A forest plot several years after wildfire

Clark Richter

Climate change and a century of fire suppression have significantly impacted forest ecosystems. But how altered fire regimes have affected plant diversity in the forest understory—the vegetation growing from the forest floor, beneath the canopy—is poorly known. A new study of yellow pine and mixed conifer forests in the Sierra Nevada, California suggests that understory diversity thrives best where fires are neither absent nor extreme.

Researchers at the University of California Davis, Stanford University, and the United States Department of Agriculture Forest Service used satellite imagery to guide the selection of 630 sampling sites that had been burned in unintentional wildfires. At each site, measured five to fourteen years after fire, every understory plant species was recorded within a circular 1/10 acre plot. Visual assessment of remaining vegetation and burn patterns was used to categorize plots by fire severity, from level 0 (unburned) to level 5 (over 90 percent plant mortality).

For the 602 plant species identified across all plots, the researchers’ graphing revealed a hump-shaped relationship. The highest total number of species were at sites with moderate (level 3) fire severity, with lower diversity at sites without fires and at those with high fire severity. “Historically, the [yellow pine and mixed conifer] system burned frequently at low and moderate intensity, so these species evolved in that context,” said lead author Clark Richter, and mature trees can survive moderate intensity fires.

Now though, high intensity fires, which can climb into the canopy and kill mature trees, occur with greater frequency and over a larger area than in the past. After high intensity fire, loss of trees gives shrubs the upper hand. As high intensity fires become more frequent, “shrubfields are becoming more common on the landscape,” said Richter. Consequently, understory diversity suffers, and this diversity loss could continue over time. The researchers suggest that fuel and fire management that mimics historical fire patterns of low and moderate intensity burns will help enhance understory plant diversity. (Ecosphere)