Precipitation exclusion array
We conducted our study using an array of 25 paired control and
precipitation exclusion sites that were established by the US Geological
Survey across Southeastern Utah (central Colorado Plateau) in 2010 to
understand the impacts of forecast increased aridity on Colorado Plateau
drylands (Hoover et al . 2015, 2017; Winkler et al . 2019).
Our set of 25 paired plots (with each pair representing one site) are a
subset of the 40 sites in the USGS study, which we selected based on
presence of well-developed biocrusts. The other 15 sites had
insufficient biocrust cover to be suitable for our research (i.e., less
than 15% cover of moss and lichen). The ca. 4500-km2 study region from
which the experimental sites were selected spanned a range of plant
communities, soil depths and parent materials, soil textures,
elevations, climates and an aridity gradient (Table 1, Supporting
Information). The climate in the ecoregion is characterized as a cold
arid to semi-arid dryland with bimodal precipitation.
At each site, pairs of similar vegetation and soil patches were
identified that represented a target plant community and treatments were
assigned randomly. Each site consisted of two paired 150 x 200 cm plots:
a control and a drought treatment. Control plots received ambient
precipitation while drought plots experienced 35% precipitation
reduction year-round using passive precipitation removal shelters from
2010 to 2018. These shelters were 230 × 310 cm and sufficiently wide to
provide a large buffer around the plots that also received the drought
treatment; their slanted roofs were composed of V-shaped plexiglass
strips that served as troughs to intercept rainfall and snow (Figure
S1.). These strips were connected to a gutter system that directed water
away from the plot. Furthermore, vinyl flashing was installed vertically
around the plot, below and a few cm above ground, but beneath the
shelters to hydrologically isolate them (Hoover et al . 2015).