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Fires change forests

Fire frequency effects on trees in forests and savannas

tree trunks with flames all around them
Fire frequency on the 29 research sites ranged from approximately one fire every decade to one fire per year, with an average of one fire every three years. Photo by Mike McMillan, USDA Forest Service.

A study spanning four continents and 67 years suggests that frequent fire is causing grasslands to replace savannas.

The effects of changing fire frequencies may take several decades to become substantial, reports the study led by Stanford University researcher Adam Pellegrini, with contributions from USDA Forest Service research plant physiologist Mary Anne Sword Sayer and emeritus scientist Dale Brockway.

The research also shows that the impact of these changes will continue and increase for many decades.

Of the 29 research plots, those that burned every year had 63 percent lower stem density than unburned plots. The annually burned plots also had 72 percent lower basal area than unburned plots.

The largest changes occurred on savannas and on sites with strong wet and dry seasons because fires are most intense in these areas. Stronger wet seasons lead to more fuels, and more severe dry seasons mean lower fuel moisture, reports the study.

Repeated fire alters tree communities by increasing the abundance of species that can better tolerate fire’s effects. Species’ functional traits support this observation. Tree species with thicker bark and deeper sapwood tolerate fire better than species without these traits. Species with dense wood are less common under recurrent fire suggesting that as fire frequency increases, conifers increase and broadleaf trees become less common. Other traits that help trees survive include relationships with fungi and bacteria. Known as symbioses, these relationships can help trees access nutrients that may be depleted by frequent burning.

Frequent burning favored trees with low nitrogen and phosphorus content and those with more efficient nitrogen uptake through ectomycorrhizal fungi associations. In other words, a history of repeated fire promotes the establishment of tree species with conservative nutrient use strategies, suggesting that fire may influence the evolution of plant nutrition strategies.

Read the study in Nature Ecology & Evolution. For more information, contact Mary Anne Sword Sayer at mary.sword@usda.gov or Dale Brockway at dale.brockway@usda.gov.

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