DOI: 10.5061/dryad.1jwstqk1x
title: Drought tolerant grassland species are generally more resistant to competition

description: 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.
description: 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 <em>t</em> was
 calculated as the total cover of a species in time
 <em>t</em>+1 divided by the total cover in time
 <em>t</em>. 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 <em>t</em> to <em>t</em>+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<sup>-1</sup>), average individual leaf area (cm2),
 leaf turgor loss point (TLP; MPa), leaf nitrogen concentration (%),
 specific leaf area (SLA; cm<sup>2</sup>
 g<sup>-1</sup>), leaf tissue density (LTD;
 cm<sup>3</sup> g<sup>-1</sup>), root nitrogen (%),
 root tissue density (RTD; cm<sup>3</sup>
 g<sup>-1</sup>), root diameter (mm), specific root length
 (SRL; m g<sup>-1</sup>), 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).
description: # Drought tolerant grassland species are generally more resistant to
 competition These data
 were analyzed and presented in the accompanying paper where we observed a
 positive correlation between low-density population growth rates in
 drought and low-density population growth rates in the presence of
 interspecific neighbors. We also found that high leaf dry matter content
 and low (more negative) leaf turgor loss point were associated with higher
 population fitness in drought and with higher neighbor competition. The
 **EDGE_covers.csv** dataset contains aggregated absolute cover estimates
 and annual population growth summarized from the Extreme Drought in
 Grassland Experiment (EDGE). The **all_pop_data.csv** contains the
 calculated population growth rates for each population and the trait data
 paired with each population. Finally, the **Suppinfo_tableS1_traits.csv**
 dataset was included as supplemental information for the publication. It
 also has the population-level traits data but includes details on the
 sources of trait data and the proximity these measurements were taken to
 the EDGE population. ## Description of the data and file structure
 **EDGE_covers.csv** contains 18066 rows where each observation describes
 the absolute cover of a focal species and the summed absolute
 interspecific cover of other species present in each unique replicate
 (subplot). "NA" = not applicable in this dataset. Columns
 include: * "site" = denotes what site in the EDGE project data
 is from. CHY = High Plains Grassland Research Station, WY; HYS = Hays
 Agricultural Research Center, KS; KNZ = Konza Prairie Biological Station,
 KS; SGS = Central Plains Experimental Range, CO; sev.blue = Sevilleta
 Wildlife Refuge, NM (shortgrass), sev.black = Sevilleta Wildlife Refuge,
 NM (desert grassland). * "year" = data was recorded. *
 "plot" = plot number. * "subplot" = subplot number/
 plot replicate number. * "trt" = drought treatment. con =
 ambient precipitation; chr = 66% reduction in mean annual precipitation. *
 "species" = Latin names for each species formatted as
 Genusspecies. * "cover" = absolute cover (%) of the species
 listed in species column. * "lambda" = population growth rate
 from the year listed in "year" column to year+1. *
 "log_lambda" = logged values from "lambda" column to
 obtain instantaneous rates of increase (r) as growth rate. *
 "surv" = binary variable used for calculating growth rates.
 Indicates if a population that was present in the year in the
 "year" column was still found on the plot in year+1. *
 "other" = summed absolute cover (%) of all the other species in
 the subplot excluding the species in "species" column. *
 "log_cover" = logged values of from "cover" column. *
 "log_other" = logged values from "other" column. *
 "cnt" = number of observations of this species across all
 subplots and years that were used to model population growth rates.
 **all_pop_data.csv** contains 113 rows where each observation contains the
 calculated low-density population growth rates in different conditions and
 trait values for each unique population (species-site combination).
 "NA" = not available in this dataset. Columns include: *
 "species" = Latin names for each species formatted as
 Genusspecies. * "site" = denotes what site in the EDGE project
 data is from. CHY = High Plains Grassland Research Station, WY; HYS = Hays
 Agricultural Research Center, KS; KNZ = Konza Prairie Biological Station,
 KS; SGS = Central Plains Experimental Range, CO; sev.blue = Sevilleta
 Wildlife Refuge, NM (shortgrass), sev.black = Sevilleta Wildlife Refuge,
 NM (desert grassland). * "Photosynthesis" = type of
 photosynthetic pathway. * "annual.Perennial" = lifespan.
 Includes perennial, annual, and biennial. * "height" = average
 maximum plant height (mm). * "rootdiam" = root diameter (mm). *
 "SRL" = specific root length (m g-1). * "RTD" = root
 tissue density (cm3 g-1). * "rootN" = root nitrogen content (%).
 * "SLA" = specific leaf area (cm2 g-1). * "LDMC" =
 leaf dry matter content (g g-1). * "LTD" = leaf tissue density
 (cm3 g-1). * "leafarea" = leaf area (cm2). * "TLP" =
 turgor loss point (Mpa). * "leafN" = leaf nitrogen content (%).
 * "intrinsicLDGRcon" = low density growth rate with 0
 interspecific cover in ambient precipitation. *
 "intrinsicLDGRchr" = low density growth rate with 0
 interspecific cover in drought. * "invasionLDGRcon" = low
 density growth rate with mean interspecific cover in ambient
 precipitation. * "invasionLDGRchr" = low density growth rate
 with mean interspecific cover in drought. * "weight" = standard
 error of models used to calculate low density growth rates. *
 "weight2" = 1/"weight" column. Used to weight
 certainty in our growth rates in subsequent models. *
 "grassland_type" = grassland community type. Includes tallgrass
 prairie, southern mixed-grass prairie, northern mixed-grass prairie,
 northern shortgrass prairie, southern shortgrass prairie, and desert
 grassland. * "grass.forb" = type of growth form grouped into
 Grass or Other. * "lifespan" = length of species lifespan.
 grouped into perennial or short-lived species.
 **Suppinfo_tableS1_traits.csv** contains 186 rows where each observation
 contains trait data for unique plant populations (species-site
 combination). Data were compiled from a number of different sources,
 source author, and location of collection are included and for each
 population. "NA" = not available, unless noted otherwise below.
 Columns include: * "Species" = Latin names for each species
 formatted as Genusspecies. * "grassland" = grassland community
 type. Includes tallgrass prairie, southern mixed-grass prairie, northern
 mixed-grass prairie, northern shortgrass prairie, southern shortgrass
 prairie, and desert grassland. * "site" = denotes what site in
 the EDGE project data is from. CHY = High Plains Grassland Research
 Station, WY; HYS = Hays Agricultural Research Center, KS; KNZ = Konza
 Prairie Biological Station, KS; SGS = Central Plains Experimental Range,
 CO; sev.blue = Sevilleta Wildlife Refuge, NM (shortgrass), sev.black =
 Sevilleta Wildlife Refuge, NM (desert grassland). *
 "grass.forb.shrub" = type of growth form. G = grass; F = forb; S
 = shrub. * "photosynthesis" = type of photosynthetic pathway. *
 "annual.perennial" = lifespan. Includes perennial, annual, and
 biennial. * "max.height (mm)" = average maximum plant height
 (mm). * "root.diameter (mm)" = root diameter (mm). * "SRL
 (m g-1)" = specific root length (m g-1). * "root.tissue.density
 (cm3 g-1)" = root tissue density (cm3 g-1). *
 "root.nitrogen.content (%)" = root nitrogen content (%). *
 "SLA (cm2 g-1)" = specific leaf area (cm2 g-1). * "LDMC (g
 g-1)" = leaf dry matter content (g g-1). * "Leaf.tissue.density
 (cm3 g-1)" = leaf tissue density (cm3 g-1). * "Leaf.area
 (cm2)" = leaf area (cm2). * "turgor.loss.point (Mpa)" =
 turgor loss point (Mpa). * "leaf.nitrogen.content (%)" = leaf
 nitrogen content (%). * "author_location.primary" = indicates
 where the primary source of published trait data for this population came
 from and where the those data were collected. Formatted as first and last
 initial of the author of the publication or dataset and then the state the
 trait data comes from, ex) DB_WY = Dana Blumenthal's publication in
 list of data sources (below) and from a population in their data collected
 in WY. Data pulled from a global dataset available on TRY written as
 TRY_global. (*"NA" = not applicable). *
 "relation.to.EDGE.primary" = describes how close the trait data
 was collected to the focal population. Local = collected at or near EDGE
 site; Regional = collected in or average between values in same grassland
 type; Global = national or general averages global values if applicable.
 (*"NA" = not applicable). *
 "author_location.secondary" = if applicable, indicates the
 secondary source of published trait data for this population came from and
 where the those data were collected. (*"NA" = not applicable). *
 "relation.to.EDGE.secondary" = indicates where the secondary
 source of published trait data for this population came from and where the
 those data were collected. Formatted as first and last initial of the
 author of the publication or dataset and then the state the trait data
 comes from, ex) DB_WY = Dana Blumenthal's publication in list of data
 sources (below) and from a population in their data collected in WY. Data
 pulled from a global dataset available on TRY written as TRY_global.
 (*"NA" = not applicable). * "author_location.tertiary"
 = if applicable, indicates the last and minor source of published trait
 data for this population came from and where the those data were
 collected. (*"NA" = not applicable). *
 "relation.to.EDGE.tertiary" = indicates where the tertiary
 source of published trait data for this population came from and where the
 those data were collected. Formatted as first and last initial of the
 author of the publication or dataset and then the state the trait data
 comes from, ex) DB_WY = Dana Blumenthal's publication in list of data
 sources (below) and from a population in their data collected in WY. Data
 pulled from a global dataset available on TRY written as TRY_global.
 (*"NA" = not applicable). ## Sharing/Access information Original
 cover estimate data from two of the EDGE sites at the Sevilleta Wildlife
 Refuge in New Mexico are publicly available at the following location: *
 
 Trait data was derived from a mix of unpublished and open-access trait
 data associated with the following publications: * Stears, A. E., Adler,
 P. B., Blumenthal, D. M., Kray, J. A., Mueller, K. E., Ocheltree, T. W.,
 Wilcox, K. R., & Laughlin, D. C. (2022). Water availability dictates
 how plant traits predict demographic rates. *Ecology*, *103*(11).
 * Tucker, S. S. (2010).
 *Morphological and physiological traits as indicators of drought tolerence
 in tallgrass prarie plants*. Pittsburg State University * Laughlin, D. C.,
 Leppert, J. J., Moore, M. M., & Sieg, C. H. (2010). A multi-trait test
 of the leaf-height-seed plant strategy scheme with 133 species from a pine
 forest flora. *Functional Ecology*, *24*(3), 493–501.
 * Farrell, A. K.
 (2018). *Effects of Management on Functional Diversity in Restored
 Tallgrass Prairie Plant Communities*. Northern Illinois University *
 Craine, J. M., Nippert, J. B., Towne, E. G., Tucker, S., Kembel, S. W.,
 Skibbe, A., & McLauchlan, K. K. (2011). Functional consequences of
 climate change-induced plant species loss in a tallgrass prairie.
 *Oecologia*, *165*(4), 1109–1117.
 * Blumenthal, D. M.,
 Mueller, K. E., Kray, J. A., Ocheltree, T. W., Augustine, D. J., &
 Wilcox, K. R. (2020). Traits link drought resistance with herbivore
 defence and plant economics in semi‐arid grasslands: The central roles of
 phenology and leaf dry matter content. *Journal of Ecology*, *108*(6),
 2336–2351. * TRY database:
 Kattge, J., Bönisch, G., Díaz, S., Lavorel, S., Prentice, L. C., &
 Leadly, P. (2019). TRY plant trait database – enhanced coverage and open
 access. *Global Chnage Biology*, *26*(1), 119–188. ## Code/Software All
 analyses were conducted in R version 4.1.3 (R Core Team, 2022). R scripts
 used to complete the analysis in the associated publication are archived
 and available on Zenodo
 ([10.5281/zenodo.10198332](https://zenodo.org/doi/10.5281/zenodo.10198332)). In the folder named "code" in this repository, there are six .R scripts containing the code for all data manipulation, analyses, and figures included in the publication and supporting information. * "Data wrangling.R" script includes code to summarize data from larger EDGE dataset, calculate lambda, and create the **EDGE_covers.csv** and **all_pop_data.csv**. It is not necessary to run this script in order to replicate the analyses, but shows how data were compiled. * "summary stats.R" contains initial summary information about the different growth rates calculated and tests for differences in growth rate related to grassland type. * "responses analysis.R" contains the analysis of the relationships between growth rate in drought and growth rate with neighbors. * "traits analysis.R" contains the analysis of the relationship between traits and population growth in different precipitation conditions and neighbor abundances. * "figures.R" contains the code to reproduce all of the figures in the manuscript. * "supplemental info.R" contains the analyses and figures for information included in the supplemental.