Climate variability supersedes grazing to determine the anatomy and physiology of a dominant grassland species

Seton Bachle,Jesse B Nippert

doi:10.1007/s00442-022-05106-x

Abstract

Grassland ecosystems are historically shaped by climate, fire, and grazing which are essential ecological drivers. These grassland drivers influence morphology and productivity of grasses via physiological processes, resulting in unique water and carbon-use strategies among species and populations. Leaf-level physiological responses in plants are constrained by the underlying anatomy, previously shown to reflect patterns of carbon assimilation and water-use in leaf tissues. However, the magnitude to which anatomy 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 (drier) and 2019 (wetter) growing seasons. We measured annual biomass and forage quality at the plot level, while collecting physiological and anatomical traits at the leaf-level in cattle grazed and ungrazed locations at each site. Effects of ambient drought conditions superseded local grazing treatments and reduced carbon assimilation and total productivity in A. gerardii. Leaf-level anatomical traits, particularly those associated with water-use, varied within and across locations and between years. Specifically, xylem area increased when water was more available (2019), while xylem resistance to cavitation was observed to increase in the drier growing season (2018). Our results highlight the importance of multi-year studies in natural systems and how trait plasticity can serve as vital tool and offer insight to understanding future grassland responses from climate change as climate played a stronger role than grazing in shaping leaf physiology and anatomy.

Highlights

The Great Plains is the largest expanse of grasslands in North America, reaching from Saskatchewan through Texas (Robinson et al 2019; Jones et al 2020)
Our data illustrated that the pattern of variation in response to wet/dry years was not uniform across locations and these responses to interannual climate had a larger effect than responses to cattle grazing
Our results emphasize the large differences in physiological and anatomical responses that can exist within a widespread C4 grass species (A. gerardii) across multiple years and locations with distinct climate and variable management histories (Fig. 1)

Summary

Introduction

The Great Plains is the largest expanse of grasslands in North America, reaching from Saskatchewan through Texas (Robinson et al 2019; Jones et al 2020). 1976; Sala et al 1988; Lura et al 2019) The impacts of these gradients are reflected in the grassland ecotones of the Great Plains (arid to mesic) that separate regions of shortgrass, mixed-grass, and tallgrass prairies (DeLuca and Zabinski 2011; Dixon et al 2014). Each of these prairie systems are dominated by a few grass species that account for a majority of annual production. These adaptations include but are not limited to: (1) large shallow rooting systems comprised of fine roots that quickly absorb water (Nippert and Knapp 2007; Nippert et al 2012); (2) belowground meristematic tissues (“bud banks”) which

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