Natural disturbance agents causing formation of canopy gaps were identified and quantified at five elevations (640, 765, 885, 1005, and 1130 m) in an old-growth spruce–fir forest. Actual and expanded gaps were measured to the inner edge of the crown or to the bole, respectively, of the live canopy trees at the gap margin. The percentage of forest canopy in gaps was lowest (6% for actual and 40% for expanded gap areas) at 1005 m elevation, near the upper limit of the spruce–fir phase, and highest (33% actual and 77% expanded) where the spruce–fir phase meets the mixed hardwood–conifer forest type at 765 m elevation. Mean gap size was smallest at the higher elevations. Expanded gap area was apportioned to tree species and disturbance agents according to basal area of affected trees in each gap. Twenty-two identified agents were involved in gap-phase mortality, and their relative importance varied strongly with elevation. Biotic diseases accounted for 66% of gap area at the lowest elevation (640 m) and only 7 and 15%, respectively, at the highest elevations (1005 and 1130 m). Root and butt rots were the most important group of biotic diseases at all elevations; their primary effect was to predispose trees to breakage of the stem near ground level. In contrast to biotic disease agents, chronic wind stress increased greatly in relative importance with elevation, accounting for 72% of gap area at 1130 m. The spruce beetle was an important disturbance agent at 640 and 765 m, where large, overmature red spruce trees had dominated the stand. Gap etiology appeared to be rarely restricted in time to a single event, and was often complex, with multiple agents occurring in the same gap and on the same tree.
Effect of shrubs on tree seedling establishment in an abandoned tropical pasture
