UBC Faculty Research and Publications

Plant diversity in old-growth and second-growth stands in the coastal rainforests of British Columbia Klinka, Karel 1997-12-31

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IntroductionOne of the human activities impacting biodiversity is the cuttingof old-growth forests.    In  response  to  the  controversysurrounding the cutting of old-growth in the coastal rainforestof BC, the  Ministries of the  Environment and Forests haveproduced biodiversity guidelines that  are to  be applied whenmanipulating stands in the provincial forest.This study  augments these  guidelines by investigating  thediversity differences between second-growth  and old-growthforests in  relation to  site  quality.  We  demonstrate how stand-level plant diversity differs between 40-year-old and old-growthstands  in  the  Very Wet Coastal Western Hemlock subzone(CWHvm) on Vancouver Island. This information is intendedto provide foresters with an understanding of the effects of age,disturbance and site quality on stand-level plant diversity, therebyallowing for  informed  professional management decisions.What is biodiversity and how is it measured?Biodiversity,  a short-form  for  biological diversity,  can beorganized at three  principal levels:  genetic, species  andecosystem.  This study examined plant diversity at the specieslevel  and targeted  understory vascular plants which comprisethe majority of plant species in coastal temperate forests.  Alpha(alpha) diversity is a measure of species diversity within an area orecosystem.  The most commonly used measure of alpha diversityis species richness (S), a simple count of the number of speciespresent within an area.  In contrast, beta (beta) diversity describesthe  diversity differences between areas or ecosystems. Betadiversity can be measured by a similarity  index,  such  as theS?renson index,  where an index  of 1 would indicate  areasidentical  in  terms  of species  composition.Alpha diversity (alpha)Species richness = number of species per areaBeta diversity (beta)                               2 (no. of species common to areas A and B)                    (no. of species in area A) + (no. of  species in area B)Does alpha diversity change over time?All levels  of diversity are subject  to  temporal  variation.Following a stand-destroying  event, whether by natural oranthropogenic disturbance, the forest will pass through a seriesof predictable developmental or successional  stages.    As itprogresses through the stages there is a shift in species diversity(Figure 1). The most dramatic shift in understory plant diversity(when  the  species  diversity declines) occurs shortly  after anold-growth stand is removed.  Diversity will then increase duringthe stand initiation stage as early successional species colonizethe  site.   When canopy closure occurs, most if  not all,  of theDoes alpha diversity vary with ageand site quality?Since all species  grow,  reproduce,  and thrive  only under alimited  range  of ecological conditions, the  diversity ofunderstory vegetation will also vary with site quality.  In thisstudy, the immature and old-growth stands were segregated intoa series  of site  associations, where each site  associationrepresents  sites  within a  relatively  narrow  range  ofenvironments that  are capable of supporting  a certain bioticcommunity.  The five site associations identified in this studywere named after the major understory indicator plant species:Salal, Blueberry, Deer fern, Foamflower and Salmonberry.  Ingeneral, along the edaphic gradient from Salal to Salmonberry,there is a change from slightly dry to very moist soil moistureconditions and a change from poor to rich nutrient conditions.Figure 1. Generalized model of stand-level diversity changeover time.S?renson indexScientia Silvica Extension Series, Number  3, 1997Plant Diversity in Old-Growth and Second-Growth Standsin the Coastal Rainforests of British Columbia=pioneer species  may disappear causing another decrease indiversity during the  stem  exclusion stage.    Species diversitywill gradually increase again as the forest moves into the old-growth stage and the late successional species reappear.Species richnessold-growth standinitiation stemexclusion understoryreinitiation old-growthstand developmental stagelate seral early seral mid-seral late seralsuccessional stageTimecanopy closuredisturbanceOur immature  study  stands  were in  the  mid-seral stage  ofsuccession, and the presence of partly open canopies indicatesthe  onset of the  understory reinitiation  stage.    As  expected,species  richness  was lower  in  immature  stands  than  in  old-growth stands with similar site quality (Figure 2).Trends in species richness were also examined across the siteassociations. The effects of site  quality on diversity differedfor immature  and old-growth stands.   The old-growth standsshowed  a clear trend  of increasing  diversity with  increasingsoil moisture and nutrient levels. Species richness was loweston water-deficient and/or nitrogen-poor sites (Salal, Blueberryand Deer fern), and highest on moist and nitrogen-rich sites(Salmonberry). In 40-year-old stands this trend was not found.Does beta diversity vary with age and site quality?When the  beta  diversity of immature  and old-growth wascompared, the understory vegetation in the old growth standswas found  to  vary more from  stand to  stand (i.e.  they  had alower S?renson similarity index) (Figure 3).  In terms of changein b  diversity from the old-growth to immature, the least changeor loss in stand to stand variation was on the Deer fern sites.Why is diversity higher in old-growth stands?Old-growth stands are more structurally diverse than immaturestands.  In the absence of major stand destroying disturbances,this  increased  structural complexity is  a natural consequenceof aging and, in fact, is the cardinal feature of old-growth.  Ahigher microsite  diversity  offers  more habitats and greateropportunities for species with different niche requirements toco-exist.  The link between high structural diversity and highunderstory vegetation diversity is  supported  by the  greaterspecies  richness  (alpha  diversity) and higher between-standvariation (beta  diversity) found  in  the  old-growth stands.    Thisstudy only investigated understory vegetation; had bryophytesand lichens been examined the differences between old-growthand immature stands would likely have been greater.Preserving stand-level biodiversityTo minimize the impacts of logging on plant speciesdiversity, as well as improving the chances for its recovery tothe pre-harvest level, it is wise to maintain as much structuralcomplexity as possible, and manage on a site specific basis.The measures recommended in the biodiversity guidebookwill help preserve both structural complexity and plantdiversity at the stand-level:? retaining snags, individual trees and/or clumps of theoriginal old-growth stands? establishing mixed-species stands with trees  distributedin a random-group pattern and with tree species that willnaturally develop a stratified stand structure (e.g.,HwCw, HwBa)? establishing and maintaining irregular canopy cover? retaining coarse woody debris? lengthening tree harvest cyclesBased on the results of this study we suggest that in the CWHvmsubzone  the  above recommendations  should  be carefullyconsidered on sites which have shown the  greatest loss  in  alphadiversity (i.e.  the  rich sites).   On poorer sites, where there  isthe greatest loss in beta diversity, more attention should be givento the plant species composition of the stands prior to harvestto ensure the maintenance of beta (stand to stand) diversity.5101520old-growth40 year-oldNumber of species per 0.04 haSalal Deer fern SalmonberryBlueberry Foam flowerIncreasing soil moisture and soil nutrient levelsFigure 2. Average species richness in 40 year-old and old-growth stands across the five site associations. growth40 year-oldSalal Deer fern SalmonberryBlueberry Foam flowerIncreasing soil moisture and soil nutrient levelsSorensons similariy indexFigure 3. Beta diversity in 40 year-old and old-growth standsacross the five site associations.Scientia Silvica  is published by the Forest Sciences Department,The University of British Columbia, ISSN 1209-952XEditor: Karel Klinka (klinka@interchange.ubc.ca)Research: Hong Qian (hqian@interchange.ubc.ca) and Karel KlinkaProduction and design: Christine Chourmouzis (chourmou@interchange.ubc.ca)Financial support: Research Branch, BC Ministry of ForestsFor more information contact: Hong Qian (hqian@interchange.ubc.ca)Copies available from:  www.forestry.ubc.ca/klinka, orK.Klinka, Forest Sciences Department, UBC,3036-2424 Main Mall, Vancouver, BC, V6T 1Z4


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