LEAF PHENOLOGY AND HERBIVORY ALONG A TEMPERATURE-GRADIENT - A SPATIALTEST OF THE PHENOLOGICAL WINDOW HYPOTHESIS

In this study, we documented patterns of variation in leaf phenology a nd leaf herbivory in scrub oak (Quercus ilicifolia Wang) along the slo pe of a closed topographical depression, 15 m deep and 400 m wide, in the pine barrens of eastern Massachusetts. Minimum temperatures over t he growing season averaged 6.5 degrees C lower at the bottom of the de pression than at the top. Bud break at the bottom of the depression wa s 2-3 weeks delayed compared with the top. In both years of this study , 1988 and 1989, leaf damage by thrips increased down slope producing differences of about 10% in the proportion of leaf area damaged betwee n the top and the bottom of the depression. Because thrips fed exclusi vely on the youngest leaves, this pattern could be attributed to a clo ser synchrony between the timing of leaf flushing and the period of in sect feeding activity towards the bottom of the depression. Average di fferences in leaf damage between years could also be accounted for by variation in leaf phenology. These patterns support the hypothesis tha t the degree of temporal overlap between availability of high quality foliage and the period of insect feeding activity may greatly determin e spatial and temporal variation in leaf damage by insect herbivores.