Research Spotlight: Vegetation Type Conversion in the Southwest -- Observations and Management Takeaways
Two new publications describe how forests, shrublands, and grasslands are being transformed into different ecosystem types in response to severe and frequent wildfire, drought, invasive species, climate, and other disturbances, a process known as vegetation type conversion.
The first paper provides a broad framework of how ecosystems may respond to these types of disturbances, from persistence of individual plants to reorganization of the ecological community, where vegetation type conversion is a type of extreme, persistent reorganization. The second paper provides examples of type conversion and management responses.
Introduction: How do Plants, Populations & Ecological Communities Respond to Disturbance?
Ecosystems are always changing in response to environmental variation. Disturbances like wildfire, drought, and pathogens are a fundamental part of the history of many ecosystems. Resilience, the ability of an ecosystem to recover following disturbance, is the result of traits and processes that operate at different levels, including persistence of individual organisms, recovery of populations, and reorganization of an ecological community when recovery fails to re-establish the pre-disturbance community.
Reorganization can consist of small changes, like a shift in the relative dominance of species present in a community, or it can be a complete transition between ecosystem type from forest to shrub or grass dominance. In some cases, these new states are transitory and ultimately revert to the pre-disturbance condition; in other cases, alternative states are persistent and potentially self-reinforcing, especially under conditions of altered climate, disturbance regimes, and invasive species. This is termed community reassembly but in the extreme, when vegetation types change, it is referred to as vegetation type conversion.
| Description | Examples | |
|---|---|---|
| Persistence |
The ability of an individual organism (e.g. a single shrub or tree) to survive a disturbance |
Thick, insulating bark prevents fire injury; self-pruning of dead branches; capacity to resprout from roots, trunk, or branches; drought-resistant xylem/hydraulic systems; managers protect a culturally important site from fire |
| Recovery |
The replacement of the pre-disturbance population through recruitment or colonization |
Seeds in the soil or in cones germinate after a fire, triggered by heat or smoke; seeds or other propagules are dispersed into the area by wind, water, or animals; managers plant trees following high severity fire |
| Reorganization |
A community of species continues to exist post-disturbance but no longer resembles the pre-existing community in one or more ways |
Species composition changes due to a warmer, drier temperatures; increased forest density following fire exclusion; managers plant drought-tolerant species after fire |
| Vegetation type conversion |
A special case of reorganization in which the change in community type and dominant plant functional types are extensive, and the alternative state is persistent and reinforced by novel interactions among climate, vegetation, and disturbances |
A forest converts to a shrubland after trees fail to recover after a large wildfire; a shrubland converts to a grassland after frequent fire and introduction of invasive annual grasses |
Ecological reorganization may occur gradually in response to changing environmental conditions or more abruptly as a result of a novel disturbance, typically wildfire or drought that causes extensive mortality of dominant vegetation. Reorganization may also occur in response to a loss of formerly frequent disturbance (e.g. fire suppression). Once a
community has reorganized, these drivers can also act to reinforce the new ecosystem state, pushing the system towards type conversion.
Vegetation Type Conversion: Examples from the Southwest
Vegetation type conversion has been increasingly observed in the southwestern United States although this process has been historically understudied and is poorly understood. To better understand the extent of type conversion and how managers can approach these transformations, managers, scientists, and practitioners from across the southwestern US came together to share and synthesize their experiences. Participants in two workshops provided 11 descriptive case studies and 61 examples of vegetation type conversion from their own field observations.
Together, the participants reported that vegetation type conversion has been extensive across the Southwest, drawing on examples from chaparral shrublands, dry coniferous forests, and sagebrush. While many factors interact to drive type conversion, high-severity fire was the predominant driver in semi-arid coniferous forests. Following a high-severity wildfire, conifer populations may fail to recover due to limited seed sources and poor conditions for seedlings. By a large margin, these forests converted to shrubland, with fewer conversions to native or non-native herbaceous communities.
Meanwhile, chaparral and sagebrush areas nearly always converted to non-native grasses through interactions among land use, climate, and fire. Chaparral requires 10–15 years following fire for recovery, while sagebrush may require several decades under favorable conditions. The non-native grasses and forbs invading these shrublands burn readily, supporting uncharacteristically frequent fire relative to historical intervals. Frequent fires facilitate further invasion, resulting in positive feedback with fire that is driving extensive vegetation type conversion.
Managing Type Conversion: Reversal, Observation, and Facilitation
While vegetation type conversion is widespread, managing these transformations can be a challenge. Land managers are responsible for maintaining ecosystems as dynamic entities, but there is currently no clear roadmap for managing type conversion, nor what the objectives should be in a rapidly changing world. Management actions can be undertaken before or during a disturbance or at any point during the process of recovery or reorganization and can be targeted toward a single species, groups of species, or the ecosystem as a whole.
Most management interventions in response to disturbance and type conversion have focused on persistence and recovery, with the goal of reversing change and restoring an ecosystem to its historical condition. In dry conifer forests, fuel-reduction and reintroduction of low-severity fire can be preventive against the severe fires that drive type conversion. Creating a heterogenous landscape through these and other management tools can serve to protect culturally important areas and habitat refuges that are managed to resist disturbance. Following a severe disturbance, tree planting can help forests recover where natural seed sources are limited. Reversing change can be resource intensive and not always successful; managers may need to prioritize sites with the highest potential for survival.
However, recovery is often challenged by increasingly stressful environmental conditions and economic and logistical barriers. Where persistence and recovery are unlikely and type conversion is likely, managers may choose to accept or even direct change. Managers could view this as an opportunity to facilitate a transformation toward a more desirable future state. For example, planting more drought-tolerant species could represent a choice to facilitate change rather than resist it. While facilitation of vegetation type conversion is the least common management response today, ideas of when, where, and how to direct changes are becoming clearer.
| Location |
Pre-type conversion vegetation type |
Post-type conversion vegetation type | Driver | Management strategy |
|---|---|---|---|---|
|
Southern California |
Chaparral shrubland | Invasive grass | High-frequency fire | None |
| Devil's Postpile | Mixed-conifer | Shrubland | High-severity fire | Reversal |
| North Rim Grand Canyon | Ponderosa pine | Shrubland | High-severity fire | Reversal |
| Great Basin | Sagebrush | Invasive grass | High-severity fire | Reversal |
| Southern CA Chaparral Fuel Breaks | Chaparral shrubland | Invasive grass | Land-use | Facilitate |
| Gila National Forest | Conifer woodland | Grassland | Drought | None |
| Coronado National Forest | Ponderosa pine | Shrubland | High-severity fire | Observe |
MANAGEMENT TAKEAWAYS
- Ecosystems respond to disturbance through persistence, recovery, and reorganization. Severe wildfire, drought, and climate changes make ecosystem reorganization increasingly likely.
- When reorganization results in a persistent shift in vegetation type, as from forest to shrubland, this is known as vegetation type conversion.
- Vegetation type conversion is widespread in the Southwest and California, with high-severity fire driving conversions from forest to shrublands and frequent fire and invasive grasses driving conversions from shrubland to grassland.
- Managers have largely approached disturbance and type conversion by focusing on prevention and reversal strategies.
- Managers may consider an approach that facilitates ecosystem reorganization given climate change, shifting fire regimes, and the spread of invasive species.
This research spotlight refers to the following publications:
- Falk, D.A., van Mantgem, P.J., Keeley, J.E., Gregg, R.M., Guiterman, C.H., Tepley, A.J., Young, D.J. and Marshall, L.A., 2022. Mechanisms of forest resilience. Forest Ecology and Management, 512, p.120129. https://doi.org/10.1016/j.foreco.2022.120129
- Guiterman, C.H., Gregg, R.M., Marshall, L.A., Beckmann, J.J., van Mantgem, P.J., Falk, D.A., Keeley, J.E., Caprio, A.C., Coop, J.D., Fornwalt, P.J. and Haffey, C., 2022. Vegetation type conversion in the US Southwest: frontline observations and management responses. Fire Ecology, 18(1), pp.1-16. https://doi.org/10.1186/s42408-022-00131-w
Click here to download a PDF version of this Research Spotlight.
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