Dynamics of sulfur exchange at the air/forest canopy interface: A review of throughfall inferred deposition rates

Abstract

A review of the literature on the throughfall/incident precipitation approach is presented as it relates to the measurement of sulfur uptake by forest canopies during non‐precipitation dry periods. This measurement approach has produced valuable data on other‐than‐wet deposition of total sulfur. The larger sulfur deposition values obtained when using annual‐budget calculations versus event‐based calculations may be attributed to enhanced deposition during wettened canopy periods or to cloud/fog interception. Substantially greater other‐than‐wet sulfur deposition values for coniferous versus deciduous forest canopies have been observed. However, there are currently too many assumptions regarding throughfall/incident measurements that have not been tested. Canopy exchange may be from about 15% to as much as 70% of the difference between throughfall and incident sulfur. The contribution of gas and particulate phases to other‐than‐wet deposition needs to be quantified. Leaching and retention need to be characterized for various types of forest canopies. Thus, it is concluded that the dry or other‐than‐wet deposition values presented in the throughfall/incident literature are generally of low accuracy.