TREE MORTALITY PATTERNS AFTER EXPERIMENTAL FRAGMENTATION OF AN OLD-GROWTH CONIFER FOREST

Five circular forest fragments of increasing size (1/16, 1/8, 1/4, 112 and 1 ha) were isolated in early 1986 on a 41-ha clearcut in north-western Sweden. I compared changes in the forest structure of these fragments with those of three control plots in adjacent unfragmented forest, during a five-year period. The site was a high-altitude, old-growth Picea abies forest. In the first autumn after isolation a period with high winds caused severe blowdown and other forest damage in all five fragments. Total tree mortality after 67 months showed a steep increase with decreasing area, from 30% in the 1-ha fragment to 98% in the 1/16-ha fragment. Uprooting was the most common type of mortality (67%) followed by stem-breakage (24%). The marked increase in wind-induced tree deaths shifted the mortality pattern, from being independent of tree size in controls to showing a strong positive relationship with diameter infragments. The study shows that completely isolated forest fragments up to at least 1 ha in size became entirely edge habitat shortly after logging. Edge-related changes in forest structure, deposition of woody debris and soil disturbance, triggered by altered climate, are key factors for understanding ecological processes operating in forest fragments. This supports the view that changes in small fragments are mainly driven by external factors.