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

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 Effects of ambient drought conditions superseded local grazing treatments and reduced carbon assimilation and total productivity in A. gerardii.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.

Keywords: Anatomy; Andropogon gerardii; Cattle grazing; Climate gradient; Ecophysiology.

Fig. 1

a Long-term mean annual precipitation for each location (1981–2019); error bars represent standard error. b Growing season precipitation from May 1st to August 10th during 2018 and 2019. Shapes denote location (open circle, S. KS; open triangle, N. KS; open square, NE), while color denotes year (gray, 2018; black, 2019)

Fig. 2

Gas-exchange collected at each site and treatment during the 2018 (gray) and 2019 (black) growing seasons. a Photosynthetic rate (A_n); b stomatal conductance to vapor (g_s); c transpiration rate (E). Thickened lines represent the median value; upper and lower edges of box represent the interquartile values (25th and 75th percentiles)

Fig. 3

Boxplots of anatomical data collected at each site and treatment during the 2018 (gray) and 2019 (black) growing season. a Mean xylem area (X_A); b mean bulliform cell area (B_A). Thickened lines represent the median value; upper and lower edges of box represent the interquartile values (25th and 75th percentiles)

Fig. 4

Linear regression relating leaf-level nitrogen content and mean photosynthetic rate at each location and year (mean ± SE). a Relationship across years; b relationship separated by treatment. Shapes denote location (circle, S. KS; triangle, N. KS square, NE), while color denotes year (gray, 2018; black, 2019)

Fig. 5

Coefficient of variation (CV) at each location and year. a Combined mean CV for the photosynthetic rate (A_n), stomatal conductance (g_s), and transpiration rate (E). b Combined mean CV for all anatomical traits (excluding redundancies). c Combined mean CV for anatomical traits that influence water storage or transport (X_A, t/b, B_A). Colors denote year of sampling (gray, 2018; black, 2019)

Fig. 6

Principal components analysis (PCA) of mean trait values for A. gerardii at each location and year. This plot provides a summary of populations in multivariate trait space using the first two PC axes, which together account for 60.9% of the trait variation. a Grouped across years; b grouped by treatment. Shapes denote location (circle, S. KS; triangle, N. KS square, NE), while color denotes grouping: a gray, 2018; black, 2019. b Light green, grazed; dark green, ungrazed. Information concerning PCA axes importance and subsequent loadings are located in Table S3