Season of prescribed fire determines grassland restoration outcomes after fire exclusion and overgrazing

Abstract

Fire exclusion and mismanaged grazing are globally important drivers of environmental change in mesic C 4 grasslands and savannas. Although interest is growing in prescribed fire for grassland restoration, we have little long-term experimental evidence of the influence of burn season on the recovery of herbaceous plant communities, encroachment by trees and shrubs, and invasion by exotic grasses. We conducted a prescribed fire experiment (seven burns between 2001 and 2019) in historically fire-excluded and overgrazed grasslands of central Texas. Sites were assigned to one of four experimental treatments: summer burns (warm season, lightning season), fall burns (early cool season), winter burns (late cool season), or unburned (fire exclusion). To assess restoration outcomes of the experiment, in 2019, we identified old-growth grasslands to serve as reference sites. Herbaceous-layer plant communities in all experimental sites were compositionally and functionally distinct from old-growth grasslands, with little recovery of perennial C 4 grasses and long-lived forbs. Unburned sites were characterized by several species of tree, shrub, and vine; summer sites were characterized by certain C 3 grasses and forbs; and fall and winter sites were intermediate in composition to the unburned and summer sites. Despite compositional differences, all treatments had comparable plot-level plant species richness (range 89-95 species/1000 m 2 ). At the localscale, summer sites (23 species/m 2 ) and old-growth grasslands (20 species/m 2 ) supported greater richness than unburned sites (15 species/m 2 ), but did not differ significantly from fall or winter sites. Among fire treatments, summer and winter burns most consistently produced the vegetation structure of old-growth grasslands (e.g., mean woody canopy cover of 9%). But whereas winter burns promoted the invasive grass Bothriochloa ischaemum by maintaining areas with low canopy cover, summer burns simultaneously limited woody encroachment and controlled B. ischaemum invasion. Our results support a growing body of literature that shows that prescribed fire alone, without the introduction of plant propagules, cannot necessarily restore old-growth grassland community composition. Nonetheless, this long-term experiment demonstrates that prescribed burns implemented in the summer can benefit restoration by preventing woody encroachment while also controlling an invasive grass. We suggest that fire season deserves greater attention in grassland restoration planning and ecological research.

Fire and herbivory – key drivers of temperate open ecosystems : Effects of consumer control on woody and herbaceous plant communities in temperate wood-pasture

“…The fire regime in this study included burning every year in early spring and was used to mimic the most recent anthropogenic use of fire, which was annually burning of meadows for improved pasture for livestock . The high reduction of woody vegetation in our spring fires somewhat contradicts results from other studies that found a higher mortality in late summer fires, due to higher fire intensity (Ansley et al, 2021;Laris et al, 2021;N'Dri et al, 2022;Novak et al, 2021). However, since all present trees were removed at the start of our experiment, there were only newly resprouted seedlings and saplings that were exposed to fire, and as discussed in Paper I, it takes time for trees to develop traits to survive fire, such as thick bark .…”

Abandoned Croplands: Drivers and Secondary Succession Trajectories under Livestock Grazing in Communal Areas of South Africa

“…In addition to light grazing, the absence of fire, particularly in abandoned fertile fields surrounded by woody vegetation in humid environments, accelerates the succession to a state of increased woody cover [84]. Bobrovsky and Khanina [85] also found that the absence of fire after abandonment results in a homogenous high-density distribution of pioneer woody species, while frequent fires maintain the succession at weedy stages for decades.…”

“…On the other hand, fire in the early stage of succession has been reported to divert the succession pattern by arresting the development towards woody vegetation resulting in dwarf shrubs [86]. Even though Novak et al [84] concludes that prescribed burns benefit restoration by arresting bush encroachment and controlling invasive herbaceous species, they must be used with other strategies, such as introducing planting propagules for effective restoration. Therefore, the use of fire-tolerant grass species in reseeding and over-sowing initiatives will quicken the restoration process.…”

Herbaceous vegetation changes along a bush encroachment intensity gradient in a montane area

“…Changes in fire frequency and elevated levels of carbon dioxide, either acting individually or interactively, are the main driving forces of bush encroachment (D'Odorico et al, 2012; Hudak, 1999; Van Auken, 2000). Bush encroachment can be accelerated by high livestock grazing pressure without resting the rangeland, leading to a reduction in grass fuel load needed for rangeland fire enhancement, thereby reducing the intensity and frequency of wildfires (Novak et al, 2021). Most bushes use C 3 photosynthetic pathway, while the majority of the herbaceous species in the rangelands are C 4 (D'Odorico et al, 2012; Orange‐Senqu River Commission (ORASECOM), 2013).…”