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Leaf traits predict water-use efficiency in U.S. Pacific Northwest grasslands under rain exclusion treatment

Does drought stress in temperate grasslands alter the relationship between plant structure and function? Here we report data from an experiment focusing on growth form and species traits that affect the critical functions of water- and nutrient-use efficiency in prairie and pasture plant communities. A total of 139 individuals of 12 species (11 genera and four families) were sampled in replicated plots maintained for three years across a 520 km latitudinal gradient in the Pacific Northwest, USA. Rain exclusion did not alter the interspecific relationship between foliar traits and stoichiometry or intrinsic water-use efficiency. Rain exclusion reduced intrinsic water-use efficiency in grasses, an effect was primarily species-specific, although leaf morphology, life history strategy, and phylogenetic distance predicted intrinsic water-use efficiency for all twelve species when analyzed together. Variation in specific leaf area explained most of the variation in intrinsic water-use efficiency between different functional groups, with annual forbs and annual grasses at opposite ends of the resource-use spectrum. Our findings are consistent with expected trait-driven tradeoffs between productivity and resource-use efficiency, and provide insight into strategies for the sustainable use and conservation of temperate grasslands. Plain language summary Scientists have previously shown that plant leaf form (e.g., shape, width, size) has a predictable relationship to leaf function (e.g., how it can perform biological processes). When we deprive plants of water, does this relationship break down? We grew prairie and pasture plants at three sites in Oregon and Washington, USA, spanning a broad range of climate and water availability. At each site, we built shelters over half our plots to keep out some of the rain, reducing how much water our plants received. Leaf form-function relationships did not change between plots with more or less water. However, each species had a different water use efficiency and nutrient content, and some grasses had an unusual response, that is, they became less efficient at using water under less rain. Overall, we were pretty good at predicting water and nutrient use based on leaf form, whether plants were annual or perennial, and how related they were. Our findings match expectations about leaf structure-function relationships and people who manage temperate grasslands can use our results to decide which plants will work best for using and conserving their systems. Key points Foliar structure-function relationships did not change under experimental drought. Leaf morphology, life history, and phylogenetics predicted resource-use for 12 species.