Phytochemical traits underlie genotypic variation in susceptibility of quaking aspen (Populus tremuloides) to browsing by a keystone forest ungulate

Abstract

Summary Overbrowsing by ungulates is a major cause of poor aspen stand regeneration across North America and Eurasia. In general, factors driving ungulate browser preferences include concentrations of plant secondary compounds and the nutritional composition (non‐structural carbohydrates, protein and minerals) of foliage. While each of these phytochemical factors has been shown to independently influence ungulate preference, the relative impact of each factor is unknown, as no study to date has examined them simultaneously. Plant fitness depends not only on the capacity of plants to resist browsing, but also on their capacity to tolerate browsing once it has occurred. Little is known of aspen tolerance to browsing, which inflicts a different form of damage than insect herbivory. We employed multiple aspen genotypes, replicate trees of which were subjected to different soil nutrient treatments, to investigate: (i) the effects of aspen genotype, nutrient treatment and genotype × nutrient interactions on susceptibility to browsing by white‐tailed deer, (ii) the phytochemical basis for the patterns observed in (i), and (iii) the effects of genotype, soil nutrients and their interaction on short‐term tolerance to deer browsing. Aspen genotypes varied markedly in their vernal susceptibility to deer browsing. Genetic variation in early season levels of non‐structural carbohydrates (sugars), protein and multiple macro‐ and trace minerals had the strongest influence on tree susceptibility to browsing. In contrast, levels of phytochemical defences had minimal effects, although the range of levels expressed in this study was small. Soil nutrient availability did not significantly influence deer preference. The extent of browsing affected post‐browse tolerance across genotypes. Soil nutrient treatment had little differential effect on tolerance, and, for the most part, genotypes did not display differential tolerance to browsing, regardless of which soil nutrient treatment they experienced. Synthesis. Genetic variation for susceptibility to browsing indicates that ungulate browsers have the potential to be agents of selection in aspen populations. In contrast with previous studies in aspen highlighting the importance of phytochemical defences in shaping preferences of browsing mammals, our results indicate that the nutritional composition of foliage (sugars, protein and mineral concentrations) can have sizable effects on preference. The observed lack of influence of soil nutrient availability on tree browsing tolerance contrasts with predictions of the limiting resource model, the prevailing model for plant tolerance.