Biocrust community resistance to climate disturbance
The final stress of our two-dimensional NMDS analysis was 0.157, indicating a good representation of the data. Community composition of biocrusts in control and treatment plots did not show clear differences (Figure 2). Neither distance nor direction of change from control to paired treatment plot were consistent among pairs of treated and control plots. We tested the magnitude and direction by using shifts in axis coordinates on both axis 1 and 2. A one sample t-test in axis 1 did not show a significant difference (p = 0.916). The same test for Axis 2 showed a significant shift toward the positive pole of the axis (p = 0.012), which correlates with the direction of the aridity gradient. Our combined test of these two results (Fisher’s C = 9.04, p = 0.06) suggested only a mild overall shift in direction and magnitude.
No site had significant TBI indices at the p<0.05 significance level after correction for 25 simultaneous tests (Supporting Information, Table S1). The overall dissimilarity was partitioned in the component indices, where species abundance losses (B) represented 49.6% of the dissimilarity and the species abundance gains (C) 50.6% (Table S2, Table S3). The mean of the differences between gains and losses was positive but not significant (p = 0.905, Table S4).
We observed positive linear slopes between control diversity and the percentage difference dissimilarity index (R2 = 0.072, p = 0.110), species abundance losses (R2 = 0.095, p = 0.078), and species abundance gains (R2 = 0.091, p = 0.083), after excluding one outlier in every model. However, multiple regression models showed that neither control plot diversity, aridity, nor their interaction had a significant effect on the overall dissimilarity of the biocrust communities or their components (Table 2, Figure 3).