Physiological Evidence from Common Garden Experiments for Local Adaptation and Adaptive Plasticity to Climate in American Live Oaks (Quercus Section Virentes): Implications for Conservation Under Global Change

Abstract

Climate is known to be a critical factor controlling the broad-scale distribution of plants but often the physiological basis for species distribution limits is not well understood, nor is the extent to which populations within species are locally adapted to climate. Reciprocal transplant experiments designed to test for local adaptation are difficult to conduct and interpret in long-lived species, like oaks. Linking the physiological tolerances of species to their climatic distributions is an alternative approach to understanding adaptation to climate, and is important in predicting future distributions of species under changing climatic conditions. Here we synthesize a series of studies in a single lineage of American oaks that span the temperate tropical divide and encompass a range of precipitation and edaphic regimes, to determine (1) the physiological basis for adaptation to seasonal winter and seasonal drought and (2) the variation among populations that associated with climate variation and can be interpreted as local adaptation. We focus primarily on a series of common gardens that allow us to determine the genetically based differences in functional and physiological traits as well as the genetically based responses to contrasting temperature or precipitation regimes. We show that variation in freezing tolerance among closely related species is greater than variation among populations within species. Nevertheless, freezing tolerance varies predictably with climate of origin and is negatively associated with growth rate. In contrast, drought tolerance mechanisms vary more among populations within a single species, at least for the most widely distributed species, Quercus oleoides, than between species. Within this species, climate of origin predicts a suite of leaf physiological traits, and there is evidence for evolutionary trade-off between desiccation avoidance and desiccation resistance. Combined, these results show evidence for local adaptation to both freezing and drought stress within species, as well as adaptive differentiation between closely related species, despite phylogenetic conservatism in functional traits and highly similar physiognomy across the American live oak clade. The results inform conservation efforts aimed at preventing extinction of tree species in the face of global change.