Woody encroachment is a global driver of grassland loss and management to counteract encroachment represents one of the most expensive conservation practices implemented in grasslands. Yet, outcomes of these practices are often unknown at large scales and this constrains practitioner’s ability to advance conservation. Here, we use new monitoring data to evaluate outcomes of grassland conservation on woody encroachment for Nebraska’s State Wildlife Action Plan, a statewide effort that targets management in Biologically Unique Landscapes (BULs) to conserve the state’s natural communities. We tracked woody cover trajectories for BULs and compared BUL trajectories with those in non-priority landscapes (non-BULs) to evaluate statewide and BUL-scale conservation outcomes more than a decade after BUL establishment. Statewide, woody cover increased by 256,653 ha (2.3%) from 2000–2017. Most BULs (71%) experienced unsustainable trends of grassland loss to woody encroachment; however, management appeared to significantly reduce BUL encroachment rates compared to non-BULs. Most BULs with early signs of encroachment lacked control strategies, while only one BUL with moderate levels of encroachment (Loess Canyons) showed evidence of a management-driven stabilization of encroachment. These results identify strategic opportunities for proactive management in grassland conservation and demonstrate how new monitoring technology can support large-scale adaptive management pursuits.
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Large‐scale fire management restores grassland bird richness for a private lands ecoregion
Of all terrestrial biomes, grasslands are losing the most biodiversity the most rapidly, so there is a critical need to document and learn from large‐scale restoration successes. In the Loess Canyons ecoregion of the Great Plains, USA, an association of private ranchers and natural resource agencies has led a multi‐decadal, ecoregion‐scale initiative to combat the loss of grasslands to woody plant encroachment by restoring large‐scale fire regimes. Here, we use 14 years of fire treatment history with 6 years of grassland bird monitoring and remotely sensed tree cover data across 136,767 ha of privately owned grassland to quantify outcomes of large‐scale grassland restoration efforts. Grassland bird richness increased across 65% (90,032 ha) of the Loess Canyons, and woody plant cover decreased up to 55% across 25% (7408 ha) of all fire‐treated areas. This was accomplished with extreme fire treatments that killed mature trees, were large (mean annual area burned was 3100 ha), spatially clustered and straddled boundaries between invasive woodlands and remaining grasslands – not heavily infested woodlands. Findings from this study provide the first evidence of human management reversing the impacts of woody encroachment on grassland birds at an ecoregion scale.
- NSF-PAR ID:
- 10445974
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Ecological Solutions and Evidence
- Volume:
- 3
- Issue:
- 1
- ISSN:
- 2688-8319
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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 grass
Bothriochloa ischaemum by maintaining areas with low canopy cover, summer burns simultaneously limited woody encroachment and controlledB. 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. -
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Abstract Ecological restoration is beneficial to ecological communities in this era of large‐scale landscape change and ecological disruption. However, restoration outcomes are notoriously variable, which makes fine‐scale decision‐making challenging. This is true for restoration efforts that follow large fires, which are increasingly common as the climate changes.
Post‐fire restoration efforts, like tree planting and seeding have shown mixed success, though the causes of the variation in restoration outcomes remain unclear. Abiotic factors such as elevation and fire severity, as well as biotic factors, such as residual canopy cover and abundance of competitive understorey grasses, can vary across a burned area and may all influence the success of restoration efforts to re‐establish trees following forest fires.
We examined the effect of these factors on the early seedling establishment of a tree species—māmane (
Sophora chrysophylla )—in a subtropical montane woodland in Hawaiʻi. Following a human‐caused wildfire, we sowed seeds of māmane as part of a restoration effort. We co‐designed a project to examine māmane seedling establishment.We found that elevation was of overriding importance, structuring total levels of plant establishment, with fewer seedlings establishing at higher elevations. Residual canopy cover was positively correlated with seedling establishment, while cover by invasive, competitive understorey grasses very weakly positively correlated with increased seedling establishment.
Our results point to specific factors structuring plant establishment following a large fire and suggest additional targeted restoration actions within this subtropical system. For example, if greater native woody recruitment is a management goal, then actions could include targeted seed placement at lower elevations where establishment is more likely, increased seeding densities at high elevation where recruitment rates are lower, and/or invasive grass removal prior to seeding. Such actions may result in faster native ecosystem recovery, which is a goal of local land managers.
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Abstract Plant populations are limited by resource availability and exhibit physiological trade‐offs in resource acquisition strategies. These trade‐offs may constrain the ability of populations to exhibit fast growth rates under water limitation and high cover of neighbours. However, traits that confer drought tolerance may also confer resistance to competition. It remains unclear how fitness responses to these abiotic conditions and biotic interactions combine to structure grassland communities and how this relationship may change along a gradient of water availability.
To address these knowledge gaps, we estimated the low‐density growth rates of populations in drought conditions with low neighbour cover and in ambient conditions with average neighbour cover for 82 species in six grassland communities across the Central Plains and Southwestern United States. We assessed the relationship between population tolerance to drought and resistance to competition and determined if this relationship was consistent across a precipitation gradient. We also tested whether population growth rates could be predicted using plant functional traits.
Across six sites, we observed a positive correlation between low‐density population growth rates in drought and in the presence of interspecific neighbours. This positive relationship was particularly strong in the grasslands of the northern Great Plains but weak in the most xeric grasslands. High leaf dry matter content and a low (more negative) leaf turgor loss point were associated with high population growth rates in drought and with neighbours in most grassland communities.
Synthesis : A better understanding of how both biotic and abiotic factors impact population fitness provides valuable insights into how grasslands will respond to extreme drought. Our results advance plant strategy theory by suggesting that drought tolerance increases population resistance to interspecific competition in grassland communities. However, this relationship is not evident in the driest grasslands, where above‐ground competition is likely less important. Leaf dry matter content and turgor loss point may help predict which populations will establish and persist based on local water availability and neighbour cover, and these predictions can be used to guide the conservation and restoration of biodiversity in grasslands. -
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Abstract Woody encroachment into grassy biomes is a global phenomenon, often resulting in a nearly complete turnover of species, with savanna specialists being replaced by forest‐adapted species. Understanding the mechanisms involved in this change is important for devising strategies for managing savannas.
We examined how isolated trees favour woody encroachment and species turnover by overcoming dispersal limitation and environmental filtering. In a savanna released from fire in south‐eastern Brazil (Cerrado), we sampled woody plants establishing under 40 tree canopies and in paired treeless plots. These trees comprised eight species selected for habitat preference (savanna or forest) and dispersal syndrome (bird dispersed or not). We recorded dimensions of each tree, dispersal syndrome and habitat preference of recruits, and quantified the physical environment within each plot, aiming at a mechanistic understanding of woody encroachment.
We found clear evidence that isolated trees cause nucleation and drive changes in functional composition of savanna. Effectiveness as nucleator differed among species, but was unrelated to their functional guilds (habitat preference or dispersal syndrome). The density of saplings in nuclei was partially explained by soil moisture (+), daily temperature amplitude (−) and sum of bases (−).
Our results indicate that isolated trees act first as perches, strongly favouring bird‐dispersed species. They then act as nurse trees, considerably changing the environment in favour of forest‐adapted recruits. In the long term, as the nuclei expand and merge, savanna specialists tend to disappear and the savanna turns into a low‐diversity forest.
Synthesis and applications . Fire suppression has allowed the nucleation process and consequently the woody encroachment and fast replacement of savanna specialists by forest species in the Cerrado. By elucidating the mechanisms behind woody encroachment, we recommend using prescribed fires to burn forest seedlings and to reduce tree canopy size wherever the management goal is to maintain the typical savanna structure and composition.
