Data and code from: Species interactions amplify functional group responses to elevated CO2 and N enrichment in a 24-year grassland experiment

Description

Notes

Methods

From the manuscript:

Experiment design: The BioCON experiment, which manipulates Biodiversity, N and CO₂, was started in 1997 at Cedar Creek Ecosystem Science Reserve (East Bethel, MN, USA, 45°40′N, 93°18′W), a Long-Term Ecological Research site. The compositionally neutral full-factorial experiment comprised 296 plots that varied in their initial planted diversity (1,4, 9 or 16 species), N (ambient or + 4 g m^-2 year^-1) and CO₂ (ambient or +180ppm) treatments applied in a split-plot design for the CO₂ treatment (Reich, Knops, et al., 2001). The diversity gradient was created by experimentally assembling plant communities with 1 species, random selection of 4 or 9 species and/or all 16 species. As 36 of the 9 species plots have since been used for a sub-experiment involving temperature and drought treatments, we excluded those plots from our analyses leaving 260 plots. The species were chosen to represent common or naturalized prairie species in the region and spanned four functional groups: C₃ grasses (Bromus inermis, Elymus repens [formerly Agropyron repens], Koeleria macrantha [formerly Koeleria cristata], and Poa pratensis), C₄ grasses (Andropogon gerardii, Bouteloua gracilis, Schizachyrium scoparium, and Sorghastrum nutans), legumes (Amorpha canescens, Lespedeza capitata, Lupinus perennis, and Petalostemum villosum), and non-legume forbs (Achillea millefolium, Anemone cylindrica, Asclepias tuberosa, and Solidago rigida). Since the species composition was randomly chosen for the 4 and 9 species plots, each replicate for these two diversity levels differs in species and FG composition. Species found in a plot other than those originally planted there were weeded annually. Additionally, to maintain the study site in a grassland state, the plots were burnt in spring for half of the years between 2000 to 2012 (Adair et al., 2009) and every fall since 2013.

Plant biomass

Late in each growing season in August in the majority of plots in almost all years, aboveground biomass in a pre-marked 10 x 100 cm strip was clipped and sorted to the species level. The part of the plot from which biomass data were collected shifted annually to reduce potential influence of clipping on a single part of the plot. Species-specific cover data from a constant predetermined and never clipped 50 x 100 cm region of the plot were also recorded for every plot each year (except for a subset of 4 species plots during 2020 as all efforts were scaled down due to the pandemic). Using cover and biomass data we then generated species-specific correlations between the two to estimate biomass data for all years based on cover data. This step allowed us to a) impute biomass values when only cover data were available; and b) obtain a more representative estimate of biomass for the plot as cover data spanned a larger area of the plot compared to clip-strip biomass data.