USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
United States Department of Agriculture Forest Service. USDA logo which links to the department's national site. Forest Service logo which links to the agency's national site.

Research Topics Fire Science

Invasive plants and fire

Weeds grow in the forest understory near a developed area of the forest.
Invasion of Scotch broom (Cytisus scoparius) in the El Dorado National Forest. Continuous patches of flammable shrubs such as these can promote transfer of fire into tree canopies. Photo by J.T. Stevens.

Many non-native plants become successful invaders in environments where fire has altered the land by removing vegetation, exposing bare soil and increasing light availability.

Evidence from California suggests that the post-fire abundance of invasive plant species tends to increase at higher fire severities, particularly in forests and chaparral. Common non-native species that invade after fire in California include bull thistle (Cirsium vulgare), salsify (Tragopogon dubius), cheatgrass (Bromus tectorum), and Scotch broom (Cytisus scoparius).

Successful post-fire invaders such as these often have weedy growth characteristics, which means that they grow quickly in high-light conditions, reaching reproductive maturity and producing large volumes of seeds within 1-3 years after establishing. However, the abundance of these invaders can decrease following regeneration of forest canopy, if subsequent fire does not disrupt the regeneration process.

Invasive plants can alter fire regimes by increasing or decreasing the frequency and severity of fires in ecosystems where they alter the fuel characteristics (the volume, moisture content, and continuity) associated with native vegetation. For instance, non-native grasses that invade arid shrublands can greatly increase fuel continuity, which can lead to more frequent and larger fires. A classic example of this in western North America, including California, is cheatgrass (Bromus tectorum) invading sagebrush (Artemesia tridentata) shrub habitat. On the other hand, when shrubs invade grassland or savanna habitat, they can often reduce fire spread and frequency because the shrubs contain higher fuel moisture in their leaves during the dry season than the grass that they are replacing. Examples of this impact include bayberry (Myrica faya) invasions in Hawaiian ecosystems.

The ability of non-native plants to alter fire regimes depends in part on the density of their invasion. When invasive plants reach high densities and occupy large contiguous areas across a landscape, their impacts on fire increase. For instance, continuous cover of flammable shrubs such as Scotch broom (Cytisus scoparius) in forest understories can increase the likelihood that natural surface fire climbs these "ladder fuels" and reaches the forest canopy. On the other hand, high densities of fire-suppressing shrubs can reduce the occurrence of surface fires where frequent fire plays an ecologically important role. Management of fire-prone ecosystems therefore may focus on early eradication of non-native plants, to prevent their populations from altering fire regimes in undesirable ways.

Publications and references:
  • Brooks, M.L.; D'Antonio, C.M.; Richardson, D.M.; Grace, J.B.; Keeley, J.E.; DiTomaso, J.M.; Hobbs, R.J.; Pellant, M.; Pyke, D. 2004. Effects of invasive alien plants on fire regimes. Bioscience. 54: 677-688.
  • D'Antonio, C.M. 2000. Fire, plant invasions, and global changes. In: Mooney, H.A.; Hobbs, R.J., eds. Invasive Species in a Changing World. Island Press, Washington D.C: 65-94.
  • Keeley, J.E.; Lubin, D.; Fotheringham, C.J. 2003. Fire and grazing impacts on plant diversity and alien plant invasions in the southern Sierra Nevada. Ecological Applications. 13: 1355-1374.
  • Mandle, L.; Bufford, J.L.; Schmidt, I.B.; Daehler, C.C. 2011. Woody exotic plant invasions and fire: reciprocal impacts and consequences for native ecosystems. Biological Invasions. 13: 1815-1827.
  • Stevens, J. T.; Latimer, A.M. 2015. Snowpack, fire, and forest disturbance: interactions affect montane invasions by non-native shrubs. Global Change Biology.