Throughfall Chemistry and Canopy Processing Mechanisms

Abstract

Forest canopies receive chemical inputs from the atmosphere by rainfall, cloud droplet capture, and the accumulation of particles and vapors by dry deposition. These chemical inputs interact with surfaces in the canopy and are released to the forest floor primarily as throughfall (TF). A quantitative understanding of chemical fluxes in TF requires examination of the mechanisms of TF processing by forest canopy components. One such mechanism involves interactions between anthropogenic acidity inputs and the forest canopy, which may increase chemical fluxes in TF. The processes that control the inorganic chemistry of coniferous forest TF include atmospheric inputs from wet and dry deposition as well as physical, chemical, and biological processes that occur on forest canopy surfaces. A review of these processes suggests that dry deposition washoff, diffusion, uptake, and cation exchange control TF chemistry. In this chapter we use these processes to generate hypothetical patterns for ion-specific TF chemical fluxes. We compare these hypotheses to short-term sequential samples of the net ionic fluxes in TF and two coniferous forests that differ in atmospheric inputs. Substantial progress has been made toward the development of a general model of TF chemistry. Experiments are proposed to address the questions that remain for the development of such a model.