New Research Weighs in on Sierra Nevada Forest Density Controversy

A new study demonstrates flaws in the methods used to argue that fire-prone Sierra Nevada forests were historically crowded with trees. Andrew Larson, University of Montana associate professor of forest ecology, is co-author of the study just published in the journal Ecological Applications.

Vertically continuous ladder fuels created by small diameter white fir

The consensus among most forest ecologists is that Sierra Nevada California, forests pre-European settlement were dominated by large trees, with sparse, open understories. This low-density forest structure was maintained by frequent low- and moderate-severity fires.

A few researchers have controversially claimed that Sierra Nevada forests were densely crowded with trees, using an untested statistical analysis of General Land Office surveys from the late 19th and early 20th centuries to support their claims. That research has even been used in lawsuits against the U.S. Forest Service that challenged forest restoration plans.

This study demonstrates flaws in the methods used to analyze the General Land Office (GLO) surveys. Larson explained the approach, "We tested how well the GLO method performed using large plots where every tree was measured and mapped places where the true forest density was known."

Results of that analysis identified issues with the GLO method. "We went through the data and showed that, in every case, this method estimated that the density of trees was two to three times higher than was the reality," said Carrie Levine, a Ph.D. student of forest ecology at University of California Berkeley and lead author of the study.

"There are multiple lines of evidence demonstrating that low-density, large-tree-dominated forest conditions were common throughout the Sierra Nevada," said Larson. Former University of Montana graduate student Molly Barth and Larson, along with collaborator James Lutz at Utah State University, published research in 2015 in the journal Forest Ecology and Management in which they reconstructed historical conditions in a Sierra Nevada mixed-conifer forest. Their findings showed historical low tree densities in the Yosemite Forest Dynamics Plot (YFDP), an old-growth, sugar pine-white fir forest in Yosemite National Park.

Larson explains the importance of the 2015 study, "The results from the YFDP reconstruction really emphasize how strongly low-severity fires controlled forest structure and density. Even at our cool, north-facing site, historical fires maintained low fuel loads and favored large-diameter trees."

Some ecologists question the need for knowledge about historical forest conditions given climate change. "It is reckless to dismiss information about historical conditions on the grounds of climate change," said Larson. Climate change is expected to bring increasing droughts, fires and insect outbreaks, stressors that forest ecologists say low-density forests will be more likely to withstand.

This new study points out flaws in the GLO method and rejects the evidence behind claims that Sierra Nevada forests were historically crowded with many flammable small trees. "Forest restoration treatments that reduce densities of small trees and use prescribed fire to reduce fuel loads are an essential part of any climate change adaptation strategy for the Sierra Nevada," said Larson.

Additional co-authors were from Harvard Forest, the USDA Forest Service Pacific Southwest Research Station, Utah State University, the University of California Davis, and the USDA Forest Service Pacific Southwest Region.

Large-diameter sugar pine with large basal fire scars from past low-severity ground fires.

Photos: Top - Vertically continuous ladder fuels created by small diameter white fir (Abies concolor) that have established during the approximately 110 years since the last widespread fire in the Yosemite Forest Dynamics Plot, Yosemite National Park, California. Photo credit: Andrew J. Larson

Bottom - Large-diameter sugar pine (Pinus lambertiana) with large basal fire scars from past low-severity ground fires.  Low-severity fires burned across the Yosemite Forest Dynamics Plot about every 30 years before 1900. Historical low-severity surface fires consumed surface fuels and killed small, fire-intolerant white fir (Abies concolor) and incense cedar (Calocedrus decurrens), favoring the survival of large sugar pine. Photo credit: Molly A.F. Barth.