A micrometeorological flux perspective on brush management in a shrub-encroached Sonoran Desert grassland

Authors: VIVONI, E.R. - Arizona State University; PEREZ-RUIZ, E.R. - Arizona State University; Scott, Russell - Russ; NAITO, A.T. - University Of Arizona; ARCHER, S.R. - University Of Arizona; Biederman, Joel; TEMPLETON, N.P. - Arizona State University

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2021
Publication Date: 12/8/2021
Citation: Vivoni, E., Perez-Ruiz, E., Scott, R.L., Naito, A., Archer, S., Biederman, J.A., Templeton, N. 2021. A micrometeorological flux perspective on brush management in a shrub-encroached Sonoran Desert grassland. Agricultural and Forest Meteorology. 313. Article 108763. https://doi.org/10.1016/j.agrformet.2021.108763.
DOI: https://doi.org/10.1016/j.agrformet.2021.108763

Interpretive Summary: Woody plant encroachment typically limits the forage productivity of managed rangelands and alters a panoply of semiarid ecosystem processes and services. Intervention strategies to reduce woody plant abundance, collectively termed “brush management”, often lack observations to quantify and interpret changes in ecosystem processes. Here, we quantify ecosystem responses to brush management after a single aerial herbicide application on a shrub-encroached grassland in southern Arizona, USA. We conducted a pre- and post-treatment comparison of the treated area and in a nearby control site, where no herbicide was applied. The comparison, spanning a seven year period, included: (1) ground, aerial, and satellite-based measurements of vegetation structure, and (2) land-atmosphere exchanges of water vapor and carbon dioxide. The herbicide treatment defoliated the dominant mesquite shrub and led to a temporary reduction in summer greening, but full foliar recovery occurred within two years. Contrary to expectations, grass cover decreased and bare soil cover increased on the treated site. Water and carbon fluxes were impacted during the 2 year post-treatment period. The fraction of precipitation lost to evapotranspiration was reduced at the treated site, while the amount of photosynthesis relative the amount of evapotranspiration rose dramatically the following spring. During mesquite recovery, photosynthesis (carbon dioxide uptake) was enhanced and accompanied by a decrease in carbon dioxide loss relative to the untreated site. Mesquite recovery was facilitated by access to deep soil water, carbohydrate reserves in rooting systems, and a lower competition from reduced perennial grass cover. These results reveal some of the consequences of mesquite brush management and show that benefits from the management can be short lived.

Technical Abstract: Woody plant encroachment typically limits the forage productivity of managed rangelands and alters a panoply of semiarid ecosystem processes and services. Intervention strategies to reduce woody plant abundance, collectively termed “brush management”, often lack observations to quantify and interpret changes in ecosystem processes. Furthermore, comparative studies between treated and untreated areas should account for heterogeneity since plant composition, microclimate, topoedaphic factors, and historical land use can substantially vary over short distances in drylands. Here, we quantify ecosystem responses to brush management after a single aerial herbicide application on an 18 hectare shrub-encroached grassland in southern Arizona, USA. We conducted a pre- and post-treatment comparison of a flux tower site in the treated area with that of a tower in a nearby control site. The comparison, spanning a seven year period, included: (1) ground, aerial, and satellite-based measurements of vegetation structure, and (2) eddy covariance measurements. The herbicide treatment defoliated the dominant shrub (velvet mesquite, Prosopis velutina) and led to a temporary reduction in summer greening, but full foliar recovery occurred within two years. Contrary to expectations, perennial grass cover decreased and bare soil cover increased on the treated site. Water and carbon fluxes were impacted during the 2 year post-treatment period. The fraction of precipitation lost to evapotranspiration was reduced at the treated site, while water use efficiency rose dramatically the following spring. During mesquite recovery, CO2 uptake was enhanced by higher gross primary productivity and accompanied by a decrease in ecosystem respiration relative to the untreated site. Mesquite recovery was facilitated by access to deep soil water, carbohydrate reserves in rooting systems, and a lower competition from reduced perennial grass cover.