Flood-driven survival and growth of dominant C4 grasses helps set their distributions along tallgrass prairie moisture gradients

Abstract

Premise: Five C₄ grasses (Bouteloua curtipendula, Schizachyrium scoparium, Andropogon gerardii, Sorghastrum nutans, Spartina pectinata) dominate different portions of a moisture gradient from dry to wet tallgrass prairies in the Upper Midwest of the United States. We hypothesized that their distributions may partly reflect differences in flooding tolerance and context-specific growth relative to each other.

Methods: We tested these ideas with greenhouse flooding and drought experiments, outdoor mesocosm experiments, and a natural experiment involving a month-long flood in two wet-mesic prairies.

Results: Bouteloua promptly succumbed to inundation, so flooding intolerance likely excludes it from wet and wet-mesic prairies. Competition is likely to exclude short-statured Bouteloua from productive mesic sites. Schizachyrium is excluded from wet prairies by low flooding tolerance, demonstrated by all experiments. Sorghastrum had low flooding tolerance in both greenhouse and natural experiments, suggesting that physiological intolerance excludes it from wet prairies. Spartina had by far the greatest growth under the wettest mesocosm conditions; this and comparisons of species growth in monocultures vs. mixtures suggests that competition helps it dominate wet prairies. Indeed, quadrat presence of Spartina increased by 57% two years after flooding of two prairies, while that of upland grasses declined by 44%. The high flooding tolerance, lack of significant differences from other species in drought tolerance, and tall stature of Andropogon suggest that broad physiological tolerance combined with competitive ability allows it to thrive across the prairie moisture gradient.

Conclusions: Flooding helps shape the distributions of dominant prairie grasses, and its effects may become more important as extreme rain events continue to increase.

Keywords: LD50; ecological distributions; flooding tolerance; logit analysis; species sorting; water table depth.

Figure 1

Logit analysis of average tissue survival after flooding for different durations. Horizontal lines are equivalent to a visual rating of 5.5. Thick curves are locally smooth regressions; the grey ribbon indicates the 95% CI. LD₅₀ values correspond to where the regressions intersect the green lines and represent the number of days of flooding required, on average, for 50% of leaf tissue to die. Reddish‐orange dots and lines are pots from greenhouse 1; turquoise dots and lines are from greenhouse 2. Species are indicated along the right‐hand edge.

Figure 2

Linear regressions of square‐root‐transformed aboveground biomass as a function of water‐table depth for Andropogon gerardii (A), Bouteloua curtipendula (B), Schizachyrium scoparium (C), Sorghastrum nutans (O), and Spartina pectinata (P) grown in monoculture in mesocosms. Based on ANCOVA, regression slopes are significantly different for Spartina vs. Bouteloua (P < 0.0001).