Growing up Aspen: ontogeny and trade-offs shape growth, defence, and reproduction in a foundation species.

BACKGROUND AND AIMS: Intraspecific variation in foundation species of forest ecosystems can shape community and ecosystem properties, particularly when that variation has a genetic basis. Traits mediating interactions with other species are predicted by simple allocation models to follow ontogenetic patterns that are rarely studied in trees. The aim of this research was to identify the roles of genotype, ontogeny and genotypic trade-offs shaping growth, defence, and reproduction in aspen. METHODS: We established a common garden replicating over 500 aspen genets in Wisconsin, USA. Trees were measured through the juvenile period into the onset of reproduction, for growth, defence chemistry (phenolic glycosides, condensed tannins), nitrogen, extrafloral nectaries, leaf morphology (specific leaf area), flower production, and foliar herbivory and disease. We also assayed the TOZ19 sex marker and heterozygosity at 10 microsatellite loci. KEY RESULTS: We found high levels of genotypic variation for all traits, and high heritabilities for both the traits and their ontogenetic trajectories. Ontogeny strongly shaped intraspecific variation, and trade-offs among growth, defence, and reproduction supported some predictions while contradicting others. Both direct resistance (chemical defence) and indirect defence (extrafloral nectaries) declined during the juvenile stage, prior to the onset of reproduction. Reproduction was higher in trees that were larger, male, and had higher individual heterozygosity. Growth was diminished by genotypic allocation to both direct and indirect defence as well as to reproduction, but we found no evidence of trade-offs between defence and reproduction. CONCLUSIONS: Key traits affecting the ecological communities of aspen have high levels of genotypic variation and heritability, strong patterns of ontogeny, and clear trade-offs among growth, defence, and reproduction. The architecture of aspen's community genetics-its ontogeny, trade-offs and especially its great variability-is shaped by both its broad range and diverse community of associates and in turn further fosters that diversity.