Widespread species often occur across a range of climatic conditions, through a combination of local genetic adaptations and phenotypic plasticity. Species with greater phenotypic plasticity are likely to be better positioned to cope with rapid anthropogenic climate changes, while those displaying strong local adaptations might benefit from translocations to assist the movement of adaptive genes as the climate changes. Eucalyptus tricarpa occurs across a climatic gradient in south-eastern Australia, a region of increasing aridity, and we hypothesized that this species would display local adaptation to climate. We measured morphological and physiological traits reflecting climate responses in nine provenances from sites of 460 to 1040mm annual rainfall, in their natural habitat and in common gardens near each end of the gradient. Local adaptation was evident in functional traits and differential growth rates in the common gardens. Some traits displayed complex combinations of plasticity and genetic divergence among provenances, including clinal variation in plasticity itself. Provenances from drier locations were more plastic in leaf thickness, whereas leaf size was more plastic in provenances from higher rainfall locations. Leaf density and stomatal physiology (as indicated by C-13 and O-18) were highly and uniformly plastic. In addition to variation in mean trait values, genetic variation in trait plasticity may play a role in climate adaptation.
Our study investigates the nature of adaptation to climate in a widespread tree species, a topic of current and growing scientific interest, given the ongoing pressures of climate change. However, surprisingly few studies to date have examined within-species patterns of phenotypic plasticity and local adaptation in climate related traits, in any detail. Our study reveals clinal variation across a rainfall gradient, not only in the values of functional traits, but in the plasticity of several morphological traits. The findings support the hypothesis that the adaptive value of plasticity depends on both the environment and the trait in question, with genetic variation for plasticity itself forming part of local adaptations to climate.