Repeated insect defoliation effects on growth, nitrogen acquisition, carbohydrates, and root demography of poplars

Large-scale outbreaks of defoliating insects are common in temperate forest s. The effects of defoliation on tree physiology are expected to cascade th rough the entire forest ecosystem, altering carbon, nitrogen, and water flu xes, and subsequently affecting nitrogen cycling and plant-herbivore intera ctions. If these post-defoliation changes are largely driven by, N deficien cy, tree root system responses to defoliation should be central to regulati ng the long-term effects of defoliation; N fertilization should reverse the effects. We examined these phenomena in a 3-year large-scale replicated ma nipulative field experiment in a hybrid poplar plantation, where we regulat ed defoliation by gypsy moths as well as nitrogen availability. To our know ledge, this is the first manipulative field experiment at this scale to exa mine the effects of severe insect defoliation on whole-tree physiology. Def oliation decreased tree growth and increased the rate of top dieback in the stand. Defoliation led to transient declines in carbon allocation to starc h in fine roots, trunk, and twigs in the year of heaviest defoliation. Root production and root mortality were unaffected by the heaviest defoliation, but nitrate and ammonium uptake were strongly depressed. N fertilization i ncreased tree growth, but did not alter defoliation effects on starch accum ulation or top dieback. Defoliation and fertilization treatments did not in teract. In this system, defoliation effects on tree recovery of leaf nitrog en lost to herbivory were primarily driven by effects on nitrogen uptake, r ather than effects on root production or mortality.