Phenological responses to prior-season defoliation and soil-nutrient availability vary among early- and late-flushing aspen (Populus tremuloides Michx.) genotypes

Abstract

Many spring-feeding insect folivores have evolved to hatch in synchrony with temporal windows of high foliar quality that occur during leaf expansion. In some host-tree species, however, budbreak phenology can shift in response to defoliation during the prior growing season. While this response could have important impacts on the performance of insect folivore species prone to multi-year outbreaks, the prevalence, underlying mechanisms, and ecological ramifications of defoliation-induced phenological shifts are poorly understood. We established a common garden of twelve trembling aspen (Populus tremuloides Michx.) genotypes, and observed the effects of prior-season defoliation and soil-nutrient availability on budbreak phenology during the following spring. Average budbreak phenology was delayed in response to both prior-season defoliation and low soil-nutrient treatments, but these responses were lower in magnitude than those previously shown to occur in response to temperature. Magnitudes of budbreak delay varied significantly among genotypes and were highest among early-flushing aspen. This negative correlation between aspen budbreak phenology and magnitudes of defoliation-induced phenological delay suggests that widespread defoliation could decrease intraspecific variation in aspen budbreak phenology during subsequent growing seasons, potentially allowing larvae to adapt to windows of optimal foliage quality during multi-year outbreaks. These results quantify additional environmental factors that may act in concert with major drivers such as temperature to mediate phenological relationships between aspen and folivorous insects.