ATMOSPHERIC DEPOSITION TO FORESTS ALONG AN ELEVATIONAL GRADIENT AT WHITEFACE-MOUNTAIN, NY, USA

Atmospheric deposition rates of pollutants are known to be greater at high elevations than at low elevations in the same region, but the pattern of the deposition rate increase with elevation has not been established. This study was conducted to estimate nutrient and pollutant transfer rates from the atmosphere to forests along an elevational gradient at a site in the Adirondack Mountains of NY, U.S.A. Widely used models of cloud droplet, dry aerosol and gas phase SO2 and HNO3 deposition processes were modified for use in this application. An extensive data set describing the elevational variation in forest canopy composition and structure and microclimatic variables was assembled from measurements made by us during seven years of research activities at Whiteface Mountain, NY. Model estimates of total atmospheric deposition of S and N increased by factors of 4 and 5, respectively, over the elevational range 600-1275 m. Steep gradients in wind speed and cloud immersion frequency contributed to a nearly exponential increase in ion deposition by cloud water interception, which was responsible for most of the increase in total ion deposition rates with elevation. An additional factor contributing to increased deposition rates at high elevations was an increasing percentage of total leaf area attributable to coniferous vegetation, which is more effective at scavenging aerosol particles and cloud droplets than broad-leaved vegetation. Dry deposition contributed 13% and cloud water 61% of the 29.5 kg N ha-1 yr-1 estimated total N deposition at 1275 m elevation as opposed to 22 and 5% of the 7 kg N ha-1 yr-1 total N deposition estimated for surrounding low-elevation (600 m) forests. The dry deposition component accounted for 5% and the cloud water component 64% of the estimated 31 kg S ha-1 yr-1 total deposition at 1275 m elevation, in comparison with a 21% dry and 5% cloud contribution to the 8.4 kg S ha-1 yr-1 estimated deposition to low-elevation forests.