Variation in tolerance to herbivory is mediated by differences in biomass allocation in aspen

1. Herbivore damage can select for tolerance in plant populations where genetic variation for tolerance exists. The causes underlying variation in tolerance are not fully resolved. We assessed the importance of two potential mechanisms for tolerance by examining its relationship with leaf photosynthetic rate and relative biomass allocation across plant organs. 2. We monitored responses of 12 aspen ( Populus tremuloides ) genotypes, grown in a common garden under two levels of nutrient availability, to defoliation in two successive seasons. Tolerance of each genotype was calculated as the difference in growth between defoliated and undefoliated trees grown under the same nutrient conditions. 3. Although light-saturated leaf photosynthesis increased in response to nutrient addition and defoliation, it did not vary among genotypes and was not correlated with tolerance. 4. Tolerance was, however, correlated with patterns of biomass allocation. Under low-nutrient conditions it was positively correlated with the proportion of biomass in stems just prior to defoliation, while under high-nutrient conditions it was correlated with greater allocation to stems in response to damage. 5. Herbivores may select for specific patterns of biomass allocation in trees, and do so differently in different environments. The positive correlation between tolerance and relative allocation to stems, as opposed to roots, runs counter to reports from studies of herbaceous species and underscores the need for further exploration of mechanisms of tolerance in woody plants.