Browse quality in quaking Aspen (Populus tremuloides): effects of genotype, nutrients, defoliation, and coppicing

The consequences of interactions among genetic, ontogenetic, and environmental factors for the quality of winter-dormant tissues as food for browsing herbivores is poorly understood. We conducted two sequential common garden studies to assess the impacts of intraspecific genetic variation, nutrient availability, prior defoliation, and ontogenetic stage on the chemical quality of winter-dormant tissue in quaking aspen (Populus tremuloides Michx.). In the first study, saplings of 12 aspen genotypes were grown under low and high soil nutrient conditions, with or without two successive seasons of defoliation. Quantity and quality of current year's twig growth were assessed. Twig production varied among genotypes and declined under low nutrient availability, but showed little response to prior defoliation. Chemical quality of sapling twigs varied substantially among genotypes, and in response to nutrient availability and prior defoliation. Overall, browse quality improved (nitrogen levels increased while phenolic glycoside and condensed tannin levels decreased) after defoliation. Growth and chemical variables exhibited low to moderate clonal repeatability (broad sense heritability) values. Our second study employed the same 12 genotypes, grown under high-nutrient conditions and with or without two seasons of defoliation. The trees were coppiced to produce root sprouts, which were chemically assessed 1 yr later. Rejuvenation via coppicing led to increased levels of nitrogen, phenolic glycosides (salicortin), and tannins in root sprouts, and the magnitude of change varied among aspen genotypes. Signatures of defoliation nearly 2 yr earlier persisted in terms of elevated levels of phenolic glycosides in root sprouts of previously defoliated trees. Aspen forests likely present browsing herbivores with chemically heterogeneous environments because of the interactions of genetic, ontogenetic, and environmental factors that vary over space and time.