Insect herbivory and fire have an interactive effect on nutrient dynamics in an oak savanna ecosystem

Nutrient cycling is known to depend on trophic interactions and disturbance, but the interactive effect of these factors is still poorly understood. In this study, we investigated 1) how herbivory by two abundant insect species (a lace bug Corythuca arcuata and an aphid Hoplochaithropsus quercicola) specializing on bur oaks (Quercus macrocarpa) is related to leaf chemistry and soil nutrient availability, and 2) how these relationships vary across a long-term manipulation of fire frequency in central Minnesota, USA. We measured soil nutrient conditions under 22 trees that were heavily infested with either lace bugs or aphids. Using mixed bed ion exchange resins, we found that soil ammonium and nitrate availability declined substantially over the course of the growing season in unburned areas, and that soil N was lower under trees infested with aphids than those with lace bugs by the end of the growing season. In contrast, N concentration declined more in leaves from lace bug trees than in those from aphid trees, particularly on high light branches where herbivory by both species is concentrated. We suggest that damage from lace bug feeding reduces leaf N content and photosynthetic rate, which in turn slows depletion of available soil N. In addition, we found that soil and bur oak leaf N levels were particularly low in frequently burned areas where lace bugs are common and aphids are rare, suggesting that fire mediation of trophic interactions can moderate the more immediate effects of fire on N cycling.