L.J. Puckett 1991 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. -from Author application/pdf en Spatial variability and collector requirements for sampling throughfall volume and chemistry under a mixed-hardwood canopy article