Western spruce budworm effects on throughfall N, P, and C fluxes and soil nutrient status in the Pacific Northwest

Abstract

Western spruce budworm (Choristoneura freemani Razowski, 2008) is the most widely distributed insect herbivore in western North American coniferous forests. By partially or completely defoliating tree crowns, budworms influence fluxes of water, nutrients, and organic carbon from forest canopies to soils and, in turn, soil chemistry. To quantify these effects, throughfall water, inorganic nitrogen (N), phosphorus (P), and dissolved organic carbon (DOC) concentrations, as well as fluxes and soil N and P concentrations, were measured in coniferous forest sites with high and background levels of budworm herbivory. Throughfall N and P concentrations and fluxes increased at sites with high budworm levels during and (or) immediately after larval-stage budworm feeding, indicating reduced uptake and (or) greater leaching from canopies as a result of budworm activity. Annual throughfall N fluxes (<67–71 g N·ha−1·year−1) and soil N concentrations were low regardless of herbivory level. In contrast, throughfall P was considerably greater at sites with high herbivory levels (2174 g P·ha−1·year−1) compared with those with background levels (1357 g P·ha−1·year−1), and this was reflected in nearly threefold higher soil P concentrations at sites with high budworm levels. Our findings suggest that by altering throughfall chemistry and soil N:P, budworms could influence elemental export from watersheds.