Social-ecological transitions in Southwestern desert grasslands: how we got here and what’s next

Seminar co-sponsored by Applied Biodiversity Science and Dept. of Ecosystem Science and Management

Presented by Brandon Bestelmeyer, Research Leader, USDA-ARS-Jornada Experimental Range (website)

The transition of grassland to shrubland states in rangelands of the Southwestern US is among the most widely recognized examples of regime shifts in terrestrial ecosystems, but is incompletely understood. I draw upon a century of records from the Jornada Experimental Range/Jornada Basin LTER and other nearby long-term research sites in the Chihuahuan Desert to challenge several misconceptions about these transitions, including (i) their control by singular “tipping points”, (ii) that they represent catastrophic degradation, and (iii) that restoration of former grassland states is impossible. Transitions have been caused by changes in multiple slow variables (ecological and societal) interacting with abrupt “trigger” events, especially drought. These transitions, however, do not represent “desertification” as is commonly assumed and net primary production is often similar between grassland and shrubland states. The apparent reversibility (or lack) of impaired soil function in many areas is revealed by grass recovery within shrubland states–with and without shrub removal. Projected climatic changes toward greater aridity, however, are likely to promote the further displacement of perennial grasses by xerophytic shrubs and make it more difficult for managers to restore desired grassland components. A key question, then, is how to promote social-ecological resilience in rangelands facing inexorable change.