Prosopis glandulosa persistence is facilitated by differential protection of buds during low- and high-energy fires

Authors: STARNS, HEATH - Texas Agrilife Research; Wonkka, Carissa; DICKINSON, MATTHEW - Us Forest Service (FS); LODGE, ALEXANDRA - Texas A&M University; TREADWELL, MORGAN - Texas A&M Agrilife; KAVANAGH, KATHLEEN L - Oregon State University; TOLLESON, DOUGLAS - Texas A&M Agrilife; TWIDWELL, DIRAC - University Of Nebraska; RODERS, WILLIAM - Texas A&M University

Submitted to: Journal of Environmental Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/20/2021
Publication Date: 11/25/2021
Citation: Starns, H.D., Wonkka, C.L., Dickinson, M.B., Lodge, A.G., Treadwell, M.L., Kavanagh, K., Tolleson, D.R., Twidwell, D., Roders, W.E. 2021. Prosopis glandulosa persistence is facilitated by differential protection of buds during low- and high-energy fires. Journal of Environmental Management. 303. Article 114141. https://doi.org/10.1016/j.jenvman.2021.114141.
DOI: https://doi.org/10.1016/j.jenvman.2021.114141

Interpretive Summary: Rangelands worldwide have experienced increases in woody plant densities and losses of grass cover over the past century. In North America these shifts are largely driven by overgrazing and fire suppression. Such shifts reduce productivity for livestock, reduce biodiversity, and are often difficult to reverse. Restoring grass dominance often involves restoring fire to the system. However, woody plants often persist following disturbance, including fire, by resprouting from protected buds, rendering fire ineffective for reducing their densities. Recent research has shown that extreme fire, defined as high-energy fires during periods when woody plants are experiencing water stress, may reduce woody plants’ capacity to resprout. This previous research did not examine whether high-energy fires alone would cause mortality of resprouting woody plants without water stress. We created an experimental framework for assessing the “buds-protection-resources” hypothesis of woody plant persistence under two different fire intensities. We exposed 48 individuals of a resprouting woody plant, honey mesquite to two levels of fire intensities (high and low) and root crown exposure (exposed vs unexposed) and evaluated resprouting capacity. We counted basal and epicormic resprouts for two years following treatment. Water stress was moderate for several months leading up to fires but low in subsequent years. Both epicormic and basal buds were protected from both low- and high-intensity fire, and transport tissues in roots were still functional even in top-killed mesquites. However, above ground buds were protected in very few mesquites subjected to high-intensity fires. High-intensity fire also decreased survival relative to low-intensity fire, caused loss of apical dominance, and left residual dead stems, which may increase the chances of mortality from future fires. Basal resprout numbers were also reduced by high-intensity fires, which may have additional implications for long-term mesquite survival. While the buds, protection, and resources components of resprouter persistence in the face of fire all played a role in mesquite resprouting, exposure to high-intensity fire decreased honey mesquite survival and reduced resprouting. This suggests that high-intensity fires trigger persistence mechanisms to different extents than low-intensity fires. In addition, high-intensity fires during normal rainfall can have negative impacts on resprouting woody plant survival and increased vulnerability tissue damage from water stress might not be a necessary precursor to honey mesquite mortality from high-intensity fire.

Technical Abstract: 1. Rangelands worldwide have experienced significant shifts from grass-dominated to woody-plant dominated states over the past century. In North America these shifts are largely driven by overgrazing and landscape-scale fire suppression. Such shifts reduce productivity for livestock, can have broad-scale impacts to biodiversity, and are often difficult to reverse. Restoring grass dominance often involves restoring fire as an ecological process. However, resprouting woody plants often persist following disturbance, including fire, by resprouting from protected buds, rendering fire ineffective for reducing resprouting woody plant density. Recent research has shown that extreme fire, defined as high-energy fires during periods when woody plants are experiencing water stress, may reduce woody plants’ capacity to resprout. This previous research did not examine whether high-energy fires alone would be sufficient to cause mortality of resprouting woody plants when water stress is low. 2. We created an experimental framework for assessing the “buds-protection-resources” hypothesis of resprouting persistence under different fire energies. In July-August 2018 we exposed 48 individuals of a dominant resprouting woody plant in the region, honey mesquite (Prosopis glandulosa) to two levels of fire energy (high and low) and root crown exposure (exposed vs unexposed) and evaluated subsequent resprouting capacity. We censused basal and epicormic resprouts for two years following treatment. Water stress was moderate for several months leading up to fires but low in subsequent years. 3. Both epicormic and basal buds were protected from both low- and high-energy fire, and transport tissues in roots were still functional even in top-killed mesquites. However, epicormic meristematic tissue was protected in very few mesquites subjected to high-energy fires. High-energy fire also decreased survival relative to low-energy fire, caused loss of apical dominance, and left residual dead stems, which may increase the chances of mortality from future fires. Basal resprout numbers were also reduced by high-energy fires, which may have additional implications for long-term mesquite survival. 4. Synthesis. While the buds, protection, and resources components of resprouter persistence in the face of fire all played a role in mesquite resprouting, exposure to high-energy fire decreased honey mesquite survival and reduced resprouting. This suggests that high-energy fires trigger persistence mechanisms to different extents than low-energy fires. In addition, high-energy fires during normal rainfall can have negative impacts on resprouting woody plant survival and increased vulnerability to cavitation from water stress might not be a necessary precursor to honey mesquite mortality from high-energy fire.