Effects of endemic densities of canopy herbivores on nutrient dynamics along a gradient in elevation in the southern Appalachians

In southern Appalachian forests, outbreaks of insect herbivores have been shown repeatedly to increase the availability of nutrients in soil and the export of nitrate in forest streams. The mechanisms underlying herbivore-induced changes in nutrient dynamics include inputs of insect frass (feces) and modification of precipitation as it passes through the forest canopy (throughfall). Here, we consider the effects of endemic (non-outbreak) populations of insect herbivores on soil processes in the southern Appalachians. We measured inputs of frass and throughfall at three elevations at the Coweeta Hydrologic Laboratory, North Carolina. We also measured soil nutrient availability and soil respiration. Inputs of total frass, frass nitrogen and frass carbon exhibited early- and late-season peaks, with those peaks occurring earlier at low elevation where leaf flush begins first. The C:N ratio of frass generally increased over time at all elevations, presumably reflecting seasonal declines in foliar nitrogen. Nitrate in throughfall generally increased over time, whereas throughfall phosphate declined and throughfall ammonium remained relatively constant. Relationships among frass deposition and throughfall nutrients varied with elevation. At low elevation, frass nitrogen was strongly correlated with throughfall nitrate, but this relationship was absent at mid and high elevation. The relationships between frass deposition and throughfall ammonium were inconsistent among elevations. The availabilities of nitrate and ammonium in soil were both related to frass deposition. For example, frass deposition in May explained about 62% of the variance in soil nitrate availability. Soil respiration exhibited summer maxima at all elevations and was related primarily to soil temperature. There was also a weak positive relationship between the C:N ratio of frass and soil respiration. Overall, we suggest that endemic densities of canopy herbivores can influence forest soil processes, but that the relationships exhibit pronounced spatial and temporal variability.