Exotic Plants May Dominate After a Fire, But Not for Long

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Exotic grasses dominated the study site soon after the fire. Photo by Matt Lavin, Wikimedia Commons.

Land managers expect that exotic invasive plants will quickly move in following a disturbance, especially after a fire. Though exotics initially might have an edge over native plants on burned ground, this may not always be so as time goes on, according to a U.S. Forest Service study.

Qinfeng Guo, a research ecologist with the Eastern Forest Environmental Threat Assessment Center, made this discovery after re-analyzing post-fire plant data from a southern California chaparral community that burned in November 1993 during the Old Topanga Fire.

Researchers had established study plots across 310 burned acres in the Santa Monica Mountains, which comprised north and south facing slopes and a variety of soil types. They recorded the number of plant species (both native and exotic) and their abundance (biomass and number of individuals per species) for four years after the fire.

“I hypothesized that the same community may show different relationships between exotic and native plants at different points in time as vegetation regrew after the fire,” says Guo. He was correct. His findings were recently published in the journal Acta Oecologica.

For two years after the fire, the number, or richness, of both native and exotic plant species increased, but exotic species made up a larger proportion of all the plant species. The richness of both exotic and native species then gradually declined, and within four years after the fire, native plant species began to dominate the larger proportion of vegetation on the site.

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Hypothetical vegetation dynamics after fire. Image courtesy of Acta Oecologica.

“The relationship between native and exotic plants has mostly been studied across space, and a growing number of studies have examined this relationship through time,” says Guo. “Rarely has the relationship between native and exotic plants been studied across time after major disturbances when the availability of resources like light, water, and nutrients can vary drastically in comparison to pre- disturbance conditions.”

Though his is a case study of just one fire and the resulting vegetation dynamics, it demonstrates a need for further investigation of post-fire plant relationships in other regions and ecosystems, especially where exotic plants can take advantage of habitat shifts due to warming temperatures and changing land use.

Any new vegetation growth after a fire can provide benefits—even exotic plants, which can protect soil from erosion and, sometimes, native plants from predation. But land managers must plan post-fire activities carefully and efficiently to ultimately reduce the threat of exotic plants to native plants and their ecosystems. “More studies like this one can help managers understand the best timing for actions that can prevent the establishment of exotic plants following fire,” says Guo.

Read the full text of the article.

For more information, email Qinfeng Guo at qguo@fs.fed.us.

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