Spatial variability and collector requirements for sampling throughfall volume and chemistry under a mixed-hardwood canopy

Abstract

Throughfall was intensively sampled during five storms in August, September, and November 1988, July 1989, and January 1990. For each storm, 50 collector bottles with funnels were positioned at random on a 0.1-ha plot under a mixed-hardwood forest canopy in northern Virginia. Concentrations and net inputs of H+, NH4+, Na+, K+, Ca2+, Mg2+, Mn2+, Cl−, NO3−, and SO42− and throughfall volume were evaluated for spatial variability both within and among storms and were used to estimate the number of collectors necessary for throughfall collection. Ion concentrations were generally less variable within storms compared with net ion input data. Concentrations and net inputs of some ions were consistently less variable than others; for example, Ca2+, NO3−, and SO42− were less variable than NH4+ and K+. These patterns of variability were consistent in comparisons both within and among storms. The relatively low variability of NO3− and SO42− is probably due to dry deposition of these ions as anthropogenic pollutants, while the low variability of Ca2+ is the result of deposition in windblown soil particles. The high variability of NH4+ and K+ is probably the result of biological processes. Ammonium is strongly retained by the canopy, and K+ is readily leached from it. Retention by, and leaching from, the canopy can induce spatial variability as a result of spatial heterogeneity in the biota. Throughfall volume also displayed low variability within and among events, requiring an average of 11 collectors to estimate the mean within 10% at the 95% confidence level.