Do vegetation-microclimate feedbacks promote shrub encroachment in the Southwestern United States?

Woody plant encroachment into grasslands is a global phenomenon that results from a variety of global change drivers. Over the last 150 years the southwestern US has undergone dramatic changes in the composition and structure of vegetation due to range expansion by Larrea and Prosopis spp. Concern about the encroachment of woody vegetation is motivated by economical losses associated with the conversion of rangelands into woodlands, regional carbon dynamics and the occurrence of erosion-driven loss of nutrient-rich soil particles in response to the decrease in grass cover, with consequent loss of ecosystem services and functioning. The relatively abrupt character of grassland-to-shrubland transitions suggests that arid and semiarid rangelands may be bistable systems, with stable states characterized by either grass or shrub dominance. Due to the presence of alternative stable states, aridland ecosystems may have limited resilience and in some cases even small changes in environmental drivers (grazing intensity, fire, rainfall patterns) can cause abrupt state transitions from grassland to shrubland. Bistable dynamics are induced by positive feedbacks between external drivers and the current system state. What remains unclear is whether feedbacks between land cover change and atmospheric boundary layer dynamics may contribute to shrub encroachment in the southwestern US. Our collaborative research includes field and modeling activities to investigate and quantify the feedbacks between encroachment by the native C3 shrub, Larrea tridentata, into native C4-dominated grassland, and the consequent changes in surface energy balance at the Sevilleta. Our research is (1) testing the hypothesis that Larrea encroachment leads to decreases in surface albedo and increases in thermal energy storage in soils with the overall net effect of increasing nighttime air temperatures that favor the establishment and growth conditions for Larrea, and (2) assessing whether and where this vegetation-microclimate feedback can induce conditions of ecological bistability in the dynamics of aridland ecosystems thereby limiting their resilience.

To test these hypotheses we recently installed a new 30m tower in the grass-shrub transition zone at the Sevilleta to measure temperature profiles, CO2, ET and energy balance. These data will compliment on-going measurements at flux towers located in pure creosotebush shrubland to the southwest of this tower and in grama grassland about 1 km north of this tower.