New Study: Broad-scale Forest Restoration Decreases Tree Mortality

Fourth study in series builds on thinning co-benefits including increased water and carbon storage

A forested meadow with green grass in Mogollan Rim near Sycamore Canyon and Williams, Arizona.
Mogollan Rim Forest This forest is located on the Mogollan Rim near Sycamore Canyon and the town of Williams, Arizona. © Tana Kappel/ TNC

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A new study by the Nature Conservancy reveals expanded forest restoration decreases tree drought vulnerability in the face of warmer, drier temperatures. Forest restoration is one of the few tools available to forest managers who face a climate and wildfire crisis.

The study focuses on tree mortality across future climate scenarios in the Four Forest Restoration Initiative (4FRI). 4FRI uses mechanical thinning and prescribed fire to reduce tree densities across one-million acres of ponderosa pine forests in northern Arizona by an average of 40% in the next 10-20 years.

“Our research reveals that thinning more trees—which lessens water competition—could reduce tree mortality under climate change conditions by 25%. In a do-nothing scenario, more trees would die in the hotter and drier conditions expected with climate change,” says Lisa McCauley, lead author of the study and spatial scientist for The Nature Conservancy in Arizona. “We also learned the more thinning we do, the less mortality there is.”

TNC in Arizona has completed a series of studies that show the many ways in which restoration can help our forests withstand climate change.

Collectively, these studies found that 4FRI restoration can increase carbon stored in our forests by 15%, increase tree growth by 30% and streamflow by 20% even under a changing and variable climate. 

“We’re on the right track,” adds Travis Woolley, forest ecologist for TNC in Arizona. “In 2021, more than 21,000 acres were thinned in the 4FRI area, the highest annual rate since the beginning in 2009. In addition to thinning thousands of acres, TNC’s key role is to reduce costs and improve efficiency of forest thinning with a combination of forestry technologies and improved business practices.”

While the new study is focused on Arizona, a companion regional drought study found that the reduction of forest densities could also reduce drought vulnerability in large areas of pine forests across the West.

The science will enable much-needed funding to scale the work in Arizona and across the region. As part of the infrastructure bill that became law in November, the U.S. Forest Service rolled out a wildfire crisis strategy which will pump $3 billion into protecting communities and building wildfire resilience. Funds will be sent to areas with the greatest need.

“Our science is clear,” says Marcos Robles, lead scientist for TNC in Arizona. “Because we and other scientists have demonstrated that forest restoration has many co-benefits—less tree death along with increased tree growth, more carbon storage and streamflow—4FRI and other large-scale forest restoration projects across the West will be considered as priority areas for the funding which could truly make a difference.”

The Nature Conservancy is a global conservation organization dedicated to conserving the lands and waters on which all life depends. Guided by science, we create innovative, on-the-ground solutions to our world’s toughest challenges so that nature and people can thrive together. We are tackling climate change, conserving lands, waters and oceans at an unprecedented scale, providing food and water sustainably and helping make cities more sustainable. Working in more than 70 countries and territories, we use a collaborative approach that engages local communities, governments, the private sector, and other partners. To learn more, visit or follow @nature_press on Twitter.