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

Supported by NSF DEB 0743678 (Univ of Virginia: P. D'Odorico, S. DeWekker & J. Fuentes, UNM: M. Litvak, W. Pockman and S. Collins, ).

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 United States has undergone dramatic changes in the composition and structure of vegetation due to invasion by Larrea spp. and Prosopis spp in southwestern deserts. 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) 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. This project will develop field and modeling activities to investigate and quantify the feedbacks between encroachment by a native C3 shrub, Larrea tridentata, into native C4 grassland, and the consequent changes in surface energy balance in a northern Chiuhuahuan desert ecosystem. To this end, the project will (1) test 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 capable of favoring the establishment and viable growth conditions for Larrea plants; (2) assess whether and where this vegetation-microclimate feedback can induce conditions of ecological bistability in the dynamics of aridland ecosystems thereby limiting their resilience.