Atmospheric deposition and corresponding variability of stemflow chemistry across temporal scales in a mid-Atlantic broadleaved deciduous forest

Despite the fact that atmospheric deposition is widely accepted to be an important process in the biogeochemical cycling of wooded ecosystems, no single study is known that has examined stemflow chemistry in relation to atmospheric deposition across time scales, from within discrete events to season, to chronicle alterations in temporal patterns of stemflow chemistry. This research partitioned stemflow solute fluxes (K+, Na+, Mg2+, Ca2+, Cl-, NO3-, and SO42-) from two tree species of differing canopy form and bark morphology into their leaching and dry deposition washoff components using a modified Kazda (1990) integration model at the intra-storm scale to examine differences within and among discrete rain events. Median annual stemflow concentrations in yellow poplar (Liriodendron tulipifera L) stemflow were higher than American beech (Fagus grandifolia Ehrh.) stemflow for all ions except NO3-. Beech median enrichment ratios were larger for all monitored ions than yellow poplar. All intra-storm stemflow ionic fluxes were initially high, exponentially decaying to a steady input, typically dominated by leaching contributions. With the exception of yellow poplar stemflow Cl- and NO3- fluxes and beech Na+ flux, all intra-storm mean ionic fluxes began with higher dry deposition percentages and transitioned to primarily leaching. Observations in the field implicate increased magnitude and rainfall intensity in the initiation of new stemflow flowpaths, evidenced by increased variability in the timing of stemflow ionic deposition and fluctuations in the proportion of washoff and leaching during some events. Beech stemflow fluxes were larger than yellow poplar for all ions during the leafed, leafless, and annual periods. Our results demonstrate: (1) the critical role of the initiation of new flowpaths and expansion and maturation of developed flowpaths on a tree's surface to solute enrichment and transport to the forest floor as a canopy wets-up: and (2) the importance of temporal scale in providing important insights into some effects of stemflow on biogeochemical cycling. (C) 2011 Elsevier Ltd. All rights reserved.