Experimental Design
At each of our six sites, transplant locations were arranged in a nested
design with three levels (plot, block, and grid), in order to
systematically sample a large and representative variety of
environmental conditions (Fig. S2). Three 5 x 5 m grids were placed
within each site. Each grid contained 25 blocks that consisted of two,
paired 0.09 m2 plots. Each paired plot received one of
two treatments: (1) ‘abiotic only’, where the entire plot was cleared of
all above ground vegetation as well the seedbank, or (2) ‘abiotic +
biotic’, where plots were left intact. We cleared plots by clipping
aboveground biomass, then disturbing the soil using trowels to remove an
inch of topsoil and its associated seedbank
(Germain et al.2017). We reduced cover by an average of 41% ± 0.89 SE, thus allowing
for contrast between intact vs relaxed biotic interactions (Fig. S3,
S4).
To test how persistence and occupancy varied across a productivity
gradient, we measured ‘greenness’ of photos of every plot taken at peak
biomass (May 2020) as a proxy for productivity at the block level
(n=450). To do so, we used the Fiji package
(Schindelin et al.2012) in Image J, choosing the ‘green band - red band’ vegetation index
to measure productivity; this index best predicts aboveground biomass in
low productivity systems, such as serpentine, on a global scale
(Prabhakara et al.2015). Although the vegetation index spanned -22 to 10 , with harsher
plots represented by higher numbers, for a more intuitive interpretation
we multiplied our index by -1 so that more productive plots had higher
numbers (Fig. S5). Although harvesting and weighing aboveground biomass
would be a more direct measure of plot productivity, doing so was not
possible as seeds were collected by local field assistants on emergency
funds during the Covid-19 lockdowns.