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Abstract Fire exclusion and mismanaged grazing are globally important drivers of environmental change in mesic C4grasslands 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 C4grasses and long‐lived forbs. Unburned sites were characterized by several species of tree, shrub, and vine; summer sites were characterized by certain C3grasses 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 grassBothriochloa ischaemumby maintaining areas with low canopy cover, summer burns simultaneously limited woody encroachment and controlledB. ischaemuminvasion. 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.
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Dispersal and persistence traits inform long-term herbaceous plant community change in encroached savannas
Savanna plant communities are highly diverse, characterized by an open-canopy structure with rich herbaceous diversity, and maintained by frequent low-intensity fire and grazing. Due to habitat loss and fragmentation, savannas are globally threatened, with less than 1% of former oak savanna land cover found in the Midwestern United States remaining. In remnant oak savannas, loss of fire and grazing has led to woody encroachment and canopy closure over the past century with cascading consequences for the taxonomic composition. Whether these taxonomic changes can be broadly predicted using species functional traits (morpho-physio-phenological characteristics that impact the fitness of a species) is a key question. We ask whether the impacts of woody encroachment on herbaceous species can be predicted from species’ abilities to persist (avoid extinction) and disperse (colonize new areas). Specifically, we pair persistence traits (e.g., clonality, belowground storage) and dispersal traits (e.g., seed mass, dispersal mode, flowering height) with a rare 60-year dataset from oak savannas in Wisconsin, USA to understand how the representation of these traits has changed in the herbaceous community over time. Over 60 years, change in species composition was explained both by dispersal abilities and persistence traits; small-seeded species reliant on unassisted dispersal and moderately clonal species experienced the greatest losses. These changes in functional composition are likely due to increased woody encroachment, which may impede propagule production and movement. Restoration efforts need to prioritize species that are dispersal limited and those that create fine fuels, which aid the persistence of fire-maintained open habitat savannas.
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- Award ID(s):
- 1754764
- PAR ID:
- 10468930
- Publisher / Repository:
- Plant Ecology
- Date Published:
- Journal Name:
- Plant Ecology
- Volume:
- 224
- Issue:
- 4
- ISSN:
- 1385-0237
- Page Range / eLocation ID:
- 361 to 371
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/s11258-023-01307-3
