Limitations to photosynthetic function across season in Larrea tridentata (creosotebush) growing on contrasting soil surfaces in the Sonoran Desert

Plant community structure in the southwestern United States co-varies with soil surface characteristics due to their role in controlling water availability. At the University of Arizona Desert Laboratory on Tumamoc Hill, we evaluated winter and summer season dynamics of photosynthesis in a dominant shrub species, Larrea tridentata (creosotebush), across soils with contrasting development of surface and sub-surface horizons. We measured pre-dawn water potential (Ψpd), stomatal conductance (gs), leaf nitrogen content (%N), and determined stable carbon isotope discrimination (Δ). There were no differences in these parameters throughout the winter, although Δ was higher and %N was lower on the clay than sandy soil early in the dry summer period, and gs was higher on the clay than sandy soil late in the summer season while precipitation decreased. We used path analysis to determine the relative influences of %N, gs and Ψpd on photosynthetic function (Δ) during the winter and summer seasons. There was no influence during the summer, although gs and %N had positive and negative impacts on Δ within the sandy soil type respectively during the winter. Across soil type, gs was the main driver of %N and Δ. Our study highlights the need to include soil type for understanding how physiological performance of L. tridentata varies over seasons within arid and semi-arid ecosystems.