Genotypic variation of defensive compounds in aspen: Implications on selective browsing and dieback

Background/Question/Methods Determining mechanisms that influence density-independent mortality is central to the understanding of population dynamics. Most studies examining aspen (Populus tremuloides Michx.) dieback have asserted the primacy of environmental factors and top-down effects (e.g., mammalian browsing) as mechanisms affecting survivorship. Although the importance of environmental factors is indisputable, especially for a species at the edge of its distribution, they often fail to explain the variation in vigor among individuals at finer spatial scales. Numerous studies have noted that neighboring aspen clones of similar age and size differ dramatically in their susceptibility to browsing and disease, resulting in a heterogeneous distribution of dieback and vigor. This suggests that genotypic variation in aspen may play a critical role influencing clonal dynamics by acting as a predisposing factor. To examine this assertion, we selected four dieback and four vigorous clones in three edaphically distinct northern fescue prairies. Annual growth, mammalian browsing and mortality of ramets were monitored for three successive years. Foliar concentrations of phenolic glycosides (salicylates), a genetically regulated group of defensive compounds in the Salicaceae, were used to assess differences among genotypes. We hypothesize that the mosaic of clone vigor and dieback may be related to inherent differences in the production of salicylates. Results/Conclusions Differences between dieback and vigorous clones were significant, with > 90% of all ramets in dieback clones showing signs of repeated browsing, increased incidence of pathogens, and higher mortality. The impacts of mammalian browsing and fungal pathogens induced dramatic changes on crown architecture, by increasing branching density and stunting vertical growth. Previously damaged ramets within dieback clones were more susceptible, indicating a possible feedback mechanism. Our results demonstrate a high degree of variation in leaf salicylate production among clones. A significant amount of this variation was accounted for by differences in clone vigor, with a negligible amount due to environmental differences. In all cases, dieback clones produced significantly lower amounts of salicylates compared to vigorous clones, suggesting that some individuals may be inherently predisposed to undergo density-independent mortality. This has important implications on the clonal dynamics of aspen, since it implies that susceptibility to stress factors (e.g., mammalian browsing and pathogen virulence) may be under genetic regulation at local scales.