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

In this study, we documented patterns of variation in leaf phenology and leaf herbivory in scrub oak (Quercus ilicifolia Wang) along the slope of a closed topographical depression, 15 m deep and 400 m wide, in the pine barrens of eastern Massachusetts. Minimum temperatures over the growing season averaged 6.5 degrees C lower at the bottom of the depression than at the top. Bud break at the bottom of the depression was 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 between the top and the bottom of the depression. Because thrips fed exclusively on the youngest leaves, this pattern could be attributed to a closer synchrony between the timing of leaf flushing and the period of insect feeding activity towards the bottom of the depression. Average differences in leaf damage between years could also be accounted for by variation in leaf phenology. These patterns support the hypothesis that the degree of temporal overlap between availability of high quality foliage and the period of insect feeding activity may greatly determine spatial and temporal variation in leaf damage by insect herbivores.