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Goal: Sustain Our Nation’s Forests and Grasslands Changes in fire frequency shift tree communities

How will forests and savannas across the globe respond to climate change? The effects of changing fire frequencies may take several decades to become substantial, but a study spanning four continents and 67 years suggests that frequent fire is causing grasslands to replace savannas. Repeated fire alters tree communities, increasing the abundance of species that can better tolerate fire's effects.

The Rim Fire burning fiercely in a California forest
Massive wildfires, such as the Rim Fire in California, are occurring with increasing frequency. Such fires can have long-lasting ecological impacts, and in some cases are causing forests to shift towards grassland ecosystems. USDA Forest Service photo by Mike McMillan.

The long-term study included 29 plots on four continents, with fire frequency ranging from every year to every decade. Plots that burned every year had 63% lower stem density than unburned plots. The annually burned plots also had 72% lower basal area than unburned plots. After 50 years of average fire frequency (fire every three years) basal area fell by 53%.

The largest changes occurred in savanna ecosystems and on sites with strong wet seasons or strong dry seasons. 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. The research also shows that the impact of these changes will continue and increase for many decades.

Frequent burning favored trees with low nitrogen and phosphorus content, and with more efficient nitrogen uptake through ectomycorrhizal (root-associated fungi) associations. Taken together, the study shows that the way forest ecosystems respond to altered fire frequencies depends on both climate and tree species characteristics.

Principal Investigators
Mary Anne Sword Sayer, Research Plant Physiologist
Dale Brockway, Research Ecologist
James D. Haywood - SRS (Retired)
4158 - Restoring and Managing Longleaf Pine Ecosystems
Strategic Program Area
Resource Management and Use
Decadal changes in fire frequencies shift tree communities and functional traits
Research Partners
Chris Swanston - Nothern Research Station
Steven T. Overby - Rocky Mountain Research Station
W. Keith Moser - Rocky Mountain Research Station
External Partners
Adam F. Pellegrini - Stanford University
Tyler Refsland - University of Nevada
Colin Averill - Institute of Integrative Biology, Zurich, Switzerland
Cesar Terrer - Stanford University & Lawrence Livermore National Laboratory, CA
A. Carla Staver - Yale University
Anthony Caprio - National Park Service
Wayne Clatterbuck - University of Tennessee
Corli Coetsee - South African National Parks, Skukuza, South Africa & Nelson Mandela Univ.
Sarah E. Hobbie - University of Minnesota
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William A. Hoffman - North Carolina State University
John Kush - Auburn University
Tom Lewis - Queensland Government, Brisbane, Australia
William A. Patterson III - University of Massachusetts
Kabir G. Peay - Stanford University
Peter B. Reich - University of Minnesota & Western Sydney University, Penrith, Australia
Casey Ryan - University of Edinburgh, Edinburgh, UK
Bryant C. Scharenbroch - University of Wisconsin
Tania Schoennagel - University of Colorado
Gabriel Reuben Smith - Stanford Univ. & Institute of Integrative Biology, Zurich, Switzerland
Kirsten Stephan - West Virginia University
Monica G. Turner - University of Wisconsin
J. Morgan Varner - Tall Timbers Research Station
Robert B. Jackson - Stanford University
T.R. Miller Woodlands Company
University of Georgia