Consequences of phenology variation and oxidative defenses in Quercus

Abstract

Ecologists are interested in consequences of changes in the timing of ecological interactions because of the potential for widespread phenological shifts under climate change. Plant–herbivore interactions may be particularly affected by phenology changes if they result in differences in leaf defensive chemistry and other aspects of host foliage quality. Recent studies of defensive leaf phenolics have focused on their ability to create oxidative stress in herbivores, but the phenology of this “oxidative capacity,” including seasonal patterns and how it is affected by leaf-out phenology, is unknown. We combined observational and experimental studies of oaks (Quercus spp.) to determine if leaf-out phenology correlates with oxidative capacity and leaf characteristics, herbivore abundance, and leaf damage, and how oxidative capacity changes through spring and summer leaf development. Oaks with earlier leaf-out had significantly lower oxidative capacity in spring and early summer, and there was a trend for higher spring leaf damage on these trees. One group of spring herbivores, leaf rollers, was more abundant on earlier-leafing trees. By mid-summer, leaf quality differences disappeared. Repeated measurements of individual trees demonstrated that oxidative capacity is highest shortly after leaf-out but declines within 2 weeks. These results partially contrast traditional views of leaf defense phenology where leaves are most susceptible shortly after leaf-out, but defenses build as proanthocyanidins accumulate in expanded leaves. Early season oxidative defenses may have a significant effect on plant growth by protecting young leaves at a time when leaf tissue loss will have the greatest consequences for later photosynthesis.