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Regeneration patterns in the Mountain hemlock zone Klinka, Karel; Brett, Bob; Chourmouzis, Christine 1997

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Scientia Silvica Extension Series, Number  6, 1997Regeneration Patterns in 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.Understanding regeneration  patterns in  the  MH zone hasbecome increasingly important as logging continues towardshigher elevations of the zone where snowpacks are deeper.Regeneration patterns in adjacent forestsRegeneration patterns on the BC coast change with elevation.At lower  elevations, within the  Coastal Western Hemlock(CWH) zone, regeneration matches the gap model: trees aremost likely to regenerate in gaps in the forest canopy causedby the death of one or more trees (Figure 2). While the gapmodel favours regeneration far from other trees, snow at highelevations inverts the pattern to favour regeneration close toother trees  (the  tree-island model,  Figure 3)  and results  inthe tree islands that define the parkland MH subzones. Treeislands are usually on raised ridges or mounds that are firstto emerge from snow and are the only microsites with a longenough growing season  to  support  tree  establishment andsurvival.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.This study examines regeneration patterns at elevations thatare intermediate  between these  two  systems,  those  withinthe forested MH subzones. It is based on results from 6 old-growth stands in Tetrahedron Provincial Park, near Sechelt,at elevations ranging from 1080-1195 m.Figure 2. The gap model. (a) A canopy gap is formed by thedeath of one or more canopy trees. (b) With the passage ofdecades or centuries, the gap is eventually filled by a newcanopy tree.  Trees are most likely to reach the canopy layerfar from other trees.timeFigure 3. The tree island model. (a) Snow blankets the groundin winter. (b) With the onset of warmer weather, snow meltsfirst near trees and on mounds. Trees are most likely toestablish and survive close to other trees.(a) (b)gap(a) (b);;;;snow;;;;;;Where does regeneration grow?As expected, we found less snow and earlier snow melt onmounds, under the forest canopy, and near canopy trees. Late-snowmelt sites had a more open canopy than early-snowmeltsites, and were more likely to be north-facing and flat thansouth-facing and steep. Most trees died standing and singlyDo regeneration patterns match the gapor the tree-island model?Regeneration patterns are more consistent with the tree-islandmodel than the gap model since trees are most likely to surviveon mounds and close to a canopy tree. Still, the presence ofsome regeneration in gaps show that the study sites supply atransition between gap and tree-island models. The tree-islandmodel is best expressed on flat, north-aspect sites that retainsnow the  longest  and so are most similar to  high-elevationsites. It is also more apparent in the regeneration of Alaskayellow-cedar than of mountain hemlock. While both speciesregenerate successfully beside other canopy trees, mountainhemlock is  much more common in  canopy gaps, especiallyon steep,  south-facing  sites.  The different regenerationpatterns of these  species may be related  to  their  ecologicalniches in  that, snow  may more strongly  restrict  theregeneration of a species near the upper limit of its elevationalrange (like Alaska yellow-cedar) than that of a species in themiddle of its range (like mountain hemlock).Management ImplicationsThis study shows that regeneration patterns reflect the impactsof elevation  and snow  and hence, indicate potentialregeneration  problems. For example, regeneration  patternsthat match the tree-island model indicate high snowpacks andsevere growing conditions. As growing conditions becomemore severe,  we expect that  regeneration  problems willincrease. Simple measures could be developed as indicatorsof the severity of growing conditions, especially the date ofsnowmelt and the proportion of understory and sub-canopytrees  growing on mounds and close to  canopy trees.  Otheruseful measures are already recorded  during standard  sitediagnosis.Some indicators of the tree-island model? snow on the ground in late May? regeneration that is mostly on mounds or close to othertrees, particularly where there are distinct tree islands? a change in species composition that excludes lower-elevation tree species (like western hemlock andPacific silver fir), especially if subalpine fir is present? an open-canopy stand with short (<25 m), heavily-tapered canopy trees, and a poorly-developed sub-canopy layer? a predominance of standing dead trees (snags) ratherthan windthrown trees? treeless gaps without standing water or stumps (late-snowmelt gaps)? large gaps containing only mountain hemlock andAlaska yellow-cedar understory trees, especially whenthey are almost prostrate and covered by moss? species that are indicators of high-elevation growingconditions such as heathers, crowberry,  partridgefoot,and white rhododendronReferenceBrett, R.B. and K. Klinka. 1998. A transition from gap totree-island regeneration patterns in the subalpine forest ofsouth-coastal British Columbia. Can. J. For. Res. 28: 1825-1831. 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 1Z4Figure 4. Most regeneration grows on undisturbed forest floor,and the proportion increases with tree height.Regeneration on undisturbed forest floor 82 %92 % 99 %< 10  10 - 129 130 +Height class (cm)so there were no large canopy openings created by multipletreefalls as are common in low-elevation stands. Though therewere many large  (up  to  600 m?), late-snowmelt  gaps,especially on flat  sites,  they  did not support  the  vigorouscolonization that would be expected if such gaps were requiredfor regeneration. Most such gaps had not supported canopytrees for many centuries (if ever) since stumps were seldompresent, and, since most regeneration was <2 m tall and buriedby snow well into June, they are unlikely to support canopytrees in the near future.Regeneration was most successful  on mounds and nearcanopy trees.  It  was unaffected by overhead canopy cover(i.e.,  the  presence or absence of a canopy gap), apparentlybecause of the  prevalence of low-angle,  diffuse light.  Incontrast to most forested  ecosystems, almost  all  seedlingsand understory trees established on undisturbed forest floorrather than on decaying wood or mineral soil (Figure 4).

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