THROUGHFALL STUDIES OF DEPOSITION TO FOREST EDGES AND GAPS IN MONTANE ECOSYSTEMS

An extensive network of bottle/funnel collectors was used to measure hydrologic, SO42- and NO3- fluxes in rain events and in throughfall beneath the canopies of several high elevation forest stands in the Great Smoky Mountains National Park during 1989-1990. The throughfall fluxes were used as deposition surrogates to quantify trends in atmospheric inputs to sapling trees growing in forest gaps and to the mature forest canopy at the edge surrounding each gap. The paired gap/edge stands were located above (1940 m) and below (1720 m) the base of the clouds typically impacting this mountain. Total hydrologic and ion fluxes beneath the edge trees during the forest growing season exceeded fluxes beneath the adjacent gap saplings by nearly a factor of three (eg. 230 vs 88 meq m-2 for SO42-) at both elevations. Water and SO42- fluxes were up to two times greater beneath the forest edge at the cloud-prone 1940 m site than at 1720 m (e.g. 230 vs 110 meq m-2 for SO42-). However, throughfall NO3- fluxes were about 30% higher at 1720 m (17 vs 13 meq m-2), because this lower site receives greater dry deposition of HNO3 due to its ridgetop location and greater wind penetration. Estimates of SO24- deposition from cloud impaction were consistent with the net throughfall flux of SO42- (throughfall flux minus rain flux) at the 1940 m forest edge, but greatly exceeded the net throughfall flux at 1940 m gap, suggesting differences in ion concentrations in cloud droplets impacting on mature edge trees and young saplings in forest gaps.