Large-scale outbreaks of defoliating insects are common in temperate forests. The effects of defoliation on tree physiology are expected to cascade through the entire forest ecosystem, altering carbon, nitrogen, and water fluxes, and subsequently affecting nitrogen cycling and plant-herbivore interactions. If these post-defoliation changes are largely driven by N deficiency, tree root system responses to defoliation should be central to regulating the long-term effects of defoliation; N fertilization should reverse the effects. We examined these phenomena in a 3-year large-scale replicated manipulative field experiment in a hybrid poplar plantation, where we regulated defoliation by gypsy moths as well as nitrogen availability. To our knowledge, this is the first manipulative field experiment at this scale to examine the effects of severe insect defoliation on whole-tree physiology. Defoliation decreased tree growth and increased the rate of top dieback in the stand. Defoliation led to transient declines in carbon allocation to starch 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 increased tree growth, but did not alter defoliation effects on starch accumulation or top dieback. Defoliation and fertilization treatments did not interact. In this system, defoliation effects on tree recovery of leaf nitrogen lost to herbivory were primarily driven by effects on nitrogen uptake, rather than effects on root production or mortality.

The amount of root mortality caused by root pathogens such as Phytophthora nicotianae (syn. Phytophthora parasitica) has typically been inferred from the net change in root length density in sequential soil cores. Because such measurements give information only on net changes in root populations, the actual rate of root turnover is often underestimated. We used minirhizotrons to track the fate of a large number of individual fine roots of mature field-grown citrus trees over a 6-month period. This method enabled us to examine the effect of P nicotianae population levels on fine-root mortality. Seasonal and genotypic variation in patterns of citrus fine root mortality were associated with variation in population levels o f P nicotianae. Fine root lifespans were shorter when populations of P nicotianae were high. Fine roots of the Phytophthora-susceptible rootstock, rough lemon (Citrus jamibhiri), had shorter median lifespans and supported larger populations of P nicotianae than the fine roots of the more tolerant rootstock, Volkamer lemon (Citrus volkameriana). Rates of root mortality were either relatively constant for roots of all ages, or increased with age; the latter pattern was most pronounced for Volkamer lemon roots. Differences in the age-dependence of root mortality may, therefore, play a role in genotypic differences in tolerance of Phytophthora root rot by these two rootstocks.

Common ragweed is an annual weed problem after winter wheat harvest in southwest Michigan. Although an interseeded cover crop of red clover is known to reduce weed populations in winter wheat stubble, the most effective rates and cultivars for weed suppression under Michigan conditions have not been identified. Three red clover cultivars were planted in March at three seeding rates in established winter wheat; after wheat harvest, a section of each plot was mowed to mimic forage harvest of clover. The experiment was repeated in 2 yr. Mowing significantly reduced common ragweed biomass each year. All cultivars and seeding rates were equally effective at significantly reducing common ragweed biomass in each year, despite the variation among years, cultivars, seeding rates, and mowing treatments in production of clover biomass.

No abstract