Climate Gradients Underlie Geographical Variations in iWUE and δ¹⁵N Values of Encelia

This study assessed variations in leaf intrinsic water use efficiency (iWUE) and δ¹⁵N values among Encelia, a genus of drought-deciduous shrubs distributed across arid regions of southwestern North America between 1972 and 1980 when climates were cooler than today. We hypothesized that geographical variations in climate would significantly influence iWUE, a response to water-related climate constraints, and δ¹⁵N values, a proxy for the balance between N₂ fixation and denitrification. Leaf samples were collected from six species of Encelia across 78 sites representing the genus range. The δ¹⁵N and δ¹³C values of these samples were measured and analyzed to identify drivers of spatial variability. Significant variations among iWUE and δ¹⁵N values were observed as a function of climate, along a spring-summer precipitation gradient. Precipitation and vapor pressure deficit (VPD) were significant drivers of variations in iWUE values, with iWUE increasing with VPD and/or decreasing precipitation, as would be predicted based on water-related constraints on leaf gas exchange. Climate values were significant drivers of variations in δ¹⁵N values, with lower δ¹⁵N values occurring in cooler temperature, spring-growing plants (northern latitudes) than in warmer summer-growing plants (southern latitudes). Encelia leaf iWUE and δ¹⁵N observations suggest few, if any, species-specific differences; but more likely that there is high plasticity in these values driven by variations in climate.