Meeting Abstract
Small ectotherms live in microclimates, which can vary in complex ways in space and time. A key factor driving microclimatic complexity is irradiance—relative sunniness or shadiness. We examined how sun versus shade drives patterns of temperature diversity in aspen canopies (Populus tremuloides), and the consequences of those patterns for small, leaf-mining caterpillars (Phyllocnistis populiella). In particular, we evaluated whether exposure to direct sunlight (1) alters larval growth rates by systematically altering distributions of daytime temperatures; or (2) increases risk of mortality from short-term exposure to lethally high temperatures without significantly altering the overall diurnal distribution of temperatures. We distinguished these possibilities by experimentally shading leaves in the field and measuring effects on leaf temperature and on larval growth and survival. We also estimated larval thermal performance curves (for growth) using short-term, lab-based exposure to constant temperatures, and upper lethal temperatures in the field using custom-built, heat-shock devices. Our data suggest that intra-canopy variation in the position, orientation, and relative irradiation of leaves drives high levels of spatial and temporal variation in leaf temperature—but that the high temperatures reached by sun-exposed leaves are transient enough that they are more important in driving risk of exposure to lethal temperatures than they are in shifting larval performance within the permissive range of temperatures.
