UBC Faculty Research and Publications

Natural regeneration on clearcuts at the lower limit of the mountain hemlock zone Klinka, Karel 1997-12-31

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Natural Regeneration on Clearcuts at theLower Limit of the Mountain Hemlock ZoneIntroductionThe Mountain Hemlock (MH)  zone includes  all subalpineforests along British Columbia?s coast. It occurs at elevationswhere most precipitation falls as snow and the growing seasonis less than 4 months long. The zone includes the continuousforest  of the  forested subzones  and the  tree  islands  of theparkland subzones (Figure 1).  Old-growth stands  arepopulated by mountain hemlock, Pacific silver fir, and Alaskayellow-cedar, and are among the least-disturbed ecosystemsin the world. Canopy trees grow slowly and are commonlyolder than 600 years, while some Alaska yellow-cedars maybe up to 2000 years old.Early regeneration  failures  followed  slashburning  and theplanting of unsuitable species. Currently, the most successfuland feasible  option for  reforesting  cutovers is  naturalregeneration with a mix of the  three  main tree  species, butuncertainties remain about the temporal and spatial pattern ofregeneration, changes in  species composition, and the  timerequired  for  stand  establishment after cutting. Our studyaddressed these concerns by examining regeneration patternson 6 sites that were clearcut 11-12 years prior to sampling andleft to regenerate naturally. The sites were located at the lowerlimits of the zone in the Tetrahedron Range, near Sechelt, atelevations from 1060-1100 m.When does regeneration establish?Most trees  taller  than  100 cm established before logging(Figure 2).  Trees  that  established 2 or more years beforelogging (residuals) accounted for 35% of all trees and theirmean height at the  time  of logging  was 50 cm. Trees  thatestablished in the 3-year window from 1 year before loggingto  1  year after  logging  (germinants)  formed  a surprisinglyhigh proportion (45%) of regeneration. Most residuals werePacific silver fir while almost half of germinants were Alaskayellow-cedar (Figure 3). There was little ingress 1 year, andnone 8 years after logging. These results show the importanceof trees  that  established in  the  previous old-growth forest,especially when clearcutting removes  all nearby seed-producing trees.1250 m950 m600 m0 m1600 mAlpine Tundra (AT)Mountain Hemlock(MH)Coastal WesternHemlock (CWH)parklandforestedmontanesubmontaneApproximate elevation above sea levelFigure 1. Elevation sequence of biogeoclimatic zones andsubzones in southern coastal British Columbia.01000200030004000frequency ( per ha)< 50 50 - 99 100 +height class (cm)residual germinant ingress01000200030004000frequency ( per ha)Pacificsilver firyellow-cedarwesternhemlockresidual germinant ingressmountainhemlockFigure 2. Age class distribution by height class. Age classesare based on year of establishment: residuals - 2 or moreyears before logging; germinants - within 1 year of logging;ingress - 2 or more years after logging.Figure 3. Age class distribution by species.Scientia Silvica Extension Series, Number  7, 1997Does blueberry impede regeneration?The dense cover of blueberry shrubs on many cutovers in theMH zone can give the impression that these shrubs impedetree establishment and survival. On our sites, however, 84%of regeneration was growing with blueberry (Figure 5) andthere was no indication that blueberry decreased survival orheight growth.  This neutral or  positive effect of  blueberrywas due to  at least  two  factors.  First, blueberry was morecommon on undisturbed substrates, which in turn were relatedto more abundant regeneration; and second, blueberry likelyhelped protect regeneration from snow damage.Management ImplicationsWith  some  exceptions, clearcut  sites  at the lower  limit  ofthe  MH zone will regenerate  naturally,  though  oftenaccompanied by a shift  to  Pacific silver  fir  and a moreclumped spatial pattern than in old-growth stands. New standswill also have lower  structural diversity since they  will beessentially single-storied.Our parallel study  in  adjacent old-growth stands  (ScientiaSilvica Extension Series, Number 6) showed that deeper snowincreases  the  proportion of regeneration  that  requires  theprotection of an overhead canopy ? a property mostcommonly associated with high-elevation tree islands. Thistendency  is  most evident where snow  melts latest,  forexample, on cool-aspect and flat  sites.  Regeneration isusually absent in large canopy gaps where snow remains intoJune.  We  can therefore  expect that  deeper snow at higherelevations will cause regeneration  problems without theprotection of an overhead canopy.Given these  climatic conditions, any form  of clear-felling(clearcutting,  patch-cutting, seed-tree  cutting, or  strip-  orgroup-shelterwood cutting) is biologically inappropriate sinceit creates adverse conditions for the establishment, survivaland growth of trees. In stands where regeneration is feasible,a simple form of selection cutting could be used that maintainsmuch of the overhead canopy by retaining some live and deadcanopy trees and most sub-canopy trees. Such cutting wouldreduce the changes in species composition and simplificationof stand structure that accompany clearcutting and would berepeated only at long intervals.ReferenceBrett, R.B. 1997. Regeneration patterns on some old-growthand clearcut sites in the Mountain Hemlock zone of southernBritish Columbia. M.Sc. thesis, Forest Sciences Department,University of British Columbia, Vancouver, BC. 96 pp.Where does natural regeneration grow?Logging resulted  in  a sharp  decrease in  the  cover ofundisturbed forest  floor  substrates  (Figure 4).  In  spite  ofcovering less  than  50% of the  ground surface,  undisturbedforest  floor  supported approximately 90% of regeneration,regardless  of age or height class. Regeneration was lesscommon on mounds than  in  adjacent old-growth  stands,probably because mounds were the microsites most likely tobe disturbed during logging.020406080percent coverundisturbedforest floorcoarse woodydebrismineral soiland rockclearcut old-growthdecayingwoodfriable forestfloor0204060percent of regenerationshorter thanblueberrywithoutblueberrytaller thanblueberryFigure 4. Percent cover of substrates on clearcuts comparedto adjacent old-growth stands.Figure 4. Percentage of trees growing with blueberry shrubs. Scientia Silvica  is published by the Forest Sciences Department,The University of British Columbia, ISSN 1209-952XEditor: Karel Klinka (klinka@interchange.ubc.ca)Research: Bob Brett (snowline@direct.ca)Production and design: Christine Chourmouzis (chourmou@interchange.ubc.ca)Financial support: Vancouver Forest Region of the BC Ministry of ForestsFor more information contact: B. BrettCopies 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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