Water limitation reveals local adaptation and plasticity in the drought tolerance strategies of Bouteloua gracilis

Authors: BUSHEY, JULIE - U.S. DEPARTMENT OF AGRICULTURE (USDA); HOFFMAN, AVA - COLORADO STATE UNIVERSITY; Gleason, Sean; SMITH, MELINDA - COLORADO STATE UNIVERSITY; OCHELTREE, TROY - COLORADO STATE UNIVERSITY

Submitted to: Ecosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/19/2020
Publication Date: 1/24/2023
Citation: Bushey, J.A., Hoffman, A.M., Gleason, S.M., Smith, M.D., Ocheltree, T.W. 2023. Water limitation reveals local adaptation and plasticity in the drought tolerance strategies of Bouteloua gracilis. Ecosphere. 14(1). Article e4335. https://doi.org/10.1002/ecs2.4335.
DOI: https://doi.org/10.1002/ecs2.4335

Interpretive Summary: As the frequency and severity of drought increases in many regions around the globe, the losses of individual species may result in changes in ways ecosystems function. Quantifying key drought tolerance traits and how these traits vary from individual to individual within the same species, as well as among species, will improve our predictions of which species are most likely to suffer from the effects of climate change. With these needs in mind, the objective of this study was to quantify the variability in drought tolerance traits and correlate this to the sensitivity of plant growth to different soil moisture levels across populations of Bouteloua gracilis (blue gramma). Twenty individuals of B. gracilis from fifteen populations along two aridity gradients (one local elevation-aridity gradient in northern Colorado and one latitude-aridity gradient from South Dakota to New Mexico) were grown under both well-watered and drought conditions in a greenhouse. We measured the rates of water loss and carbon dioxide (required for growth) uptake. We also measured the internal pressure inside the water conducting tissues during the middle of the day (hereafter "xylem water potential") as well as the ability of a plant to draw water into its cells in the face of this pressure (hereafter "osmotic potential"). We should expect both lower xylem water potential and osmotic potential when plants are grown under drought conditions. The weight of the plants after the experiment was also measured. These traits were measured for all 15 populations of B. gracilis. Plants taken from arid populations weighed more when grown under well-watered conditions, but these populations also experienced the greatest reductions in growth when subjected to drought. Rates of water loss and carbon dioxide intake were not associated with these differences in plant weight. Plants from wet ecosystems adjusted their osmotic potential in the drought treatment. In contrast to this, plants from more arid ecosystems did not adjust their osmotic potential, but also experienced large declines in xylem water potential. Our results suggest that there is an unexpected shift in the water-use strategies among populations of B. gracilis. Populations from more arid sites were the most sensitive to precipitation, likely because they did not exhibit any drought tolerance adjustments similar to populations from wetter sites.

Technical Abstract: Background and Aims: As the frequency and severity of drought increases in many regions around the globe, changes in ecosystem function may follow the loss (or introduction) of individual species. Quantification of key drought tolerance traits and the intraspecific variability of these traits will improve predictions of the vulnerability of a species to drought conditions. With these needs in mind, the objective of this study was to quantify the variability in drought tolerance traits and correlate this to the sensitivity of plant growth to different soil moisture levels across populations of Bouteloua gracilis. Methods: Twenty individuals of B. gracilis from fifteen populations along two aridity gradients (one local elevation-aridity gradient in northern Colorado and one latitude-aridity gradient from South Dakota to New Mexico) were clonally propagated and planted in both a high and low soil moisture treatment in a greenhouse. Gas exchange, midday water potential ('mid), osmotic potential ('osm), biomass production, and morphological traits were examined to quantify the drought tolerance of each population. Key Results: Biomass productivity of the most arid populations was the highest under well-watered conditions, but these populations also experienced the greatest reductions in productivity when grown under water-limited conditions. Leaf morphological traits and gas exchange did not explain these patterns. Plants from more mesic ecosystems adjusted their 'osm in the water-limited treatment in order to stabilize 'mid between the two watering treatments. In contrast, plants from the most arid regions did not adjust 'osm and experienced reductions in 'mid under water-limited conditions. Conclusions: Our results suggest that there is an unexpected shift in the water-use strategies among populations of B. gracilis. Populations from more arid sites were the most sensitive to precipitation, likely because they did not exhibit any drought tolerance adjustments like populations from mesic sites.