North American grasslands have experienced increased relative abundance of shrubs and trees over the last 150 yr. Alterations in herbivore composition, abundance, and grazing pressure along with changes in fire frequency are drivers that can regulate the transition from grassland to shrubland or woodland (a process known as woody encroachment). Historically, North American grasslands had a suite of large herbivores that grazed and/or browsed (i.e., bison, elk, pronghorn, deer), as well as frequent and intense fires. In the tallgrass prairie, many large native ungulates were extirpated by the 1860s, corresponding with increased homesteading (which led to decreased fire frequencies and intensities). Changes in the frequency and intensity of these two drivers (browsing and fire) have coincided with woody encroachment in tallgrass prairie. Within tallgrass prairie, woody encroachment can be categorized in to two groups: non‐resprouting species that can be killed with fire and resprouting species that cannot be killed with fire. Resprouting species require additional active management strategies to decrease abundance and eventually be removed from the ecosystem. In this study, we investigated plant cover, ramet density, and physiological effects of continuous simulated browsing and prescribed fire on
Effects of Fire Intensity and Abiotic Factors on Persistence of an Encroaching Woody Species
Over the past century, rangelands worldwide have experienced changes in vegetation cover and structure, many transitioning from grass-dominated to shrub-dominated systems (Archer et al. 2017; Fuhlendorf et al. 2017). In North America, such transitions are primarily a consequence of livestock management and fire exclusion practices of Euro-American settlers (Bray 1904; Archer 1989; Fuhlendorf and Smeins 1997). These shrub-dominated systems are often less productive for wildlife and livestock and may have crossed a threshold which cannot be reversed via common restoration practices such as prescribed fire (Ansley and Castellano 2006; Ratajczak et al. 2016). Oftentimes, the inability of prescribed fire to succeed at crossing this threshold is the result of insufficient fuel loading or inadequate fire intensity due to prescription parameters (Havstad and James 2010; Twidwell et al. 2016). However, recent work has demonstrated that burning under more extreme conditions (e.g. higher temperatures, lower fine fuel moisture) can slow or change the course of encroachment (Twidwell et al. 2013; Twidwell et al. 2016).
Many encroaching shrub species are capable of persisting after fire via resprouting from protected buds (Bond and Midgley 2001). Such mechanisms pose challenges for land managers, particularly because resprouting often results in a higher number of stems per individual plant. Mesquite (Prosopis spp.) shrubs are well-known for their ability to persist to varying degrees following disturbance due to fire, chemical, and mechanical treatments. Due to historical livestock management and fire suppression practices, honey mesquite (Prosopis glandulosa) has increased in dominance and abundance in the southern Great Plains since the beginning of Euro-American settlement (Bray 1904; Archer 1989). Although prescribed fire has increased in acceptance as a method to reduce encroachment of mesquite, low-intensity fires performed during the dormant season rarely cause mortality (Wright and Bailey 1980; Ansley et al. 1998), especially when they are performed as a single treatment rather than as part of a comprehensive management plan. However, recent studies have demonstrated that more intense fires conducted outside the dormant season are capable of reducing resprouters (including mesquite), particularly during periods of drought (Twidwell et al. 2016). We evaluated impacts of fire intensity and abiotic factors on persistence of honey mesquite, a species of concern for managers in the southern Great Plains.
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- NSF-PAR ID:
- 10139334
- Date Published:
- Journal Name:
- Proceedings of the Albuquerque Fire Behavior and Fuels Conference
- Page Range / eLocation ID:
- 1-6
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Cornus drummondii C.A. Mey, a resprouting clonal native shrub species. Browsing reducedC. drummondii canopy cover and increased grass cover. We also observed decreased ramet density, which allowed for more infilling of grasses. Photosynthetic rates between browsed and unbrowsed control shrubs did not increase in 2015 or 2016. In 2017, photosynthetic rates for browsed shrubs were higher in the unburned site than the unbrowsed control shrubs at the end of the growing season. Additionally, after the prescribed fire, browsed shrubs had ~90% decreased cover, ~50% reduced ramet density, and grass cover increased by ~80%. In the roots of browsed shrubs after the prescribed fire, nonstructural carbohydrates (NSC) experienced a twofold reduction in glucose and a threefold reduction in both sucrose and starch. The combined effects of browsing and fire show strong potential as a successful management tool to decrease the abundance of clonal‐resprouting woody plants in mesic grasslands and illustrate the potential significance of browsers as a key driver in this ecosystem. -
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