Integrated Adaptations to Moisture Supply and Cross-over in Whole-plant Growth among Eucalyptus Species Along an Australian Rainfall Gradient

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Land plants face a fundamental tradeoff: to acquire the carbon dioxide needed to photosynthesize and grow, they must expose their moist, living cells to a dry atmosphere, which causes enormous losses of water. Any adjustment to minimize water loss comes at the price of slower intake of carbon. Theory suggests that roughly 20 traits involving plant morphology, physiology, and leaf structure should vary with moisture supply in order to maximize whole-plant height growth and survival. This study will examine all these traits in ten species of Eucalyptus (one of the most widely planted genera of trees on Earth) that differ in their distribution across a rainfall gradient from temperate rain forest to semi-desert in Victoria, Australia. Plants will be grown in four common gardens that span this gradient. Measurements of photosynthetic and water transport capacity will be made regularly and periodic harvests will be used to assess morphology and rates of growth. This project is pioneering for understanding adaptation to moisture supply, because it combines a novel theoretical approach with the use of common gardens to test for potential forces shaping species distributions. This project will train a postdoc, at least one graduate student, and multiple undergraduates. The results will help to understand plant response to prolonged and severe droughts, and distribution in arid regions. A multimedia presentation on Eucalyptus adaptation to moisture supply will be made for public television, and a capstone field course will be offered through UW-Madison and U Sydney that will focus on plant adaptation and community ecology.Plant survival, growth, adaptation, and integration of roughly 20 traits involving photosynthesis, hydraulics, and resource allocation will be studied in ten Eucalyptus species, stratified phylogenetically and by dominance of successive bands of annual rainfall in Victoria, Australia. Measurements will be made over two 2-year experiments at four common gardens spanning this gradient. Phylogenetically structured tests of adaptation and functional integration will be conducted, asking whether the observed shifts tend to maximize rates of height growth, whether patterns of plasticity within species in response to moisture supply conflict with those seen across species, and whether species show evidence of adaptive cross-over, with taxa having an advantage in height growth or survival only under conditions similar to those they dominate in nature. A direct experimental test of competitive success at each common garden will be compared with expectations based on growth rates of uncrowded monocultures and theoretical predictions based on plant traits. The proposed research is novel for studying plant adaptation to moisture supply, in terms of the range of traits whose adaptation and integration will be examined, the use of common gardens to test for cross-over in height growth as support for adaptation and a potential force shaping species distributions, and the use of growth models to identify key traits and trait aggregates that affect plant growth.