Throughfall Studies of Deposition to Forest Edges and Gaps in Montane Ecosystems

Lindberg, Steve; Owens, J

ABSTRACT: Abstract. An extensive network of bottle/funnel collectors was used to measure hydrologic, SO2-, and NO3 fluxes in rain events and in throughfall beneath the canopies of several elevation forest stands in the Great Smoky Mountains National Park during 1989-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 surrounding each gap. The paired gap/edge stands were located above (1940 m) and (1720 m) the base of the clouds typically impacting this mountain. Total hydrologic fluxes beneath the edge trees during the forest growing season exceeded fluxes beneath adjacent gap saplings by nearly a factor of three (e.g. 230 vs 88 meq m-2 for S02-) elevations. Water and SO2- fluxes were up to two times greater beneath the forest the cloud-prone 1940 m site than at 1720 m (e.g. 230 vs 110 meq m-2 for SO2-). However, throughfall NO3 fluxes were about 30% higher at 1720 m (17 vs 13 meq m-2), because lower site receives greater dry deposition of HNO3 due to its ridgetop location and wind penetration. Estimates of SO2- deposition from cloud impaction were consistent the net throughfall flux of SO2- (throughfall flux minus rain flux) at the 1940 m forest but greatly exceeded the net throughfall flux at 1940 m gap, suggesting differences concentrations in cloud droplets impacting on mature edge trees and young saplings forest gaps.