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Tree species response to gap formation in partially-cut interior cedar-hemlock forests of northwestern British Columbia Coates, Kenneth David


Partial cutting is a type of small- to intermediate-scale disturbance that affects forest community dynamics and ecosystem processes by creating canopy gaps of various sizes through the harvesting of trees. Studies of canopy gap dynamics have contributed significantly to our understanding of the role of disturbance in forests, but have been little used by foresters for predicting ecosystem response to partial cutting. The gap dynamics literature was reviewed for insights into ecosystem responses to partial cutting and a series of gap-based studies was undertaken in the interior cedar - hemlock forests of northwestern British Columbia to quantify: (1) environmental conditions after gap formation; (2) species-specific responses to light environments; (3) natural regeneration and growth of planted seedlings in different gap sizes and gap positions; (4) tree species composition and growth in 30-40 year old gaps; and (5) windthrow in partially cut forests. Light levels and soil temperature varied by gap size and gap position, increasing from south to north gap positions, but not dramatically so in larger gaps away from the shady south end. Soil temperature in gaps rose above 10° C early in the growing season and remained there until early autumn. Fifth year size and recent growth of five planted conifer species was quite similar from low to high light. Under field conditions, there was no meaningful variation in whole-plant light compensation points among the species. Low light performance of lodgepole pine was similar to or better than that of the other species. Growth rates at high light were broadly overlapping and there was considerable variability within species. Greatest variability among species was at intermediate light levels (30-70% full sun) where careful matching of tree species to light environment can maximize growth rates. Natural regeneration was abundant in gaps but poor in clearcuts and the forest understory. Density of all species increased from the north to south end of gaps, while growth was best in the middle to northerly positions. There was no evidence of gap partitioning by species during the regeneration phase. Instead, gap partitioning appears to take place gradually through differential mortality in response to gap size and gap position. Partial cutting had little effect on wind damage to merchantable trees (> 17.5 cm diameter). In the partial cuts, 2.2% of the trees were damaged compared to 1.1% in unlogged areas. The greatest wind damage occurred in old-growth stands whether partially cut or not. For 8 of the 9 tree species examined, no individual tree characteristics seemed to predispose them to wind damage. Amabilis fir, trembling aspen and subalpine fir were the most susceptible to windthrow.

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