Data Preparation
We used species relative cover and aboveground biomass to estimate per species biomass in two ways. Most sites sorted sampled biomass into broad functional groups (e.g., graminoid, forb, legume), and have identified these groups for every species in compositional cover data. In sites and years when biomass was sorted into functional groups, the species percentage cover was summed within those same functional groups and the relative cover of each species within a functional group was multiplied by the sorted biomass of that functional group to estimate per-species biomass (Axmanová et al. 2012). This relates the species cover to biomass for different functional groups (Figure S3a), and accounts for differences in the mass to cover relationships among different life forms. For example, broadleaf forbs will likely have a higher cover to mass relationship as their leaves are more horizontal.
In sites and years where biomass was not sorted to functional groups, or in plots where samples of functional groups were not matched between cover and biomass data (e.g., a legume recorded in cover measurements but not in biomass samples), total live biomass values were used to estimate per species biomass. In these cases, cover of each species relative to the whole plot was multiplied by the total live biomass for the plot (Axmanová et al. 2012; Hautier et al. 2014; Isbell et al. 2015) (Figure S3b). We expect that the first method provides more accurate species-level estimates, so this method was used wherever possible. These approaches use the best available data from destructively sampled biomass strips to estimate species-level biomass from percent cover data. We acknowledge that this is not an exact measure of per species biomass, and introduces some uncertainty in our analyses. However, we compared both methods and found no major differences in estimates of overall biomass change associated with components of diversity change between major functional groups (Figure S3c). In addition, we examined whether using species’ percent cover instead of biomass as a response altered our inferences (Figure S4). Changes in percent cover through time were broadly qualitatively consistent with those estimated using biomass. However, cover is a constrained and two-dimensional measure that does not fully describe growth in a plant community. We find that the rate of change in cover does not change as much in response to NPK, but still demonstrates turnover within communities, so when we relate biomass measures to cover to estimate per species biomass, biomass estimates are moderated by cover and likely underestimated due to these differences (Figure S4).