Biocrusts are compositionally resistant to climate perturbation
We found a general compositional resistance indicated by the lack of
substantial shifts in biocrust species composition in response to the
imposed 8 year drought. Our analysis based on the NMDS ordination
suggested only weak evidence of compositional shift; the lack of a
stronger trend could be due to the heterogeneity of the communities, the
variance in successional state among communities, and the often strong
random component of the community responses to perturbations (Schmitz
1997). Part of the heterogeneity of the sites involves the long history
of disturbances of the Colorado Plateau, like grazing by domestic
livestock and wildfire, that are associated with a slow recovery rate
due to the low productivity of the system (Schwinning et al .
2008). This may have resulted in a landscape where our experimental
sites had different initial biocrust states and communities did not
start at their maximum potential, having a realistic mosaic of
communities of the region. Nonetheless, the weak direction of the
compositional shifts correlated with the aridity gradient suggests that
drought could slowly drive biocrusts to a species composition
characteristic of more arid environments, underlying the common
successional retrogression response of community maturity previously
shown. This could occur both through alterations to the pace of recovery
following disturbance and through unique successional trajectories
caused by an altered climate. These minor shifts in biocrust
composition, here measured as dissimilarity indices, were partitioned
into two components: species gains and species losses. The biocrust
species gains and losses indices were on average equivalent, showing
that neither of the two processes was dominant in the compositional
changes, regardless of the indication given by successional
retrogression in which a consistent species loss would be expected as
late successional cover decreases. However, we consider that perhaps a
longer experimental time period than our eight-year drought could be
necessary to shift composition, as biocrust species reach environmental
conditions outside their tolerance range.
We expected that diversity would increase biocrust resistance to drought
based on our second hypothesis. However, our models showed that the
dissimilarity between control and droughted communities was not driven
by the control diversity or by site aridity. Although diversity is
rarely the primary driver of stability (Worm & Duffy 2003), finding no
predictive power of diversity on the compositional resistance was
unexpected, and leads us to infer that the degree of the compositional
resistance may be driven by other community attributes rather than
taxonomic diversity.
Additionally, we found a weak but positive relationship between overall
dissimilarity and control plot diversity, suggesting that more diverse
biocrusts are slightly less resistant due to greater shifts in
composition as they gain and lose species (Figure 3). These results do
not support our initial diversity-stability hypothesis; however,
resistance, as a component of stability, refers to the difference in a
system property from its unperturbed condition to a perturbed one. If
under undisturbed conditions a property increases with diversity, the
post-disturbance relationship is likely to have a less positive slope.
Because species-poor systems already have low values of such properties
under undisturbed conditions, they may be less affected by disturbance
than species-rich systems (Pfisterer & Schmid 2002). Past studies have
documented a negative relationship between diversity and resistance in
systems with diversity-dependent productivity (Pfisterer & Schmid 2002;
Vogel et al . 2012). This rationale explains the stronger negative
impact of drought on richer biocrust communities, where the change in
composition accounts for changes in relative abundance or cover (a
productivity proxy), in a system that has shown richness to enhance
productivity and functionality (Maestre et al . 2012; Bowkeret al . 2017).
While previous studies have shown aridity to drive biocrust composition
and functionality (Bowker & Belnap 2008; Delgado-Baquerizo et
al . 2016), which could create variations in the strength of biotic
interactions (Bowker et al . 2010; Maestre et al . 2010), we
found no support here for the effect of aridity in biocrust
compositional resistance. This lack of an aridity effect could also
reflect a strong adaptation of the biocrust community to ambient
aridity. Similarly, previous work at these same study sites found a
surprising lack of interaction between drought and soil attributes in
determining plant response (Hoover et al . 2015), which led to the
suggestion that vegetation communities are adapted to their local soil
conditions such that drought responses are similar across contrasting
soil types. However, it is also possible that the range of our
environmental gradient, our sample size, varied land use history, or the
experimental duration, was not broad enough to capture the interaction
of aridity and drought on biocrust composition resistance, or that other
environmental factors not considered could have a stronger effect on the
impacts of global change in our system.