Inter-Annual Climate Differences Supersede Grazing Effects on the Anatomy and Physiology of a Dominant Grass Species.

Grassland ecosystems are historically shaped by climate, fire, and grazing as essential ecological drivers. These grassland drivers influence morphology and productivity via physiological processes, resulting in unique water and carbon use strategies among species and populations. Leaf-level physiological responses in plants are framed by the underlying microanatomy, previously shown to reflect patterns of carbon assimilation and water-use in leaf tissues. However, the magnitude to which microanatomy and physiology are impacted by grassland drivers, remains unstudied. To address this knowledge gap, we sampled from three locations along a latitudinal gradient in the mesic grassland region of the central Great Plains, USA during the 2018 and 2019 growing seasons. We measured annual biomass and forage quality at the plot level, while collecting physiological and microanatomical traits at the leaf-level in cattle grazed and ungrazed locations at each site. Leaf-level measurements were focused on the dominant grass species Andropogon gerardii (big bluestem) because of its high abundance, continental-scale distribution, and forage value. The two sampling seasons received markedly different levels of precipitation: drought conditions in 2018 and excessive early season precipitation in 2019. Ambient drought conditions negatively impacted A. gerardii physiology and drastically reduced productivity regardless of grazing. Leaf-level microanatomical traits, particularly those associated with water-use, varied within and across locations and between years. Our results highlight how trait plasticity can serve as an important tool for predicting future grassland responses to climate change and variable disturbances. Specifically, climate played a stronger role than grazing in shaping above-ground processes in microanatomy and physiology.