Changes in Southern Appalachian Canopy Tree Gaps Sampled Thrice

Abstract

Species responses to disturbance (mortality of dominant individuals within a community) influence many aspects of that ecological community. To trace the responses of vegetation to one particular type of disturbance, I sampled vegetation in 250 canopy gaps in 1976/1977, 1983, and 1990/1991. These gaps were located in three sites in the southern Appalachians of eastern North America: Great Smoky Mountains National Park, Walker Cove Research Natural Area, and Joyce Kilmer Wilderness Area. Each gap was sampled thrice for sapling composition (stems ≥1 cm in diameter at breast height). Additional measurements included the extension growth of border trees into the gap, recent mortality rates of border trees, deterioration rates of gap makers that had been standing dead in earlier samples, and the composition of understory plots. The mean extension (branch) growth of border trees was 12 cm/yr, with slower growth by longer branches. Gap makers that had been tall stumps tended to deteriorate, although 35% stayed intact over the 14 yr of the study. Border-tree mortality averaged 0.60%/yr, with higher rates for larger stems and with much interspecific variation. Stem density of saplings in gaps increased during the first sampling interval and decreased during the second as self-thinning counteracted increased establishment. Basal area increased during both sampling intervals. The death of border trees increased basal area per unit gap area. The four main species (Acer saccharum, Tsuga canadensis, Fagus grandifolia, and Halesia carolina) showed different patterns of correlation to gap size and age. Species in general showed more correlations with gap age for the first sample than afterwards; gap size was more consistently related to species importance. Species patterns also were affected by the presence or absence of border-tree mortality. The stands studied seem nearly at equilibrium. Some small changes are likely to occur, but the species present dominated all size classes: gap saplings, border trees, other canopy trees not related to gaps, and understory saplings. Species differed in their relative growth rates in the understory and in gaps of different sizes. Species also differed in their survival rates in the understory and in the canopy.