Abstract
AbstractFire exclusion and mismanaged grazing are globally important drivers of environmental change in mesic C4 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 C4 grasses and long‐lived forbs. Unburned sites were characterized by several species of tree, shrub, and vine; summer sites were characterized by certain C3 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 m2). At the local‐scale, summer sites (23 species/m2) and old‐growth grasslands (20 species/m2) supported greater richness than unburned sites (15 species/m2), 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.
Highlights
Prescribed fire—burning by land managers under specific conditions to induce desired ecosystem responses—is an essential element of efforts to conserve and restore the biodiversity of Earth’s mesic C4 grasslands
Significant species associated with old-growth grasslands in the ordination were as follows: perennial C4 grasses, A. gerardii, B. curtipendula, Bouteloua rigidiseta, S. scoparium, and S. nutans; perennial forbs, Brickellia cylindracea, Convolvulus equitans, Glandularia bipinnatifida, Hedyotis nigricans, Lespedeza texana, Liatris mucronata, Rhynchosia senna, Stillingia texana, and Wedelia texana; and annual forbs, Chamaesyce serpens, Evax verna, Galium virgatum, Hedeoma acinoides, Hymenopappus tenuifolius, Tetraneuris linearifolia, and Warnockia scutellarioides (Fig. 1B, C)
Among the seasonal fire treatments, non-metric multidimensional scaling (NMDS) axis 2 separated the unburned sites from the summer sites (Fig. 1A), with the fall and winter sites appearing intermediate in composition
Summary
Prescribed fire—burning by land managers under specific conditions to induce desired ecosystem responses—is an essential element of efforts to conserve and restore the biodiversity of Earth’s mesic C4 grasslands (including savannas and open-canopy grassy woodlands; Menges and Gordon 2010, Veldman et al 2015, Buisson et al 2019). In humid tropical and subtropical climates, such a correspondence between reproductive effort and fire season suggests a long evolutionary history between grassland species and lightning-ignited fires (Simon et al 2009, Maurin et al 2014), which historically occurred during seasons commonly referred to as the growing season, rainy season, or warm season (e.g., Ramos-Neto and Pivello 2000, Noss et al 2015) While this evidence suggests a need to consider prescribed fire season in restoration efforts, we lack long-term experimental evidence of whether burning in particular seasons can restore grasslands toward an old-growth ecosystem state after chronic overgrazing and fire exclusion. Long-term experiments on the effects of different fire seasons have been limited to old-growth grasslands that were not in need of restoration (e.g., Towne and Craine 2014), reconstructed grasslands planted on former agricultural land (Howe 2011, Dickson et al 2019), or study designs that did not assess restoration in reference to old-growth grassland communities (Taylor et al 2012, Boughton et al 2013)
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