Resilience mechanisms and recovery in a Chihuahuan Desert rangeland ecosystem

Background/Question/Methods

Desert grassland ecosystems of the Chihuahuan Desert, and other parts of the southwestern U.S., underwent significant changes from the late 1800s through the 1950s. Most often, grassland states were converted to shrubland states. Such transitions have been associated with diminished livestock production, erosion, reduced soil water retention and diminished net primary production, and biodiversity loss. Synthesizing across a number of data sources, we first describe the social and ecological mechanisms that precipitated transitions. We then describe several elements of resilience in the Chihuahuan Desert that have been discovered and their implications for ongoing management.

Results/Conclusions

Evidence from the Jornada Basin LTER indicates that transitions from grassland to shrublands were triggered by a failure to reduce livestock numbers during periods of low rainfall. These events caused initial perennial grass mortality and subsequent feedbacks with soil degradation, paralleled by shrub establishment and survival. Theory and observation suggests that a threshold is crossed once sufficient grass biomass is lost and feedbacks with soil erosion and reduced water availability drive remaining grasses extinct. Recent work, however, indicates several factors that have contributed to grassland resilience in the face of these processes. First, the occurrence of transitions across the landscape was buffered by variation in soil and landscape properties that influence soil water availability such that grasses were preserved in many areas. Second, net primary productivity of eroding shrubland states is, remarkably, not lower than that of grasslands although the response of these states to rainfall differs. Third, a recent large precipitation event revealed that remnant and colonizing grasses can reverse perennial grass loss, even if temporarily, on what were thought to be irreversibly degraded soils. Finally, degradation episodes in the first half of the 20^th century have radically transformed institutions and perceptions such that livestock management is more responsive to high-risk conditions. Here, as in other arid ecosystems, scientists have focused largely on the mechanisms of degradation rather than strategies to exploit resilience. The utility of such strategies for management is now being realized.