Testing the apparent resistance of three dominant plants to chronic drought on the Colorado Plateau

Abstract

Summary Many drylands, including the south‐western United States, are projected to become more water‐limited as these regions become warmer and drier with climate change. Such chronic drought may push individual species or plant functional types beyond key thresholds leading to reduced growth or even mortality. Indeed, recent observational and experimental evidence from the Colorado Plateau suggests that C3 grasses are the most vulnerable to chronic drought, while C4 grasses and C3 shrubs appear to have greater resistance. The effects of chronic, or press‐drought are predicted to begin at the physiological level and translate up to higher hierarchical levels. To date, the drought resistance of C4 grasses and C3 shrubs in this region has been only evaluated at the community level and thus we lack information on whether there are sensitivities to drought at lower hierarchical levels. In this study, we tested the apparent drought resistance of three dominant species (Pleuraphis jamesii, a C4 rhizomatous grass; Coleogyne ramosissima, a C3 drought‐deciduous shrub; and Ephedra viridis, a C3 evergreen shrub) to an ongoing experimental press‐drought (‐35% precipitation) by comparing individual‐level responses (ecophysiology and growth dynamics) to community‐level responses (plant cover). For all three species, we observed consistent responses across all hierarchical levels: P. jamesii was sensitive to drought across all measured variables, while the shrubs C. ramosissima and E. viridis had little to no responses to the experimental press‐drought at any given level. Synthesis. Our findings suggest that the apparent drought resistance at higher hierarchical levels, such as cover, may serve as good proxies for lower‐level responses. Furthermore, it appears the shrubs are avoiding drought, possibly by utilizing moisture at deeper soil layers, while the grasses are limited to shallower layers and must endure the drought conditions. Give this differential sensitivity to drought, a future with less precipitation and higher temperatures may increase the dominance of shrubs on the Colorado Plateau, as grasses succumb to chronic water stress.

Shrub persistence and increased grass mortality in response to drought in dryland systems

“…This is similar to previous results that show parent materials are important for mitigating drought in our study area, but the effect size of soils and parent material is smaller than expected (Hoover et al, 2015(Hoover et al, , 2017. That is, species are generally well adapted to the soils and climate at which they occur, even under the long-term moderate drought imposed here.…”

“…A. hymenoides is a critical source of forage for domestic livestock and wildlife in the Colorado Plateau Desert, yet the results from our study suggest a low drought threshold for this plant, indicating the species may continue to see strong population declines in response to the increased drought predicted for the Southwest (Cook et al, 2015). This low tolerance may be the result of decreased physiological performance in response to drought that has been shown to ultimately reduce A. hymenoides carbon gain and decrease cover, and is likely related to the shallow root profiles of these graminoid species (Hoover et al, 2017;Wan, Sosebee, & McMichael, 1993).…”

“…Winkler, Conver, et al, 2018). Even short-term drought experiments have shown strong effects on plant mortality (Adams et al, 2009;Hoover et al, 2015Hoover et al, , 2017Munson, Bunting, Bradford, Butterfield, & Gremer, 2018;O'Brien, Aviles, & Powers, 2018), but our study suggests that additional, and potentially cumulative effects, may go unnoticed without additional years of data. Our long-term drought experiment is one of the first to track individuals of different species throughout a continuous period, capturing ambient changes as well as responses to chronic reductions in precipitation.…”

“…This increasing aridity is occurring simultaneously with increasing temperature, further reducing soil water availability (Seager et al, 2007;Wuebbles, Fahey, & Hibbard, 2017) and resulting in major forest dieoffs and shifts in community composition (Allen & Breshears, 1998;Breshears et al, 2005;Munson et al, 2011;Swetnam & Betancourt, 1998). These and related studies suggest that an increase in woody shrubs and a decrease in native perennial grasses is likely to occur under future conditions (Hoover et al, 2015;Hoover, Duniway, & Belnap, 2017;Munson et al, 2011;Wertin, Reed, & Belnap, 2015).…”

Precipitation reduction alters herbaceous community structure and composition in a savanna

“…Interestingly, our data also showed that average height and percentage cover markedly decreased from the control to 30% precipitation reduction, but little change was observed following further reductions in precipitation (Figure ), which indicated the existence of very low thresholds for the changes in height and cover of the herbaceous community under varying levels of precipitation reduction. This finding can be primarily attributed to the sensitivity of plants to environmental stress (Hoover, Duniway, & Belnap, ). Generally, the height and cover of plants (especially those in arid and semi‐arid regions) are much more sensitive than other traits under conditions of stress (Qin, Sun, & Wang, ).…”