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Regeneration, growth and productivity of trees within gaps of old-growth forests on the outer coast (CWHvh2).. Klinka, Karel; Kayahara, Gordon J.; Chourmouzis, Christine 2001

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Regeneration, Growth and Productivity of Trees Within Gaps of Old-Growth Forests on the outer coast (CWHvh2) of British ColumbiaScientia Silvica Extension Series, Number 42, 2001IntroductionCentral to the issue of harvest feasibility on the outer BC coast (CWHvh) is the question of whether sites, once harvested,can be regenerated, and whether the time period for replacement and subsequent growth is economically and environmentallyacceptable.  Since low productivity sites have not been harvested in the past, there is a lack of data to answer this question.We tried to provide an answer by assessing regeneration following natural disturbances.  Small scale gap disturbances arethe norm within old-growth stands.  If regeneration is not a problem in gaps, then we have some evidence that regenerationshould not be a problem upon implementation of our management practices.  The objectives of this study were: (1) todevelop baseline information on the mechanisms and the patterns of regeneration across a sequence of forest types; (2) toassess regeneration success with respect to productivity; and (3) to estimate future growth and productivity.Study sites and methodsData was collected from canopy gaps in five different site series near Prince Rupert representing a gradation of productivityand drainage.  Gaps were located in five site series:  i) [04] HwSs-Lanky moss, ii) [01] CwHw-Salal, iii) [11] CwYc-Goldthread, iv) [12] PlYc-Sphagnum, and v) [32] Sphagnum bog (Figure 1).  Five gaps per site series were sampled in theDiana Lake transect and three gaps per site series were sampled in the Smith Island transect. Plot size varied from 100m2in site series 04 to 50m2 in site series 01 and 11, and 25m2 in site series 12 and 32.  For all trees, saplings, and seedlings,height, diameter and rooting substrate were measured.  Stem analysis was carried out on all trees and saplings (>1.3 mtall).  Five randomly selected seedlings (< 1.3 m tall) of each species were cut at the root collar and collected for analysis.For the Diana Lake transect, one dominant canopy tree of each species was sampled near each gap. The dominant treeswere felled and sections cut every 0.5 m to 5.3 m in height and every 1 m thereafter. At the Lachmach Channel site, wesampled 5 faller created gaps in a 23 year old partial cut. For the preliminary analyses, only the age and height for theDiana Lake transect are presented.Figure 1. Position of the five series onthe edatopic grid and on thelandscape at the Diana Lake site.verypoor poorveryrichSoil Nutrient RegimeSoil Moisture Regime0201030405111231, 3210,13slightlydryfreshmoistverymoistwet06,0807,08,09040111, 1231,32Results (preliminary)The gaps in the four site series have increasing light environments from the lowest in gaps of site series 04 (productive),to highest in site series 01 (unproductive zonal) and 11 (bog forest).  This light gradient is a function of the height anddensity of the trees surrounding the gap.  Gaps of site series 04 are surrounded by a forest of full crown closure with trees40-50 m tall.  Gaps of site series 01 and 11, are surrounded by open forests without crown closure with trees 20-25 m talland 15 m tall, respectively.  Gaps of site series 12 are openings in a forest having widely dispersed trees.Within all gaps, both seedlings and trees are much older than would be expected from their size.  The mean seedling age(< 1.3 m tall) was > 30 years, and some were > 100 years (Table 1).  Seedlings are not only growing under the saplings butalso under a heavy cover of Vaccinium up to 1.5 m tall.  The mean age of trees (> 1.3 m tall) was > 100 years (Table 2).  Ittook well over 100 years for the trees in the gap to reach approximately 4.5 m for site series 04, 3.5 m for site series 01, 3.0m for site series 11, and 2.5 m for site series 12.  However, the spread in ages within gaps was very large.  In general, theyoungest ages within the gaps were > 50 years, with the oldest 200 to > 400.  Age (years) Site series  Plot mean SD range 04 1 29 12 11-57 04 2 ? ? ? 04 3 39 31 11-102 04 4 26 15 4-56 04 5 21 10 7-42 01 1 48 18 30-86 01 2 57 31 18-122 01 3 44 25 12-115 01 4 38 16 13-77 01 5 23 13 7-66 11 1 33 25 2-97 11 2 45 17 14-85 11 3 47 24 12-113 11 4 54 32 19-118 11 5 45 20 7-96 12 1 48 24 24-117 12 2 79 37 27-162 12 3 54 26 16-107 Age (years)    Height (m) Site series  Plot  Number of trees  mean SD  range    mean SD  range 04 1 7 161 115 39-357  7.20 6.67 1.40-20.80 04 2 12 150 39 77-227  4.53 3.62 1.33-15.80 04 3 14 128 63 52-308  4.03 3.67 1.35-17.90 04 4 8 193 45 88-267  4.72 2.88 1.43-10.42 04 5 9 129 109 27-465  4.67 6.22 1.35-25.80 01 1 14 148 91 46-374  3.67 4.21 1.39-17.20 01 2 11 181 112 69-425  5.28 4.11 1.92-13.50 01 3 26 145 70 68-414  2.99 2.34 1.51-13.48 01 4 57 71 15 50-115  2.48 1.48 1.40-10.74 01 5 22 120 69 49-253  3.58 3.08 1.47-12.40 11 1 31 125 28 77-197  2.80 1.39 1.39-7.13 11 2 35 143 66 56-456  3.00 2.10 1.42-10.95 11 3 33 133 86 47-504  2.98 1.70 1.44-6.98 11 4 23 163 93 82-440  3.18 2.22 1.37-10.73 11 5 26 153 54 73-306  3.32 2.38 1.41-10.70 12 1 48 129 40 48-216  1.98 0.67 1.30-3.59 12 2 52 131 47 72-231  2.29 .85 1.39-4.39 12 3 52 118 34 60-189  2.82 1.80 1.36-7.80  Table1.  Age comparisons for site series 04, 01, 11 and 12for seedlings (< 1.3 m height) growing in canopy gaps.Values are given for each plot in the Diana Lake transect.Table 2.  Age and height comparisons for site series 04, 01, 11 and 12 for trees (> 1.3 m height) growingin canopy gaps.  Values are given for each plot in the Diana Lake transect and number of trees are per50m2.Site series 04 showed definite, although variable, patterns of release of trees within gaps.  There were instances of agradual ingress and release of most trees within a plot, and patterns where trees were suppressed for 100+ years butexhibited strong release to become the dominants within the gap.  In such cases, two height cohorts were created, one ofa few dominants, and another of a large number of shorter saplings which represent ingress into the gap.  The pattern ofingress and release exhibited for site series 04 was also generally exhibited for site series 01, although the degree ofrelease was not as dramatic and showed more of a steady increase.  Despite similar ages between site series 01 and 04,and the former having greater light availability in the gap, trees in site series 01 were shorter and the double height cohortpattern of site series 04 was weaker.  One or two trees may dominate, but generally, all the regeneration was increasing inheight at similar rates.  For site series 11, the pattern of release was much more subdued.  Most trees had fairly steady,constant, and slow growth.  This site series receives more light in the understory than either site series 01 or 04, so thelight levels of the pre-gap understory would not increase as much as would happen in site series 04.  This more constantlight level would explain the gradual increase in height growth exhibited by most trees in the bog forest.  Further, muchof the light during the growing season is diffuse thus increasing the similarity between open grown and open-canopycover conditions.  The light conditions for the bog woodland are different than the three previous site series.  Here, thesituation is not really classic gap dynamics since the trees are so widespread that the conditions more resemble full lightlevels.  Despite the increased light availability, height growth was slow, reflecting the site quality.  The maximum heightwas 7.80 m, yet the ages range in the hundreds of years.DiscussionThree main points are emphasized.  The first is that regeneration in canopy gaps is not a problem.  Since experience insite series 04 suggests that regeneration under clearcut conditions is not a problem, than by extrapolation, it appears thatregeneration should not be a problem under management scenarios for site series 01 or 11, despite the prevalence of aheavy Vaccinium cover.  However, this statement is made under the condition that the hydrological conditions of the siteis not altered under a clearcut scenario.  The second point is that although there is a lot of regeneration, the establishmentand growth within these gaps is extremely slow.  The gap trees are hundreds of years old.  The lack of trees < 1.3 m in thepast 50+ years shows a very large regeneration delay.  The tall and heavy Vaccinium cover may be part of the explanation,but even in site series 04, which does not have the associated cover of ericaceous species, there also is a lack of trees < 1.3m in the past 50 years.  It takes well over 100 years for saplings to grow 2-5 m high.  The third point is the recognition ofthe variation in height development (and probably species development as well, which will be analyzed in the comingyear) within the gaps. Even within one site series gap sizes differ, the size, density, and pattern of the trees surroundingthe gaps differ, the seed source differs, and the substrates within the gap differs.  All of these factors, as well as others,combine to form varied gap dynamics which can not be generalized.These three points lead to some preliminary management concerns.  It appears that, if hydrological conditions do notchange, regeneration is achievable upon clearcutting on site series 01 and 11.  However, if some sort of variable retentionharvesting system is used with dependence on natural regeneration, then forest managers must allow for a very longregeneration delay.  Even in the productive site series 04, the regeneration is much older than initially thought.  Finally,if an aggregate retention system (the traditional patch cut) is implemented followed by planting to overcome the regenerationdelay, we have to recognize that we are still simplifying the natural system.  Even within the concept of creating gaps, thevariability of growth rates is large.  Research is needed to assess whether the simplification of regeneration within gapswill have any negative impact upon biodiversity concerns.ReferenceChourmouzis, C. and G. J. Kayahara. 1998.Regeneration dynamics of old-growth forests onthe north coast of British Columbia. NorthwestScience 72:57-59.Scientia Silvica  is published by the Forest Sciences Department,The University of British Columbia, ISSN 1209-952XEditor: Karel Klinka (klinka@interchange.ubc.ca)Research: Gordon J. Kayahara (gordon.kayahara@mnr.gov.on.ca), ChristineChourmouzis (chourmou@interchange.ubc.ca), and K. KlinkaProduction and design: C.  ChourmouzisFinancial support: Forest Renew British ColumbiaFor more information contact:  G. KayaharaCopies 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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