Climate change and physical disturbance cause similar community shifts in biological soil crusts

Abstract

Biological soil crusts (biocrusts)—communities of mosses, lichens, cyanobacteria, and heterotrophs living at the soil surface—are fundamental components of drylands worldwide, and destruction of biocrusts dramatically alters biogeochemical processes, hydrology, surface energy balance, and vegetation cover. Although there has been long-standing concern over impacts of physical disturbances on biocrusts (e.g., trampling by livestock, damage from vehicles), there is increasing concern over the potential for climate change to alter biocrust community structure. Using long-term data from the Colorado Plateau, we examined the effects of 10 y of experimental warming and altered precipitation (in full-factorial design) on biocrust communities and compared the effects of altered climate with those of long-term physical disturbance (>10 y of replicated human trampling). Surprisingly, altered climate and physical disturbance treatments had similar effects on biocrust community structure. Warming, altered precipitation frequency [an increase of small (1.2 mm) summer rainfall events], and physical disturbance from trampling all promoted early successional community states marked by dramatic declines in moss cover and increases in cyanobacteria cover, with more variable effects on lichens. Although the pace of community change varied significantly among treatments, our results suggest that multiple aspects of climate change will affect biocrusts to the same degree as physical disturbance. This is particularly disconcerting in the context of warming, as temperatures for drylands are projected to increase beyond those imposed as treatments in our study.

Keywords: alternate states; biocrusts; community structure; secondary succession; warming.

Fig. 1.

Biocrusts can locally regulate ecosystem processes and cover large portions of dryland ecosystems as in A (photo by Bill Bowman). Biocrusts are sensitive to physical disturbances from vehicles and trampling by livestock or people as depicted in B, which shows an experimentally trampled plot (foreground) bordered by undisturbed biocrust (background).

Fig. 2.

PRCs (Left) showing temporal responses of biocrust communities (log scale) to climate manipulation treatments relative to controls (green, zero line). Taxon weights (Right; log scale) indicate the relative contribution of taxa to community shifts: Weight >0 indicates increased abundance, and weight <0 indicates decreased abundance. Overall, climate treatments are moving biocrusts away from moss-dominated (S. caninervis) to cyanobacteria-dominated (M. vaginatus) communities. Symbols above the x axis indicate when communities within a treatment first differed (P < 0.05) from controls; * and † indicate a shift in biocrust communities of watered and warmed + watered treatments, respectively, and ‡ indicates a shift in the warmed biocrust community.

Fig. 3.

Temporal changes in the relative cover of moss (Top), lichen (Middle), and cyanobacteria (Bottom) in response to climate manipulation treatments. Values shown are means ± 1 SE. Repeated-measures, mixed-effect models (treatment, time, and treatment × time as fixed effects, and blocks as a random effect) revealed significant effects of time and treatment × time (P < 0.05) on all groups.

Fig. 4.

RCCI for moss (Top), lichen (Middle), and cyanobacteria (Bottom) in biocrusts subjected to climate manipulations or physical disturbance from repeated human trampling. RCCI shows changes in biotic cover relative to controls (Methods). The RCCI value ranges from +1 (100% increase in cover in response to treatment) to –1 (100% decrease in cover in response to treatment). Bars are means ± 1 SE, P values are probability of type I error (Kruskal–Wallis tests), and lettering indicates significant differences via Steel–Dwass nonparametric pairwise comparisons.