to zoning were (1) to have a range of emphaseson timber production and biodiversity conservationFig. 3 VRAM dispersed retention experiment at Horseshoe Lake near Powe10%, 5%, 30% retention (left to right, middle). Photo by W.J. Beesezones with varying emphasis on conservation and inten-sive management. TRIAD zoning is growing in popularitywithin Canada as a means of fulfilling diverse objectives(Côté et al. 2010; MacLean et al. 2008; Montigny andMacLean 2006).Western monitors the achievement of targets for thepercentage of harvest area completed using the retentionsystem and the stand-level retention percentage for eachof the Forest Stewardship Zones. Results are evaluated an-nually based on a 5-year rolling average. For 2014–2018,the company exceeded both targets for all zones. Percentll River, BC. Treatments are uncut (lower), clearcut (upper center) andBeese et al. Ecological Processes (2019) 8:33 Page 8 of 22retention system targets were exceeded by 5% to 17%among zones. Average stand-level retention for zonesranged from 27 to 33% against targets of 10% to 20%.Implementation monitoring is carried out on a 5-yearinterval unless a change occurs which warrants a morefrequent sampling. The purpose of implementation mon-itoring, using random field checks, is to ensure the in-tent of retention is being met. A LiDAR-based tool hasbeen developed to calculate the percent forest influencefor each cutblock (i.e. the area within one tree length ofa timber edge)—a key ‘rule of thumb’ metric for the re-tention system.Other special land use plans in BCOther AM and monitoring initiatives in coastal BC in-clude the Clayoquot Sound region, managed under theClayoquot Scientific Panel recommendations (CSP 1995),the Great Bear Rainforest managed under ecosystem-based management guidelines (Price et al. 2009) and TaanForest in Haida Gwaii. Forests in Clayoquot have beenmanaged by First Nations owned forest companies (IisaakForest Resources Ltd., MaMook Natural ResourcesLimited) created after the CSP. Monitoring activities haveincluded windthrow and residual stand damage, and thedistribution and abundance of wildlife trees, coarse woodydebris and native plants; unfortunately, monitoring hasbeen very limited (Bunsha 2012). Through the CoastalStewardship Network, First Nations in the Great BearRainforest are engaged in collaborative monitoring acti-vities using traditional Nation-based Guardian Watchmanprograms (Coast Funds 2019). A regional monitoringstrategy was developed to collect standardized field datathat feeds into a database for the entire land use plan area,largely focused on the marine environment and streamhabitat. Taan Forest and BC Timber Sales have a moni-toring program supporting FSC certification in HaidaGwaii. Stand-level biodiversity indicators include attri-butes such as retention level, retention patch size andcoarse woody debris.Forest and Range Evaluation ProgramThe Forest and Range Evaluation Program (FREP) moni-tors forest practices in BC under the Forest and RangePractices Act (BC-FRPA 2002). There are 11 resourcevalues (e.g. biodiversity, cultural heritage, recreation, soils,timber, visual quality, water) identified under FRPA, andFREP has the responsibility for implementation and effec-tiveness monitoring for these values. FREP is a founda-tional element of the results-based approach in FRPA byproviding science-based evidence to guide continualimprovement of practices, policies and legislation (FREP2016). Monitoring protocols have been developed formost resource values, including stand-level biodiversity.Field data are collected by government field staff fromrandomly selected, recently harvested cutblocks. Dataincludes the quality and quantity of tree retention andcoarse woody debris.Review of key findings from monitoring andresearchResearch and monitoring of indicator (focal) species areused to assess whether or not strategies for leaving re-presentative unmanaged areas and habitat structuresactually result in viable populations of forest-dwellingspecies. Informative focal species are forest-dwelling,sensitive to forest practices, practical to monitor andprovide information that can guide management (Bunnelland Dunsworth 2004, 2009). A review of the biodiversityaspects of retention from 214 studies found beneficialeffects of tree retention on species diversity compared toclearcutting (Rosenvald and Lõhmus 2008). A growingnumber of studies are documenting significant benefits ofthe VR approach (e.g. Baker et al. 2013, 2015). A meta-analysis of 78 studies comparing retention harvests withclearcuts found that retention cutblocks supported higherrichness and a greater abundance of forest species thanclearcuts (Fedrowitz et al. 2014); however, retention cut-ting had negative impacts on some species compared touncut forest. Here, we review key findings from studies inBC and adjacent US states, including some as yet un-published results from the VRAM experiments. Unlessotherwise noted, percent retention refers to area whenreferring to aggregated (group) retention and basal areawhen referring to dispersed (single tree) retention.Structural retention and habitat attributesMonitoring of VRAM sites and operational cutblocks byWestern and predecessor companies have documentedthe structural attributes (i.e. biological legacies and specifichabitat elements) provided by retention in relation tobenchmark natural forests in coastal BC. Long-termretention in operational cutblocks (reported annually andverified with a random sample) averaged over 20%,primarily designed as aggregated or mixed retention(Beese et al. 2003). Stand-level retention ranged from 5 to55% during VR phase-in, with almost 75% of cutblocks inthe 11–30% range (Bunnell and Dunsworth 2009). From1999 to 2005, transects were established to monitorretained vegetation structure in 193 VR blocks, 98 uncutsites and 52 other sites (Huggard et al. 2009b). Measuredstructural elements included live trees; snags (dead stand-ing trees) and coarse woody debris (species, diameter,height and decay class); cover layers (canopy, small tree,shrub, herb, moss, litter and mineral soil); and dominantshrub and herb species (Huggard et al. 2009a; seeAdditional file 1: Appendix B for methodology). Detailedanalyses examined 63 different habitat elements. Initialcomparisons assessed whether retention patches provideBeese et al. Ecological Processes (2019) 8:33 Page 9 of 22similar levels of attributes to those found in unlogged(benchmark) forests. Overall, retention tended to havelower levels of some important elements, especially largetrees, total basal area and some vegetation layers, com-pared to benchmark sites in the same biogeoclimatic unit(Huggard et al. 2009b). Snag density was lower for bench-mark sites than retention patches in drier biogeoclimaticsubzones, but higher for benchmark sites than retentionpatches in wetter subzones. In some areas, tall snags wereless common in retention patches, especially near theedges, because of removal of hazard trees during harvest-ing. The abundance of live and dead tree habitat elementswas proportional to percent retention for aggregatedretention, which had higher amounts of many elementscompared to dispersed retention (Huggard et al. 2009b).Cutblocks were resampled to assess five-year post-harvestchanges. Standing tree density in retention declined about20% over 5 years due to windthrow and other mortality.Both growth and mortality rates of trees were higher onthe edges of retention patches, mostly within 10m of theedge, with some effects up to 20 m into the patch. Otherthan for small trees, tree mortality was similar in dispersedand aggregated retention (Huggard 2005, 2006). Post-harvest structure sampling was completed for two VRAMexperimental sites in 2006, representing different amountsof dispersed (Lois/Horseshoe Lake) and aggregated(Tsitika) retention (Additional file 1: Table B2).Densmore (2011) completed a detailed analysis ofFREP stand-level biodiversity monitoring on over 400cutblocks from 2006 to 2009 for the major biogeocli-matic subzones of the BC coast. Average retention was22.8% of the total gross area of the sampled cutblocks.Quality of retention, as measured by large snags, largelive trees and number of tree species, was consistentlyhigh for all but one of the six subzones. The volume ofcoarse woody debris in harvested areas was equal orgreater to baseline amounts in retained patches; how-ever, piece size was lower than the baseline for half ofthe subzones.For the South Coast and West Coast Regions, FREPmonitoring of cutblocks harvested from 2007 to 2014found that 94–97% met the 3.5% minimum retentionlevel with average retention over 20% and 17%, respec-tively. Density of large diameter live and dead trees inretention was typically below baseline levels for wetterbiogeoclimatic subzones but similar to unlogged baselinedata for drier subzones on the mainland coast. Thevolume of coarse woody debris on cutblocks was similaror somewhat higher than the baseline for most sub-zones, although the contribution of large diameter stemswas somewhat lower in some units (FREP 2016).In Haida Gwaii, results from FREP monitoring of TaanForest for 33 cutblocks between 2006 and 2010 showedan average of 13% stand-level retention; this increased to27% for ten cutblocks assessed between 2011 and 2015,and 34% for 2016 (Taan Forest 2018). Other indicatorsof stand-level biodiversity that were monitored but arenot reported here are average patch size, the amount oflarge snags and coarse woody debris relative to naturalbenchmarks and percent windthrow.In neighboring Washington and Oregon, the Demon-stration of Ecosystem Management Options (DEMO)study characterized structural differences among variableretention treatments and found that they were highlyinfluenced by initial stand conditions (Maguire et al. 2007).Dispersed treatments maintained significantly greater ca-nopy cover than aggregated treatments at the same reten-tion level. A retrospective study of structural attributesretained by partial cutting over the past century in south-east Alaska found that a tree species composition similarto that in uncut old-growth stands was maintained (Dealand Tappeiner 2002). It is easier to maintain a range ofstand attributes more similar to pre-harvest conditionsusing aggregated retention than with dispersed retention.Wind damage to retention and cutblock edgesWind damage has the potential to significantly alterstand structure in the initial years following variableretention harvesting. The maritime, hypermaritime andmontane rainforest ecosystems of coastal Washington,BC and Alaska experience a strongly seasonal climatewith mild, dry summers and wet windy winters. Large-scale counter-clockwise-rotating extratropical cycloneswhich form over the Pacific Ocean produce strongsoutherly winds as they reach the coast, and northwestwinds as they cross inland. Orographic effects lead tovery heavy precipitation as these systems cross thecoastal mountain ranges. Wind damage from uprooting(windthrow) and breakage of trees is a major and recur-rent natural disturbance process in coastal rainforests,ranging from individual tree to whole-stand replacement(Mitchell 2013). Nowacki and Kramer (1998) describerecurrent storm-induced whole stand replacement incoastal Alaska, which leads to distinctive soil chemistryand microtopography (Bormann et al. 1995; Krameret al. 2001, 2004; Leckie et al. 2004).Trees which are abruptly exposed by harvesting ofadjacent trees are vulnerable to damage from annualwinter storms, and monitoring of operational cutblocksand VR trials reveals the impact of harvest intensity andarrangement (Table 1). Across these studies, windthrowranges from 2 to 53% of retained trees. Damage increasesas harvesting intensity increases, and is higher in smalleraggregates than in larger ones. Wind damage variesregionally, with high rates of loss in Haida Gwaii and theexposed west coast of Vancouver Island (Beese et al. 2019)and lower rates in the more sheltered inland biogeocli-matic zones (Steventon 2011). Local variation in standtbars6–165terhestriBeese et al. Ecological Processes (2019) 8:33 Page 10 of 22and site conditions also influence damage rates. InClayoquot Sound, Scott and Mitchell (2005) reportedTable 1 Windthrow in variable retention trials and operational cuStudy name Ecological conditionsa Retention type YeOperational VR, ClayoquotSound, BCHypermaritimeCWHDispersed,Aggregated,Mixed1–STEMS 2– CampbellRiver, BCVery DryMaritime CWHDispersed,Aggregated,Other systems2MASS—CampbellRiver, BCMontane CWH Dispersed,AggregatedShelterwood6Operational VR—KispioxForest District, BCMoist Cold ICHWet SubmaritimeMontane CWHAggregated 12Operational VR,Weyerhaeuser/Western,AM Monitoring, Coastal BCHypermaritime,Maritime,Montane CWHAggregated,Mixed2–aBiogeoclimatic zones and subzones: CWH Coastal Western Hemlock zone, ICH Incoastal except the ICH portion of Steventon (2011), which has forests similar to tbYears post-harvest when wind damage was assessedcPercent damage to retained stems within plots, entire aggregates or a definedstudy areas or treatments for the cited studiesthat the likelihood of being windthrown increasedwith mean annual wind speed and soil conditions.Windthrow decreased with greater depth of permeablesoil, and was lower on mineral soil than on organic soil.There were no differences in windthrow levels betweenuniform or multi-storied stands. Trees with small livecrown ratios and greater slenderness were more vulne-rable to damage. The latter result was also found by Byrneand Mitchell (2013) in the uniform Douglas-fir dominatedsecond growth stands at the Silvicultural Treatments forEcosystem Management in the Sayward (STEMS 2) trialnear Campbell River BC (Montigny and Nigh 2009). Dam-age can continue for a number of years following harvest.In the Montane Alternative Silvicultural Systems (MASS)project on east-central Vancouver Island, Beese (2001) ob-served that a high wind event 7 years following harvestcaused substantial damage to the patches and clear-cutedges.Beese et al. (2019) conducted windthrow monitoring of172 VR cutblocks occurred throughout Western ForestProducts’ BC coastal operations over a 6-year period fromsouthern Vancouver Island (VI) to Haida Gwaii (QueenCharlotte Islands). They found that windward-facingcutblock edges were more vulnerable to windthrow thanother boundary exposures and damage differed signifi-cantly among biogeoclimatic subzones. Topographicallyexposed locations such as ridge crests and upper slopesexperienced more wind damage than middle and lowerslopes. The amount of wind damage also increased withlocks in British Columbiab Wind damagec Citation16.5% of 1215 stems in various typesIncreased with percent of trees harvested;Dispersed > AggregatedScott and Mitchell 2005Dispersed 40 stems/ha (5%): 24%Small aggregates (0.05 ha): 54%Thinning (64% volume removed): 18%Group selection (25% removed): 5%Patch cuts (25% removed): 6%Control (unharvested): 8%Aggregated > DisperseddeMontigny andNigh 2009, Byrneand Mitchell 2013Dispersed 25 stems/ha: 29%Large aggregates (1.5 ha): 2%Shelterwood (25% basal area retained): 10%Dispersed > AggregatedBeese 2001Aggregates (0.8 ha avg.): 9%Less windthrow in larger aggregatesSteventon 2011Small to medium aggregates(0.22 ha avg.): 37% (23–53%)Large aggregates (> 1 ha): 24% (16–45%)Less windthrow in larger aggregatesBeese et al. 2019ior Cedar Hemlock zone; subzone names from Klinka et al. (1991); all areas areCWH zone; other interior sites (Bulkley/Morice) in Steventon not shownp along larger aggregates; percentages show the average and (range) amongincreasing stand height and fetch distance. Contrary tomany studies, windthrow increased with soil depth—afinding the authors attributed to stand height supersedingsoil factors as a causal agent for windthrow over a broadlandscape. The distance that windthrow penetrated astand edge was affected by some of the same factors aspercent damage. Penetration along cutblock edges variedregionally from 6 to 20 m. The results suggest that design-ing variable retention for local site, stand and wind expo-sure conditions can reduce the potential for wind damage.Effects on tree growth and understory vegetationThe greatest impacts on early growth of residual trees arefrom mortality due to wind damage and from woundingduring harvesting. Impacts from both sources are typicallygreater for dispersed retention than for aggregated reten-tion or group removal (Phillips 1996; Aubry et al. 2004).Studies in southeast Alaska found that dispersed retentionresulted in greater tree wounding during timber harvestingthan aggregated retention or group selection (McClellanand Hennon 2005); however, a decay projection modelestimated that volume losses at the end of an 80-yearrotation would be minimal (Hennon and DeMars 1997).Wind damage has more significant impacts on residualtrees in coastal BC than tree wounding during harvesting.The main silvicultural concern about variable retentionis the potential impact on future stand growth. PermanentBeese et al. Ecological Processes (2019) 8:33 Page 11 of 22plots were established on VRAM sites and supplementalareas to measure the impact of the amount and spatialdistribution of retention on tree growth. An innovative‘sector’ sampling scheme was developed in order to obtaina proportional sample of retained aggregates and sur-rounding cut areas (Iles and Smith 2006). A model wasdeveloped (N.J. Smith, pers. com.) that tracks growingseason light and moisture to predict growth for westernhemlock and Douglas-fir. Findings to date suggest thatmost of the impact of aggregated retention on growth ofregenerating trees is restricted to 10 m from the standedge into the opening. Growth was highly correlated withsurficial moisture, suggesting that the impact of residualtrees on regeneration is due to moisture competitionrather than shading. For a given retention level, asmeasured by percent canopy, growth impacts on re-generation were greater for dispersed retention thanaggregated retention (N. J. Smith, unpublished data).For the VRAM dispersed retention comparison, Douglas-fir basal diameter growth was reduced 5 to 6 years afterplanting under 30% dispersed retention compared to aclearcut, but not for 5–15% dispersed retention (Smithand Beese 2012).Ten-year results from planted regeneration studies atthe MASS study in coastal BC found reduced growthcompared to clearcutting for amabilis fir and westernhemlock in a shelterwood with 25% residual basal area,but little impact on growth for 5% dispersed retentionand 1.5 ha patch cut treatments (Mitchell et al. 2007).Growth rates of regeneration 10 years after single treeand group selection in western hemlock-amabilis fir old-growth stand on the west coast of Vancouver Islandwere greater for group selection than single tree selec-tion, where initial volume removal was 25% for bothsystems (D’Anjou 2003). At the DEMO sites in Washing-ton and Oregon, a study comparing the effects of differentlevels and patterns of retention on planted and naturallyregenerating conifers found that after 12 years dispersedretention reduced growth of planted Douglas-fir and otherspecies more than aggregated retention for a given reten-tion level, and dispersed treatments had greater density ofnatural regeneration (Urgenson et al. 2013). Examiningedge effects at DEMO after 20 years, Whyte and Halpern(2019) found that density of natural conifer regenerationincreased with closer proximity to the edge of aggregatesand unharvested forest edges, but height tended todecrease closer to edges, although there was variation inresponse among conifer species and age of regeneration.Monitoring of VRAM sites and operational cutblocksdocumented changes to understory vegetation 5 years afterharvesting (Huggard 2005, 2006). Attributes measured ontransects within dispersed retention areas, up to 50 m intoretained aggregates and 50 m into adjacent cut areasincluded height and cover of dominant shrubs, herbs andmoss (Additional file 1: Appendix B, Figure B1, B2 andTable B1). In general, aggregated retention maintainedvegetation elements that were more similar to uncutbenchmark sites than did dispersed retention. Edge effectson herb and shrub layers within aggregates and adjacentcut areas were minor, and were concentrated within 10 mof the stand edge. Shrub cover in cut areas, with or withoutdispersed retention, was about 70% less compared toretained aggregates and benchmarks. Herb and moss coverwas similar for retention aggregates compared to uncutbenchmarks. Herb cover was reduced initially in cut areasand moss was almost eliminated. Dispersed retention,which was practiced primarily in drier biogeoclimatic units(i.e. Coastal Douglas Fir zone; and Coastal WesternHemlock (CWH) zone, very dry maritime, dry mari-time and moist maritime subzones; Klinka et al. 1991)generally had higher levels of mineral soil exposurethan the cut areas of aggregated retention sites.Early results from permanent understory vegetation plotsat the MASS study near Campbell River found that ashelterwood with 25% basal area retention had greaterpost-harvest abundance (frequency per replicate) of under-story trees, shrubs and bryophytes compared to a clearcut;patch cut (1.5 ha cut areas) and dispersed retention (5%basal area) treatments were not significantly different fromthe clearcut (Beese and Bryant 1999). Richness, diversity(Shannon-Wiener H′) and evenness did not differ sig-nificantly pre- or post-harvest among treatments. A post-harvest decline in species associated with undisturbedforests and increase in early seral species was leastpronounced in the shelterwood treatment, therebymaintaining the greatest similarity to the old-growth con-trol in terms of overall species and life-form composition.Species gains exceeded losses on treatment plots; however,bryophytes and herbs (e.g. orchids, lilies) that prefer moist,shaded habitats generally decreased after harvesting.Fifteen-year results from the same study at MASS (Beeseet al. 2009, unpublished data2) showed no significantdifferences in diversity measures among treatments, butshelterwood, patch cut and dispersed retention treatmentshad greater diversity at 15-year post-harvest than pre-harvest (p < 0.05). Retention studies in many forest typesreport increased species richness or diversity after harvest-ing from and influx of early-seral species in cut areas plusspecies maintained from the pre-harvest stand (e.g.Halpern et al. 2012; Roberts et al. 2016; Lilles et al. 2018),but not in all cases (Craig and Macdonald 2009).Of the primary issues associated with biodiversity andforest harvesting, of greater concern than overall diversityis the impact on species associated with older forests.Herbs whose presence decreased with disturbance atMASS were mostly Orchidaceae (Platanthera dilatata,Listera caurina) and Pyrolaceae (Moneses uniflora, Orthi-lia secunda, Chimaphila menziesii), which prefer moist,Beese et al. Ecological Processes (2019) 8:33 Page 12 of 22shaded habitats. At the DEMO study areas in Oregon andWashington, Halpern et al. (2012) found that late-seralherbs were sensitive to both the amount and pattern(aggregated vs. dispersed) of retention. They observedlocal extirpation of late-seral associated herbs was greatestin the harvested areas between aggregates and at thelowest level of dispersed retention (15%).Bryophytes decreased after harvesting at both MASSand DEMO (Halpern et al. 2012). By year 15, several bryo-phytes that were reduced after harvesting at MASS beganto recover. In a BC interior cedar-hemlock forest, Lilleset al. (2018) documented that recovery of non-vascularplants over a 24-year period increased with retention level.The bryophyte study at MASS was limited to about 20species that occur on forest floor and mineral soil. Muchof the diversity in bryophytes, particularly liverworts, isassociated with decaying wood and lower tree trunks—habitats that are lost or significantly altered after forestharvesting (Baldwin and Bradfield 2005). Aubry et al.(2009) suggest that retaining trees in 1 ha aggregates canprovide short-term refugia for late-seral herbs and bryo-phytes, but note the susceptibility of aggregates to edgeeffects. Other studies suggest that fungi and lichens havemore species associated with old forests and are of greaterconcern for losses from harvesting (Sillett et al. 2000;Coxson and Stevenson 2007; Molina et al. 2001;Trofymow et al. 2003).Effects on birdsOn the BC coast, aggregated retention was found to bebeneficial for bird species typically found in uncut for-ests, and bird occurrence was positively correlated withpercent retention (Preston and Harestad 2007). Overallresults to date from bird studies at VRAM sites showthat all guilds (groups of species with similar habitatpreference) and about one-third of individual birdspecies were affected by retention level (Chan-McLeod2008). Mature-forest dwelling species were generallymore abundant at higher VRAM retention levels com-pared to 10% retention or clearcuts. Removal of 31% ofthe original forest in various aggregate sizes (69% reten-tion, at Memekay VRAM) had little impact on forestbird communities. Dispersed retention levels of 5% to10% showed no significant overall difference in birdabundance from clearcuts, which favored shrub nestersand ‘open’ habitat species. Aggregate size appeared tohave an impact on some species, with large aggregates(1 ha) preferred over small- to medium-size aggregates(0.25 to 0.5 ha), although results with retention type andlevel were highly variable. In Australia, a study designedto test the impact of aggregate size on birds found nosignificant size effects on bird species richness whencomparing retention of three 0.5 ha aggregates to one1.5 ha aggregate (Lindenmayer et al. 2015).Huggard and Bunnell (2007, extension brochure3) ex-amined study results for 69 species of birds and concludedthat many less-sensitive species decreased substantiallybelow 15% to 20% retention, and some sensitive speciesdeclined more abruptly below 35% to 40% retention. Ameta-analysis of studies comparing green tree retentionlevels up to 50% did not demonstrate statistical differencesin species richness and abundance for a variety of taxaamong four classes of retention level because of highvariability (Rosenvald and Lõhmus 2008); however, thisreview found a positive relationship between persistenceof several species groups and tree density in about two-thirds of the relevant studies and retention pattern in halfof the studies. Fedrowitz et al. (2014) found that pattern ofretention (dispersed vs. aggregated) had no effect on thedifferences between clearcuts and retention cuts, althoughthey noted that limited data was a factor in their analysis.A recent meta-analysis of 30 studies on the response ofbird guilds to retention (Basile et al. 2019) found no diffe-rences in overall species richness and abundance betweenretention and uncut forest; however, for forest specialistbirds, their findings suggest that 40–60% retention may beneeded to maintain the same bird assemblage found inunharvested forests.In southeast Alaska, DellaSala et al. (1996) studied forestbird response to treatment of young forests (thinning andgap creation) and compared bird abundance in young for-ests to old-growth. Although the study did not examinevariable retention, the findings are relevant. They recom-mended leaving patches of old forest during harvesting toenhance habitat of breeding and wintering birds associ-ated with old-growth. Three of 16 common breeding birdswere more abundant in old-growth than in treated oruntreated young forests: Red-breasted Sapsucker, Pacific-slope Flycatcher and Golden-crowned Kinglet. BrownCreeper had a low frequency of detection but was foundexclusively in old forests. Higher use of old-growth bywintering birds, such as the Golden-crowned Kinglet, wasrelated to winter severity because of snow interception byold-growth canopies (Kirchoff and Schoen 1987). Simi-larly, at high elevations on eastern Vancouver Island,Beese and Bryant (1999) found 85% of winter residentbirds in retention aggregates and unharvested old-growth.DellaSala et al. (1996) observed that four bird species werenegatively associated with old-growth and others did notdiffer between young and old forest, which illustrates thefact that not all bird species will benefit from retention. InWashington and Oregon, a study of small passerine birdsat the DEMO sites comparing different levels and patternsof retention found that species richness declined sig-nificantly 1–3 years after logging for the lowest level (15%)of both aggregated and dispersed retention (Wilk 2019).Post-harvest Shannon-Weiner diversity was higher for15% aggregated vs. dispersed retention. TreatmentBeese et al. Ecological Processes (2019) 8:33 Page 13 of 22response varied for canopy, cavity-nesting and under-story species.Effects on other vertebratesBesides birds, the vertebrates most sensitive to stand-level effects from forest harvesting are small mammals(Bunnell and Dunsworth 2009). Studies of the effect ofvariable retention on voles in Douglas-fir–lodgepole pinestands in the southern interior of BC found that reten-tion maintained habitat for southern red-backed voles(Myodes gapperi), a species found in mature and old for-est (Sullivan and Sullivan 2011). Aggregated retentionmaintained red-backed vole populations similar to uncutforest 20 years after harvesting, but dispersed retentiondid not (Sullivan and Sullivan 2018). Compared to clear-cuts, dispersed retention (73–127 trees/ha of overstory,> 10–20 m height) did not have significantly lower popu-lations of Microtus spp., a genus of voles responsible forfeeding damage on seedlings from elevated populationsin herb and shrub dominated habitat after harvesting.Sullivan and Sullivan (2011) suggest that higher levels ofretention are needed (i.e. enough to suppress understoryvegetation) in order to help reduce outbreaks of smallmammal damage to tree regeneration. At the DEMOstudy sites, Gitzen et al. (2007) found that the similarityin relative abundance of 12 small mammal speciesbetween harvested areas and uncut forest decreased withdecreasing amounts of retention. Capture rates, however,were similar for both aggregated and dispersed retention,and responses among early- and late-successional specieswere not always as predicted. For the northern flyingsquirrel, a study at DEMO found that squirrel numberswere higher for uncut forest and 75% retention treatmentsthan lower levels of retention (40% and 15%), supportingprevious studies showing their sensitivity to forest harvest-ing (Holloway et al. 2012).The impact of forest harvesting on species that aresensitive to microclimatic changes and are less mobile isan important aspect of biodiversity conservation that wasinvestigated on VRAM sites and operational cutblocks(e.g. amphibians; also carabid beetles and gastropods, seebelow). Wind (2008) examined the effects of harvestingand buffers on amphibian use of small wetlands. Four am-phibian species were found breeding in small ponds: long-toed salamander, rough-skinned newt, Pacific chorus frogand red-legged frog. Two and three years after harvesting,all four species have continued breeding at small wetlandsat each site and standing water has persisted longer in theseason than before harvesting. Frogs utilized more wet-lands for breeding after harvesting than before harvesting.Wetlands with no canopy cover appeared to have a greaterinflux of breeding post-harvest than wetlands with reten-tion. Based on these findings, a field card for identifyingimportant wetland sites was developed (Wind and Beese2008). Maguire et al. (2005b) did not find a relationshipamong four species of salamanders and level and patternof tree retention at the DEMO study; however, salamanderabundance was positively correlated to high levels ofcoarse woody debris and herbaceous cover.Effects on invertebrates and below-ground diversityInvertebratesCarabid (ground-dwelling) beetles have been widely usedas indicators of environmental change due to their rapidresponse to disturbance and a high degree of habitatspecialization (Pearce and Venier 2006). Monitoring ofcarabid beetle response to variable retention harvestinghas occurred in coastal and northern BC. One yearafter harvest at the Tsitika VRAM site (Additional file 1:Figure A1), aggregated retention maintained carabidbeetle communities similar to those found in uncut(old-growth) stands, with community similarity, relativeto the uncut condition, increasing with retention level(Additional file 1: Figure C1, from Pearsall 2008, un-published data4). This pattern also developed shortlyafter harvest in aggregated retention sites in Piceaengelmannii x glauca—Abies lasiocarpa forests in north-ern BC near Smithers (Lemieux and Lindgren 2004). AtTsitika, these patterns were largely driven by the loss ofthe forest specialist Scaphinotus angusticollis with in-creased tree removal. However, 6 years after harvest,carabid community diversity metrics were similar acrossclearcut, aggregated retention and old-growth samplinglocations, and lower overall distances in cluster analysisindicated similar community structure among treatments(Additional file 1: Figure C1). Thus, carabid communitiesat this site appeared to recover regardless of retentionlevel, though the loss of Zacotus matthewsii, an old-growth specialist, was noted at all treatments (Pearsall2008, unpublished data4).Some inconsistencies in the effects of variable retentionharvesting on carabid populations were found at someVRAM sites, most likely attributable to site differencesand post-harvest windthrow. Four years after harvest at aVRAM aggregated retention site (Goat Island), the 10%retention area showed high carabid community similarityto the uncut second growth control, whereas the 20% and30% retention areas (which experienced high levels ofwindthrow damage) had carabid communities moresimilar to the clearcut. Despite this, carabid beetle samplescollected within or at the edge of retention patchesclustered together and separately from the cut matrixsamples, suggesting that aggregates maintained carabidcommunities distinct from those found in cut areas(Additional file 1: Figure C2, Pearsall 20095). Three yearsafter harvest at a riparian aggregated retention VRAM site(Lewis Lake), the samples from the 15% site showed moresimilarity to the uncut second growth than the 50% site,Beese et al. Ecological Processes (2019) 8:33 Page 14 of 22which was located on a dry site at the crest of a hill (Add-itional file 1: Figure C3). At the Moakwa riparian retentionVRAM site, both retention levels (15% and 50%) werelargely unsuccessful at maintaining mature-forest carabids(Additional file 1: Figure C4). Importantly, the old-growthcontrol stand at Moakwa had high abundance of old-growth specialists Z. matthewsii, Scaphinotus johnsoni andother forest specialists, which may have driven site se-paration between cut areas and the uncut stand. Despitethe limited efficacy of retention patches at Moakwa formaintaining mature forest carabids, there was evidence oflife-boating at this site; the forest specialist S. angusticolliswas more abundant in the cut areas of retention treat-ments relative to the clearcut treatment (Pearsall andBeese 2009).The effect of aggregate size on carabid beetles was in-vestigated on operational cutblocks and on two VRAMgroup size comparison sites on western (Klanawa, SW ofPt. Alberni) and northern (Cluxewe, near Pt. McNeill)Vancouver Island from 1 to 6 years post-harvest (Pearsall2007). For both old-growth and second-growth forests,there were higher total catches per trap of forest specialistcarabids in larger aggregates (0.8–1.4 ha) than in smalleraggregates (0.1–0.4 ha). Old-growth retention aggre-gates were generally successful at retaining forest spe-cialists, while small second-growth aggregates were verypoor at retaining species such as Pterostichus crenicol-lis, S. angusticollis and Z. matthewsii. Species-aggregatearea curves for these three forest specialists showed apositive response to aggregate size for P. crenicollisand S. angusticollis, but not Z. matthewsii (Additionalfile 1: Figure C6, C7 and C8, from Pearsall 2007).While aggregated retention has shown varied success forcarabid beetles, monitoring of dispersed retention sites incoastal BC has demonstrated this method of harvestingmay be unsuitable for retention of carabid beetles. At adispersed retention VRAM site (Fig. 3, Additional file 1:Figure A1), clearcut and dispersed retention treatmentsshared a similar carabid community composition 1 yearafter harvest, with abundance declining in these treatments,but not in uncut stands, 6 years after harvest (Additionalfile 1: Figure C5, Pearsall 2008, unpublished data4).Gastropods are particularly sensitive to harvest due totheir limited dispersal ability and strict moisture require-ments. The response of gastropods to VR harvesting wasassessed at six VRAM sites (Ovaska et al. 2016). Theabundance of Pristoloma species, small snails (as a group),Haplotrema vancouverense and Striatura pugetensis waslower in all harvesting treatments after 2 to 4 years, rela-tive to pre-harvest and uncut controls at four VRAM siteswith varying levels of retained aggregates, showing nobenefit over clearcutting. Similarly, at the site shown inFig. 3, the three levels of dispersed retention reduced theabundance of five gastropods and all small snails relativeto the control. However, the highest dispersed retentionlevel (30%) showed higher abundance of some species(Planigyra clappi, Vertigo species and Vespericolacolumbianus) relative to the other harvested treatments.At a VRAM aggregate size comparison, the large aggregate-size treatment supported high abundances of Pristilomaspecies, S. pugetensis, and small snails combined, and hadoverall higher gastropod richness relative to the smaller-sized aggregates and the clearcut treatment (in most cases).Overall, larger aggregates (> 0.8 ha) anchored on riparianareas and high levels of dispersed retention (30%) arerecommended for improved conservation of gastropods.Soil biodiversityAggregated and dispersed retention harvesting has success-fully maintained fungal communities, especially root-associated ectomycorrhizal fungi, at several sites in coastalBC. At two sites on Vancouver Island, Outerbridge andTrofymow (2004) found increased ectomycorrhizal diver-sity and percent root colonization on the roots of plantedDouglas-fir seedlings with increasing proximity to retentionpatches. At the dispersed retention VRAM site (Fig. 3),using comparable methodology, Outerbridge and Trofy-mow (2009) found similar trends; they also concluded thatincreased retention levels had improved outcomes forectomycorrhizal diversity. A multidisciplinary group ofstudies is examining soil microbial and faunal diversity andfunction in response to a wide range of silvicultural systems(Table 1; www.for.gov.bc.ca/hre/stems) at the STEMSresearch site on Vancouver Island (Grayston et al. 2005).At STEMS2, Jones et al. (2008) found no differences inectomycorrhizal fungal communities between uncut standsand a range of aggregate sizes (5–40 m in diameter),4–6 months after harvest, but the influence of thepatch in maintaining ectomycorrhizal diversity dis-appeared 10 m from the patch edge. However, 10 yearsafter harvest at the STEMS1 research sites, Kranabetteret al. (2013) found reduced abundance and fruiting of ecto-mycorrhizal fungi in patch sizes < 20 m in diameter relativeto uncut stands. These authors noted that although noneof the patch sizes successfully maintained ectomycorrhizaldiversity relative to continuous forest, the reductions indiversity were modest and populations of prominent fungiwere stabilized in patch sizes of at least 0.2 ha. Whenconsidering all fungal guilds in decomposing fine roots,Philpott et al. (2018) found that 6 years after harvest atSTEMS3, aggregated retention maintained fungal com-munities similar to those found in uncut stands, whereasfungal communities in the dispersed retention treatmentwere more similar to those found in the clearcut. However,rapid post-harvest recovery occurred regardless of harvestmethod at the STEMS1 site, where fungal communitycomposition did not differ among retention, clearcut anduncut treatments 13 years after harvest.Beese et al. Ecological Processes (2019) 8:33 Page 15 of 22The ability of variable retention harvesting to maintainoverall belowground biodiversity has also been assessed atSTEMS1 and STEMS3 using phospholipid fatty acid ana-lysis (PLFA), a broad-scale microbial community profilingtechnique. Five years after harvest at STEMS 1, Dewi(2009) found increased abundance of fungal biomarkers inthe forest floor of retention patches and in the dispersedretention treatment relative to the clearcut treatment.Fungal biomarkers declined with distance from aggre-gates, whereas microbial community structure was main-tained evenly across the cutblock in the dispersedretention treatment. This same trend was also observedfor total microbial biomass. Two years after harvest atSTEMS3, Churchland (2013) found reductions in fungalbiomarkers in the cut matrix surrounding retentionpatches, and in the dispersed retention treatment.Although these results are somewhat contradictory,the additional time since harvest in the Dewi (2009)study may have allowed for sufficient recovery of fungalcommunities in the dispersed retention treatment. Over-all, these results indicate that dispersed retention is moreeffective at maintaining microbial communities over theentire treatment unit, likely due to overlap in root systemsin this treatment, whereas benefits are restricted to within10 m of aggregates.Some benefits of aggregated retention for soil fauna(arthropods) have also been reported at STEMS. One yearafter harvest at STEMS2, most soil fauna showed no differ-ences in abundance in samples taken from the patch centre,edge or in the cut matrix, but pauropods and soil mitesdeclined with distance from retention patches (Grayston2007, unpublished data6). Collembolan communities insamples taken from the centre or the edge of patches werealso similar to collembolan communities found pre-harvest.Importantly, most (95%) of the soil faunal population wasfound in the forest floor, with soil disturbance (exposedmineral soil) causing significant reductions in the abun-dance of symphylans, diptera larvae, pseudoscorpions,pauropods and collembola. Thus, aggregated retentionlikely benefits soil fauna by retaining an undisturbed forestfloor, which may serve as refugia after harvest.Public perception and visual aestheticsSocial acceptability of harvesting is an important consi-deration, particularly on public forest lands. Studies ofpublic perceptions of clearcutting, VR and traditionalsilvicultural systems’ effects on scenic beauty or visualquality in Pacific coastal forests show consistent trends.For a given retention level, treatments that leave dispersedindividual trees receive consistently higher ratings thanaggregated retention (Clausen and Schroeder 2004;Sheppard and Meitner 2005; Ribe 2009). Low levels ofretention (under 15%) of any kind are generally perceivedonly slightly better than clearcutting.ConclusionsWhat have we learned from variable retentionimplementation?Many factors need to be considered when choosingstand-level practices to meet social, biological and eco-nomic goals. The final choice represents a balanceamong complementary and competing objectives. Assuch, it is not possible to determine precise and optimalfigures for the proportion of different silvicultural sys-tems, the amount of retention in each cutblock or thespatial distribution of retained aggregates or single trees.Nevertheless, such standards must be chosen based onthe best available information, theoretical concepts andpractical considerations. Experience over the past twodecades in coastal BC suggests the following conclusionsthat are relevant for applying stand-level retention:1. The landscape context determines what is necessaryor appropriate for stand-level retention in relationto biodiversity conservation goals.2. It is neither practical, nor likely even possible, toprecisely mimic natural disturbance patterns.3. Riparian networks are a governing factor forretention patterns because of existing reserverequirements and high-value habitat features formany forest organisms.4. Wind damage is a significant challenge fordispersing stand-level retention, making aggregatedretention or clearcutting with reserves the mostviable options for some sites in windy landscapes.The potential long-term benefit of retention forenhancing diversity of live trees in future forestsmay be compromised by wind damage in somelocations.5. Retention provides habitat that allows for someforest-associated organisms to persist afterharvesting (i.e. life-boating), with a positivecorrelation between retention amount and speciespersistence, greater benefits with aggregatescompared to dispersed single trees for some speciesand some evidence for a positive correlationbetween aggregate size and species survival.6. Growth impacts on forest regeneration increasewith percent canopy retention, and greaterdispersion of single trees or small aggregates.7. Typical retention levels (under 25%) do little todistinguish VR from clearcutting in the eyes of thepublic unless trees are distributed as dispersedindividuals or small aggregates.We also learned a lot about the practical logistics ofimplementing VR. A preference for aggregated retentionover dispersed retention for many habitat values alignswith safety and cost-efficiency (Zielke et al. 2008; BeeseBeese et al. Ecological Processes (2019) 8:33 Page 16 of 222013). Aside from the biological benefits of VR, market-place demands and public perceptions are important in-fluences on forestry practices that make retention a costof doing business in many areas.Is variable retention providing benefits for differentorganisms?While VR does not benefit every organism, the studiesdescribed above indicate positive effects on many forest-dwelling organisms compared to conventional clear-cutting. An important caveat on these findings is therelatively short duration of most studies to date, raisingthe question of whether or not the apparent ‘life-boating’effect for some species will persist, or whether or notsome differences between regenerating areas of retentionblocks and clearcuts will increase or diminish over time.The follow-up question—‘What amount and pattern ofretention is appropriate for different species?’—is muchmore complex. There is no single answer to this ques-tion except, when faced with uncertainty, ‘don’t do thesame thing everywhere’ (Bunnell et al. 1999). Differentspecies require different habitats, so it is intuitive tokeep the retention ‘variable’. The amount and pattern ofretention should be planned to suit the nature of theecosystems, the objectives and the species involved.In general, research results from early post-loggingstudies suggest that amount of retention is more import-ant than pattern for many species (Maguire et al. 2005a;Fedrowitz et al. 2014). For a given retention level,however, the monitoring and research results described inthis paper suggest that leaving aggregates (groups or largerpatches of trees) has more ecological value (generally) fora range of organisms than leaving dispersed individualtrees; it is also more feasible for forest harvesting. Inmountainous areas exposed to severe winds, using clear-cuts with adjacent reserves has been successful for redu-cing wind damage to retention. Monitoring birds, groundbeetles, gastropods and other species groups has shownthat retention can help many organisms survive and dis-perse in regrowth stands. There also appears to be a posi-tive correlation between the size of the aggregates and theecological benefits, but only for some species and eco-systems. While retaining forest that is representative ofthe original stand is important, exceptions should be madefor features of unusual biological significance (e.g. swampsor other wetlands with forested buffers, rare habitat types,snags). For extreme wind-hazard areas, a minimum aggre-gate size of 1 ha is appropriate. In many stands, thelocation of riparian habitats, which are typically the mostproductive in the area, is the primary influence on thefinal amount and distribution of retention.In forests where timber production is a major goal,both stand-level retention and landscape-level reservesfacilitate biodiversity conservation across the landscape(Lindenmayer and Franklin 2002). Neither approachalone is likely to be as effective or efficient as the two incombination (Bunnell et al. 1998). An essential elementof a broad biodiversity conservation strategy is a networkof protected areas and other reserves. The primary func-tion of these landscape-level reserves is to represent thefull range of ecosystems within the non-harvestable landbase to maintain lesser known species and ecological pro-cesses (Kremsater et al. 2003). Variable retention in theharvested ‘matrix’ between reserves may perform a com-plementary role for species that do not require forestinterior conditions but rely on certain structural attributes.In fact, a fundamental premise of VR is that more specieswill thrive in the area if mature forest elements are distri-buted throughout the production-forest landscape ratherthan adding an equivalent amount of mature forest to thereserve system. For some birds, retaining 15% to 20% ofthe original forest distributed over a cutblock can main-tain half the population abundance of the uncut forestcondition; however, populations of other bird speciesdecline proportionally or more rapidly as the percentageof suitable habitat is reduced (Huggard and Bunnell20073). In other words, the habitat for some species maybe improved significantly over a wider area than the sumof the retention area. For other species, adding a largepatch to a reserve would have greater benefit than dis-persing the equivalent forest area as retention.The role of VR is particularly important in landscapesthat are unlikely to have enough protected areas andreserves to maintain biodiversity on their own. At theother extreme, if a high proportion of a landscape isreserved, then stand-level retention may not be veryimportant. Consequently, the higher the level of landscape-level reserves, the less important VR or matrix managementmay be for sustaining viable populations of forest-dwellingspecies. Furthermore, there are many other factors toconsider besides what to reserve from harvesting. Franklinet al. (2018) suggest that intermediate stand treatments toenhance heterogeneity, and recovery periods betweenregeneration harvests are equally important considerationsas retention of biological legacies for an ecological forestryapproach. Stand treatments may not be as important inlandscapes with a lot of natural heterogeneity (e.g. having amix of forest ages, productivity classes, wetlands and com-plex coastline), as is the case in many parts of coastal BC.Ecosystems can be both resistant and resilient tochanges from natural disturbances or cutting (Keenan andKimmins 1993). Multiple pathways and patterns can occurwithin the limits of ecosystem processes and the range ofnatural variation. Therefore, even though emulating na-ture has emerged as a forest management strategy forsustaining both products and biodiversity, it may not benecessary to attempt to truly mimic natural disturbances.The very nature of commercial forestry (i.e. removal ofBeese et al. Ecological Processes (2019) 8:33 Page 17 of 22wood) makes precise imitation of natural disturbanceimpossible (Palik et al. 2002). Unfortunately, past practiceson the BC coast have had little regard for natural dis-turbances. Pearson (2010) estimated that stand-replacingnatural disturbances affected 3.1% of the forested areaover the past 140 years on BC’s central coast. Althoughharvesting affected 5.4% of this same area overall, 59% ofvalley bottom areas were harvested, illustrating that bothspatial pattern and scale must be considered when asses-sing the impact of forestry on ecosystems. Observation ofthe stand structures and patterns of stands across land-scapes produced by natural disturbances provides poten-tial models for the desired stand and landscape condition(e.g. Daniels and Gray 2006), but it may not be practicalor desirable to emulate the full range of the type, scale andseverity of disturbances present. Simplification of naturalstructures and patterns may be necessary to improve theefficiency of management or to meet other societal objec-tives (Mitchell and Beese 2002).Should variable retention be used everywhere?BC has a high diversity of forest ecosystems and distur-bance regimes, as well as social and economic consider-ations that vary from place to place. Forest managementpractices need to vary in response to this diversity. Nosingle harvesting or silvicultural system is appropriateeverywhere. Clearcut, seed tree, retention, shelterwood orselection systems may be ecologically, economically andsocially appropriate in the right context. Using a mixtureof systems can help achieve a range of patch sizes andstructures within stands and across landscapes. Neverthe-less, the variable retention approach is intentionally broadenough to encompass a wide range of forest conditions,harvesting systems and regeneration requirements. Forestpractitioners must choose the most appropriate silvicul-tural system, based on safety, ecological and operationalfactors. Perhaps most importantly, foresters must recon-cile or balance the often competing goals of silvicultureand biodiversity conservation. While the increase instand-level retention in BC over the past 20 years isencouraging, the resistance to adoption of new termi-nology (i.e. the retention system rather than clearcut withreserves) is disappointing. Use of the retention system insilvicultural prescriptions is currently regional at best, withlittle use outside of the BC coast. The commitment inlegislation to wildlife tree retention should be supportedby silvicultural prescriptions that clearly articulate thelong-term intent of such reserve trees.How can we improve monitoring and adaptive management?It is popular to recommend adaptive management, butdoes it work in practice? Unfortunately, most examples ofapplying formal adaptive management in natural resourcemanagement have failed because of three main problems:(1) lack of funding for monitoring, (2) failure of decisionmakers to embrace the AM concept and (3) lack of leader-ship to sustain a complex process (paraphrased fromWalters 2007). Nevertheless, the Forest Strategy success-fully closed the AM loop with changes to stewardshipzones and VR guidelines as a result of monitoring (Beese2013). For example, guidelines for VR were updated toencourage greater use of larger aggregates, and zoningstandards for stand-level retention were changed forwindthrow-prone areas. Details on AM approaches andthe challenges for implementation are discussed for anumber of cases in natural resource management (e.g.Stankey et al. 2005; Greig et al. 2013). In the past 20 years,research and monitoring studies have obtained usefulresults to inform variable retention practices; however,many aspects of biological monitoring require longer timeframes for assessment of impacts (Bunnell and Dunsworth2009). Monitoring species and forest structure are ex-pected to provide data that will help us demonstratesustainability and biodiversity conservation. Futurechallenges for forestry in BC include dealing with theuncertainties of climate change, survival in the worldmarketplace and our ability to adapt practices to meetchanging goals and expectations. Applying adaptivemanagement to these challenges requires a greaterlong-term funding commitment on the part of govern-ment and industry.Monitoring is an essential part of AM and is composedof three main elements: implementation or performance(have you met the stated targets and standards?), effective-ness (have you achieved the desired outcomes?) and vali-dation (are the assumptions correct?). Validation asks‘why’ a practice or strategy is or is not working and alsocan be considered refinement monitoring or research.Linking monitoring back to management action is funda-mental for an effective operational AM program. Unfortu-nately, much of the current monitoring in BC is simplyimplementation monitoring and needs to be refined toadequately address effectiveness and validation. Moreexperimental installations like the VRAM sites are war-ranted as well as AM comparisons at the watershed scale.Endnotes1For posterity, the authors note that Dr. Ken Lertzman(Simon Fraser University), a member of the ClayoquotScientific Panel, came up with the term “variable reten-tion” that was adopted by the CSP to replace “green treeretention”, the term commonly used up to that point. Dr.Steve Mitchell (University of British Columbia) suggestedthe idea of making a distinction between the variableretention approach, which can be accomplished withmodifications to traditional silvicultural systems, and anew “retention silvicultural system” designed specificallyto meet the goals of variable retention.Beese et al. Ecological Processes (2019) 8:33 Page 18 of 222Beese, WJ, Sandford, JS, Koppenaal, R, Mitchell, AK,Goodmanson, G, Bown, T, Harrison, ML (2009) Conifergrowth and understory vegetation response to alternativesilvicultural treatments and systems: 15-year results.Final Report, BC Forest Science Program, Project No.Y0911149, 43 p.3Huggard, DJ, Bunnell, FL (2007) Stand-level retentionand forest birds: a synthesis of studies. University of BC,Centre for Applied Conservation Research, Forest SciencesCentre, Vancouver, BC (brochure)4Pearsall, IA (2008) Study to assess the efficacy ofground beetles (Coleoptera: Carabidae) as ecologicalindicators in two variable-retention experimental sites: Atemporal comparison. Pearsall Ecological Consulting,Nanaimo, BC. Final Contract Report to Western ForestProducts, 166 p.5Pearsall, IA (2009) Effect of stand-level structures andconfigurations on carabid beetles in coastal B.C. VR sites:comparison of riparian retention and group retention VRmethods. Final Report, BC Forest Science Program,Project No. Y091065.6Grayston, SJ, (2007) Green tree retention: a tool tomaintain ecosystem health and function. Final Tech-nical Report, BC Forest Science Program, Project No.FSP Y073049.Additional fileAdditional file 1: Supplementary materials, Appendices A - D. (PDF 809 kb)AbbreviationsAAC: Allowable annual cut; AM: Adaptive management; BC: British Columbia;BCMFLNRO: BC Ministry of Forests, Lands and Natural Resource Operations;BCMFML: BC Ministry of Forests, Mines and Lands; CSA: Canadian StandardsAssociation; CSP: Clayoquot Scientific Panel; CWH: Coastal Western Hemlock;DEMO: Demonstration of Ecosystem Management Options; FPPR: ForestPlanning and Practices Regulation; FREP: Forest and Range EvaluationProgram; FRPA: Forest and Range Practices Act; FSC: Forest StewardshipCouncil; MASS: Montane Alternative Silvicultural Systems; MB: MacMillanBloedel; SFI: Sustainable Forestry Initiative; STEMS: Silvicultural Treatments forEcosystem Management in the Sayward; VI: Vancouver Island; VR: Variableretention; VRAM: Variable Retention Adaptive Management;Western: Western Forest ProductsAcknowledgmentsThe authors thank the foresters, forestry engineers, planners and forestworkers in BC for successfully taking up the challenge to implement variableretention and to do it safely. We are grateful to the many authors of citedstudies for their valuable contributions to this topic, especially Sue Grayston,Isobel Pearsall, Dave Huggard, Anne Chan-McLeod, Louise deMontigny,Nancy Densmore, Christina Ovaska, Colin Peters, Michael Preston, TerryRollerson, Jeff Sandford, Nick Smith, Lennart Sopuck and Elke Wind. We alsothank Bryce Bancroft, Fred Bunnell, Laurie Kremsater, Ken Lertzman, KenZielke and many others who have worked to bring practical changes tosilvicultural practices in BC’s forests. We thank Carlos Acosta for preparing thediagrams of the VRAM experiments in Fig. A1. Funding for many studiescited was provided by the forest companies involved, the BC Forest InvestmentAccount, Land Based Investment Program and Forest Science Program. Finally,we thank Guillermo Martínez Pastur and Jerry Franklin for encouraging us towrite this overview paper, and we thank two anonymous reviewers forsuggestions that improved the manuscript.FundingFunding for reported studies with which the authors are or have beenassociated was provided by the Province of British Columbia (BC Ministry ofForests, Lands, Natural Resource Operations and Rural Development; BCForest Investment Account, Land Based Investment Program; BC ForestScience Program), the University of British Columbia and forest companies(Cascadia Forest Products, MacMillan Bloedel Limited, Western ForestProducts Inc., Weyerhaeuser Company). Other than the scientists and fieldpersonnel directly involved in the research, the funding bodies did not havea role in the design of the studies, in the collection, analysis, and interpretationof data, and in writing the manuscript.Availability of data and materialsThe datasets used and/or analyzed during the current study are available fromthe corresponding author on reasonable request.Authors’ contributionsWJB was the lead author in writing and editing the manuscript andpreparing figures, except Fig. A1 as noted in the acknowledgements; JDprovided text and data for Western Forest Products’ Forest Strategy,particularly ‘History of VR adoption and spread in BC’ and ‘The ForestStrategy—ongoing implementation by Western Forest Products Inc.’; BGDwas the primary author of ‘The Forest Project/Coast Forest Strategy’ on thedesign of adaptive management and monitoring and a major contributor tothe conclusions; SJM was the primary author of ‘Wind damage to retentionand cutblock edges’ and was a major contributor to the introduction and toediting of the entire manuscript; TJP was the primary author of ‘Effects oninvertebrates and below-ground diversity’. All authors read and approved thefinal manuscript.Ethics approval and consent to participateNot applicable.Consent for publicationThe authors consent to publish the data included in this paper.Competing interestsThe authors declare that they have no competing interests.Author details1Vancouver Island University, 900 Fifth Street, Nanaimo, BC V9R 5S5, Canada.2Western Forest Products, 118–1334 Island Hwy., Campbell River, BC V9W8C9, Canada. 3Glen Dunsworth Ecological Consulting, 7857 Shangri-La Rd.,Lantzville, BC V0R 2H0, Canada. 4University of British Columbia, 2424 MainMall, Vancouver, BC V6T 1Z4, Canada. 5BC Min. of Forests, Lands and Nat. Res.Oper, 400–640 Borland St., Williams Lake, BC V2G 4T1, Canada.Received: 11 April 2019 Accepted: 6 June 2019ReferencesArnott JT, Beese WJ (1997) Alternatives to clearcutting in BC coastal montaneforests. 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Article
British Columbia
eng
Reviewed
Vancouver : University of British Columbia Library
Springer Berlin Heidelberg
Attribution 4.0 International (CC BY 4.0)
Faculty
Alternative silvicultural systems
Clearcutting
Biological legacies
Forest biodiversity
Old-growth
Variable retention
Two decades of variable retention in British Columbia: a review of its implementation and effectiveness for biodiversity conservation
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