Drought tolerant grassland species are generally more resistant to competition

Plant populations are limited by resource availability and exhibit physiological trade-offs in resource acquisition strategies. These trade-offs may constrain the ability of populations to exhibit fast growth rates under water limitation and high cover of neighbors. However, traits that confer drought tolerance may also confer resistance to competition. It remains unclear how fitness responses to these abiotic conditions and biotic interactions combine to structure grassland communities and how this relationship may change along a gradient of water availability.
To address these knowledge gaps, we estimated the low-density growth rates of populations in drought conditions with low neighbor cover and in ambient conditions with average neighbor cover for 82 species in six grassland communities across the Central Plains and Southwestern United States. We assessed the relationship between population tolerance to drought and resistance to competition and determined if this relationship was consistent across a precipitation gradient. We also tested whether population growth rates could be predicted using plant functional traits.
Across six sites, we observed a positive correlation between low-density population growth rates in drought and in the presence of interspecific neighbors. This positive relationship was particularly strong in grasslands of the northern Great Plains but weak in the most xeric grasslands. High leaf dry matter content and low (more negative) leaf turgor loss point were associated with high population growth rates in drought and with neighbors in most grassland communities.

Synthesis: A better understanding of how both biotic and abiotic factors impact population fitness provides valuable insights into how grasslands will respond to extreme drought. Our results advance plant strategy theory by suggesting that drought tolerance increases population resistance to interspecific competition in grassland communities. However, this relationship is not evident in the driest grasslands where aboveground competition is likely less important. Leaf dry matter content and turgor loss point may help predict which populations will establish and persist based on local water availability and neighbor cover, and these predictions can be used to guide the conservation and restoration of biodiversity in grasslands.

Methods
Cover data
These data include a subset of 82 species (113 species-site combinations) that were monitored annually as part of the Extreme Drought in Grasslands Experiment (EDGE). Topographically unform and hydrologically isolated plots were set up across six grassland types (tallgrass prairie, southern mixed-grass prairie, northern mixed-grass prairie, northern shortgrass prairie, southern shortgrass prairie, and desert grassland) and absolute cover of all species in four 1 x 1 m quadrats was estimated yearly from 2012–2017. At each site, ten control plots at each site received ambient rainfall over the experimental period, and ten treatment plots experienced a 66% reduction in growing season precipitation (equivalent to roughly 40–50% over the whole year) using greenhouse rainout shelters equipped with strips of clear corrugated polycarbonate. Additional site and experimental design details are available in Griffin‐Nolan et al., (2019). 

Population growth rates
Percent cover was used as a measure of population size for each species at the quadrat level. Population growth rate at time t was calculated as the total cover of a species in time t+1 divided by the total cover in time t. The natural logarithm of this value (intrinsic rate of increase) for a species in a quadrat describes whether the population increased (positive value) or decreased (negative value) in the transition from year t to t+1. Population growth rates were calculated for each species in each quadrat in each annual transition. Because we use species cover instead of counts of individuals to measure population size, intraspecific cover is equal to the cumulative cover of a species in a quadrat. Interspecific cover in each quadrat is defined as the cumulative cover of all non-focal species in a quadrat. We calculated low-density growth rates for populations of each species at each site to assess fitness in two different conditions: mean neighbor abundance under ambient rainfall and minimum neighbor abundance under extreme drought. Minimum and mean neighbor abundances were averaged across all five years of the experiment. To estimate these growth rates, we fit linear models predicting intrinsic rate of increase for each species in each grassland as a function of drought treatment, intraspecific neighborhood cover, and interspecific neighborhood cover across years. 

Functional traits
Species-level trait data were assembled from several publications and trait databases and these eleven included leaf dry matter content (LDMC; g g-1), average individual leaf area (cm2), leaf turgor loss point (TLP; MPa), leaf nitrogen concentration (%), specific leaf area (SLA; cm2 g-1), leaf tissue density (LTD; cm3 g-1), root nitrogen (%), root tissue density (RTD; cm3 g-1), root diameter (mm), specific root length (SRL; m g-1), and average maximum height (mm). Trait data were compiled for each species at the site level where available (Table S1). Trait values measured at, or nearby, EDGE sites were considered the closest estimate for species traits. For this we used a mix of unpublished and open-access trait data from individual researchers (Blumenthal et al., 2020; Craine et al., 2011; Farrell, 2018; Laughlin et al., 2010; Stears et al., 2022; Tucker, 2010). Grassland communities that did not have data available at the local scale were filled in by progressively broader estimates using regional averages and eventually global estimates provided by the TRY database as needed (Kattge et al., 2019).