Increased understanding of how mechanical thinning, prescribed burning, and wildfire affect subsequent wildfire severity is urgently needed as people and forests face a growing wildfire crisis. In response, we reviewed scientific literature for the US West and completed a meta-analysis that answered three questions: (1) How much do treatments reduce wildfire severity within treated areas? (2) How do the effects vary with treatment type, treatment age, and forest type? (3) How does fire weather moderate the effects of treatments? We found overwhelming evidence that mechanical thinning with prescribed burning, mechanical thinning with pile burning, and prescribed burning only are effective at reducing subsequent wildfire severity, resulting in reductions in severity between 62% and 72% relative to untreated areas. In comparison, thinning only was less effective – underscoring the importance of treating surface fuels when mitigating wildfire severity is the management goal. The efficacy of these treatments did not vary among forest types assessed in this study and was high across a range of fire weather conditions. Prior wildfire had more complex impacts on subsequent wildfire severity, which varied with forest type and initial wildfire severity. Across treatment types, we found that effectiveness of treatments declined over time, with the mean reduction in wildfire severity decreasing more than twofold when wildfire occurred greater than 10 years after initial treatment. Our meta-analysis provides up-to-date information on the extent to which active forest management reduces wildfire severity and facilitates better outcomes for people and forests during future wildfire events.

(ALL RIGHTS GRANTED TNC) View of Escudilla Mountain. Fall colors heighten the natural beauty of forest and meadow in the White Mountains, one of Arizona’s last wide-open spaces and where TNC works to protect the headwaters of three major Arizona rivers: the Salt, Gila, and Little Colorado and their greenbelts – riparian habitats critical for wildlife and water quality – as well as restore healthy forests within the largest ponderosa pine community in the world, save rare and unique wildlife and plant species, and control non-native, invasive species, such as crayfish, Arizona. © Betsy D. Warner/TNC

Aerial view of the Parks West restoration site overseen by TNC. Taken March 16, 2020.

Conservancy staffers, Dale Turner and Amanda Rebore, helping to map the Sabn Pedro River in Arizona. They use a GPS unit to denote the end of the water flow on the LowerSan Pedro River. They hiked through the Conservancy’s San Pedro Preserve.

Our Approach to Science

The Nature Conservancy’s conservation science program in Arizona engages stakeholders and expertise in applied science and policy to develop new information, ideas, and tools that can help solve some of our most pressing challenges affecting people and nature

Explore Our Science

Future Forests

We are working with partners and stakeholders to accelerate the pace and scale of forest restoration for a healthy Arizona

Mapping the Status of River Streams

Wet/dry mapping provides a low-cost, river-wide snapshot of hydrologic conditions for rivers with interrupted perennial surface flows.

Our Science in Action

Forests

Future Forests

Accelerating the pace and scale of forest restoration for a healthy Arizona.

Rivers & Water

Wet/Dry Mapping

Snapshot of hydrologic conditions for rivers with interrupted perennial surface flows.

Rivers & Water

Arizona Rivers & Water

Achieving sustainable water management.

Deserts & Grasslands

Las Ciénegas Adaptive Management

Collaboration with the Bureau of Land Management at Las Ciénegas National Conservation Area.

Conservation

Ecoregion Assessments

Conservation of biodiversity across western North America.

Forests

Southwest Forest Assessment Project

Restoring ecosystems throughout Arizona and New Mexico’s National Forest.

Deserts & Grasslands

Southwest Grasslands

Assessing the state of Arizona’s grasslands.

Conservation

Natural Infrastructure

Developing a natural infrastructure dataset by integrating studies on wildlife habitat and open space.

Explore Our Science

Dec 2024

Maps

BLM, NGO

Freshwater Assessment

Riparian-aquatic

San Pedro River

Dale Turner, Brooke Bushman, Lisa McCauley, Patrick Hellmann

San Pedro River Wet/Dry Map Animation

Every June groups of volunteers walk the entire 170 mile length of the San Pedro River and record where it is wet and where it is dry during the hottest, driest time of the year. Twenty years’ worth of data on summertime surface flows in the San Pedro River within the San Pedro Riparian National C[…]

Dec 2024

Maps

BLM, NGO

Freshwater Assessment

Riparian-aquatic

San Pedro River

Turner, D., L. McCauley, P. Hellmann

San Pedro River Wet-Dry Maps

The San Pedro River wet/dry mapping dataset is a community effort to track the river’s health by monitoring the persistence of surface water during the driest time of each year. It is created by recording the end points of every wet section of the San Pedro River during June each year. Maps depict[…]

Apr 2024

Papers

Collaboration, USFS

Ecosystem, Forest

Fire, Forest Restoration Benefits, Topic

Geo Region, Western U.S.

Kimberley T. Davis, Jamie Peeler, Joseph Fargione, Ryan D. Haugo, Kerry L. Metlen, Marcos D. Robles, Travis Woolley

Tamm review: A meta-analysis of thinning, prescribed fire, and wildfire effects on subsequent wildfire severity in conifer dominated forests of the Western US