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Gap-phase community dynamics in a sub-alpine old growth forest Lertzman, Kenneth Peter


Small-scale natural disturbances involving the death of one to a few trees and creating gaps in the forest canopy are critical to the population and community ecology of many forest types. I studied the role of canopy gaps in the structure and dynamics of a high-elevation, old growth forest in coastal British Columbia. The research was conducted in four stands at Cypress Provincial Park (~1,100 m) occupying the transition from the upper montane ecosystems of the Coastal Western Hemlock Zone to the lower elevations of the Mountain Hemlock Zone. They contained four tree species: Pacific silver fir (Abies amabilis), western hemlock (Tsuga heterophylla). mountain hemlock (Tsuga mertensiana). and Alaska yellow-cedar (Chamaecyparis nootkatensis). Though the stands varied in the proportions of each species, all had a similar distribution of area underdosed canopy (29%) and in gaps (52% expanded gap; 18% canopy gap). The overstory of two stands was dominated by Pacific silver fir and western hemlock, and two had a more equitable distribution of species. Pacific silver fir was much more dominant in the sapling layer (81%) than the canopy layer (43%) in all stands. Western hemlock was the next most frequent species among saplings (13%). Growth rate among Pacific silver fir saplings was greater in both classes of gap than it was under closed canopy. The distribution of saplings was independent of canopy classes, but there was a significant interaction between sapling species and rooting substrate. Western hemlock saplings were largely restricted to stumps, whereas firs occurred on stumps and on the forest floor. Most fir saplings (81 %) showed evidence of suppression, and 23% had experienced multiple periods of suppression and release. Most gaps had more than one gapmaker (90%). Half of all gapmakers died standing, and only 13% were windthrown. Pacific silver fir was represented among gapmakers in a much higher proportion than among canopy trees in general (64% vs. 45%). Median canopy and expanded gap areas were 41 and 203 m², respectively. The estimated forest turnover time varied from 280-1000 years depending on assumptions about the time taken for gaps to be filled. The most likely range is 600-700 years. I examined methods for calculating turnover time by comparing estimates from the literature and from simulations. The two common methods for estimating forest turnover time produce different estimates for stands where both can be calculated. The inverse of the rate of creation of new gap area (TT1) is consistently lower than the estimate based on total gap area and the time taken for gaps to fill (TT2). Tropical forests appear to have faster turnover times than do temperate ones, but comparisons are confounded by differences in estimation method. I found both TT1 and TT2 to be sensitive to assumptions about the equilibrium structure of the forest. TT2 reaches its steady state value later in stand development, but TT1 is more sensitive to short term fluctuations in gap creation rate. I examined the species composition of gapfillers with respect to two sets of hypotheses. The first set related the species of each gapfiller to the species of the gapmaker it is replacing. I did not find evidence that self-replacement or reciprocal-replacement act to maintain the current community composition. Gapmaker-gapfiller comparisons indicated preferential replacement of all species by Pacific silver fir, suggesting that the community is in a state of change. The second set of hypotheses related the species composition of gapfillers to features of the gap environment. I did not find evidence to support the ideas that gap size, location within a gap, or local canopy composition exert a strong influence on the species composition of regeneration within the gap. The only circumstance where Pacific silver fir was not overwhelmingly dominant among gapfillers was on stumps, where almost all successful western hemlock gapfillers were located. These patterns suggest that neither species-specific interactions between gapmakers and gapfillers nor variability in gap environments is adequate to maintain the current composition of the forest canopy. I used the matrix of species-specific transitions between gapmakers and gapfillers as a basis for modelling the longer term consequences of these replacement patterns. The models incorporated species-specific mortality rates to express differential longevity and variation in the transition probabilities to represent climatic fluctuations. I found that differential longevity among species exerted a strong influence on the equilibrium species composition, on the rate of community change, and the time to equilibrium. When the differential in mortality rates among species is proportional to their representation among gapmakers, the equilibrium composition is close to the current canopy composition. Because of its higher representation among gapmakers, Pacific silver fir may not be increasing in the canopy, despite its dominance among gapfillers. The simulations with varying climate produced 4 main results. 1) Unless the duration of climate fluctuations is either very long or very short, forest composition is in a continual state of disequilibrium. 2) Because of differential longevity, species have different response times to changes in climate. 3) Because of the difference in response times, the mean abundance of each species under a varying climate scenario is different than the composition expected from the mean climate state. 4) The rare, long-lived species, Alaska yellow-cedar, was favored by climatic fluctuations at the expense of the more common shorter lived species, Pacific silver fir. In this system, current canopy composition could be maintained by a combination of differential longevity among species and climatic fluctuations allowing periodic recruitment of the less common species.

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