DOI: 10.5061/dryad.h6r35
title: Data from: Seasonality of precipitation interacts with exotic species to alter composition and phenology of a semi-arid grassland

description: While modeling efforts suggest that invasive species will track climate
 changes, empirical studies are few. A relevant and largely unaddressed
 research question is: how will the presence of exotic species interact
 with precipitation change to alter ecosystem structure and function? We
 studied the effects of changes in seasonal timing of precipitation on
 species composition and resource availability in a grassland community in
 Colorado, USA. We examined how seasonal precipitation patterns affect the
 abundance of historically present (native) and recently-arrived (exotic)
 plant species, as well as soil moisture, nitrogen, and above-ground
 biomass. Over four years, we applied four precipitation treatments based
 on climate model predictions for the study area:
 winter-wet/summer-ambient, winter-wet/summer-dry, winter-wet/summer-wet
 and winter-dry/summer-wet. Cover of exotic winter-active grasses was
 greater in winter-wet treatments than in control or winter-dry treatments.
 Cover of native warm-season grasses and forbs was greatest in the
 winter-dry/summer-wet treatment, and lowest in the winter-wet/summer-dry
 treatment. These results support the expectation that increased winter
 precipitation benefits new arrivals, whereas increased summer
 precipitation benefits later-growing native plants. Structural equation
 models showed that interactive effects of increased winter precipitation
 and increased cover of winter-active grasses reduced growing season soil
 water content and species diversity. In addition, the dominant
 winter-active species, Bromus tectorum, flowered and senesced earlier in
 plots receiving increased winter precipitation and reduced summer
 precipitation, suggesting that earlier growth of winter-active grasses
 decreases available soil resources and impacts later-growing native
 plants. Peak above-ground biomass was lowest in the treatment receiving
 reduced summer precipitation, but only in years with dry springs.
 Plant-available nitrogen in spring was lower in plots receiving
 supplemental winter precipitation, and highest in plots with reduced
 winter precipitation. Synthesis. Our results indicate that altering the
 seasonality of precipitation can have large direct effects on plant
 community composition and phenology, as well as significant indirect
 effects, mediated through exotic species, on plant-available resources and
 plant interactions.
description: precipitation_experiment_cover_dataThis data file contains absolute percent cover measurements of vegetation in plots of a precipitation experiment in a mixed-grass prairie in Colorado. Cover measurements were made in April, June, and August 2011-2013. The 'treatment' column heading corresponds to the precipitation treatment received by the plot, ‘winwet’ = winter wet, ‘winwet_sumdry’ = winter wet/summer dry, ‘windry_sumwet’ = winter dry/summer wet, and ‘windry_sumwet’ = winter dry/summer wet. ‘Ex_grass’ = percent cover of exotic grasses, ‘ex_forbs’ = cover of exotic forbs, ‘cool_grass’ = cover of native cool season grasses, ‘warm_grass’ = cover of native warm season grasses, and ‘total_veg’ = cover of all live vegetation.precipitation_experiment_resourcesThis data file contains measurements of soil resources and plant community characteristics from a precipitation experiment in a mixed-grass prairie in Colorado. All measurements were made from 2011-2013. The 'treatment' column heading corresponds to the precipitation treatment received by the plot; ‘winwet’ = winter wet, ‘winwet_sumdry’ = winter wet/summer dry, ‘windry_sumwet’ = winter dry/summer wet, and ‘windry_sumwet’ = winter dry/summer wet. ‘Spring_nitrogen’ = total nitrogen extracted from resin bags buried in plots at 5 cm depth from April-May (units are µg N / gram resin). ‘summer_nitrogen’ = total nitrogen extracted from resin bags from June- September, ‘winter_nitrogen’ = total nitrogen extracted from resin bags from October-March. Units for aboveground biomass are g/m2. ‘VWC’ = soil volumetric water content (%) from 0 – 12 cm depth. ‘Diversity’ = Shannon-Weiner diversity index. ‘April_B_tectorum’ = % cover of Bromus tectorum in plots in April.