Format: ORAL

Duncan D. Smith1, Mark A. Adams2,3, Amanda M. Salvi1, Ccile An4, Katherine A. McCulloh1, Thomas J. Givnish1

1 Department of Botany, University of Wisconsin-Madison, Madison, USA 2 Faculty of Science, Engineering, & Technology, Swinburne University of Technology, Hawthorn, Australia 3 School of Ecosystem and Forest Sciences, University of Melbourne, Creswick, Australia 4 Department of Statistics, University of Wisconsin-Madison, Madison, USA

Tradeoffs between the energetic benefits and costs of traits can shape the distributions of species and traits along environmental gradients. Here we test predictions based on tradeoffs involving photosynthesis and water loss using survival, growth, and 50 photosynthetic, hydraulic, and allocational traits of ten Eucalyptus species grown in four common gardens along an 8-fold gradient in relative moisture supply as measured by precipitation/pan evaporation (P/Ep) from mallee to tall wet sclerophyll forest in Victoria, Australia. Phylogenetically structured tests show that most trait-environment relationships accord qualitatively with theory. Most traits appear adaptive across species within gardens (indicating fixed genetic differences) and within species across gardens (indicating plasticity). However, species from moister climates have lower stomatal conductance than others grown under the same conditions. Responses in stomatal conductance and five related traits appear to reflect greater mesophyll photosynthetic sensitivity of mesic species to lower leaf water potential. Our data support adaptive crossover, with realized height growth of most species exceeding that of others in climates they dominate. Our findings show that pervasive physiological, hydraulic, and allocational adaptations shape the distributions of dominant Eucalyptus species along a subcontinental climatic moisture gradient, driven by rapid divergence in species P/Ep and associated adaptations over the past 4 million years. Across the entire genus, the rate of net species diversification has increased substantially over this period, especially in areas of low climatic moisture availability, which are often associated with smaller species ranges.