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).