Biocrust community survey techniques
In April of 2018, we used a non-destructive method to survey the paired
plots in the 25 selected sites, the cover of individual biocrust
bryophyte and lichen species, cyanobacterial community types (i.e.,
“dark” and “light”; see below) were measured, and other ground cover
at the soil surface such as rocks, litter, and scat were noted.
Cyanobacterial biocrust cover was not classified taxonomically but
categorized as lightly-pigmented vs. darkly-pigmented because of the
impossibility of species identification of cyanobacteria in the field; a
darker level of pigmentation is indicative of successional development
and function (Belnap et al . 2008; Lan et al . 2017). To
collect cover data, we placed ten 25 x 25 cm gridded point intercept
quadrats per plot, recording 20 intersections for each (Jonasson 1983).
To place the quadrats, each plot was divided into 12 sections of 0.5 x
0.5 m. We placed quadrats in the bottom left corner of the 10 outer
sections. Within each gridded quadrat, we aligned a pin flag with each
grid intersection and extended it downward to the soil surface,
recording the species or ground cover element intercepted. An
intersection accounts for 5% cover within that framework. To account
for species or cover types that the pin flag did not hit, but were
within the quadrat, we searched the quadrat for additional species and
estimated a visual percentage of cover to the nearest one percent. When
a moss or lichen species was unknown, we assigned a genus or consistent
morphospecies name to separate it from other species and take it into
account in our diversity estimates. All evaluations were made when
biocrust species were dry and inactive. Because our assessment was
non-destructive, specimen collection was not possible, and some species
were not identified to the species level. For all 10 sections per plot,
we calculated the percentage cover of all visible components. We
subtracted the cover of non-colonizable soil surface (i.e., plant
litter, rock) and recalculated the cover of all major functional groups:
bryophytes, lichens, cyanobacteria, and bare ground, as well as the
Shannon diversity index (Shannon & Weaver 1949) based on available
habitat. We then averaged the values of the 10 cover frames to calculate
cover at the plot level. In the absence of a biocrust diversity survey
of the plots at the time of installation, we used the diversity data of
the control plots as our best available proxy for the initial biocrust
diversity, due to the proximity of the plots and similar environmental
conditions.