Drought, fire suppression techniques and changes in land use have made California forests denser and more susceptible to fast-moving wildfires, a study to be released Tuesday has found.
Researchers at UC Berkeley, UC Davis and the U.S. Geological Survey compared tree surveys conducted between 1929 and 1936 with surveys conducted between 2001 and 2010. They found that large-tree density fell across California, with declines of up to 50% in the Sierra Nevada highlands, the south and central coast ranges, and Northern California. At the same time, the density of smaller trees increased dramatically.
Drier conditions caused by drought reduce water available for trees to grow while making it easier for fires to start and spread. Scientists say the changes raise critical questions about how California manages its forest land to prevent and control wildfires as temperatures increase.
“The current drought in California highlights our need to understand the role of water balance in these systems and how it will be affected by global temperature rise,” said the study's lead author, Patrick McIntyre. “Forests and woodlands cover a third of California, so this has important implications for our state.”
The study, to be published in the Proceedings of the National Academy of Sciences, found that since 1930, as water stress (when there is more demand for water than there is water available) increased, the density of large trees decreased.
Large trees, which are important for storing carbon from the air and as sources of food and habitat, can be more vulnerable to drought and water stress, McIntyre said.
“Declines in their numbers are concerning," he said.
Researchers found that densities of small trees increased in almost every region of California surveyed. Small-tree density within the Sierra Nevada highlands more than doubled, and it increased over 50% in the Sierra Nevada foothills, the north coast region and the transverse and peninsular ranges.
Oak trees, which thrive in warm, dry climates, increased in density as pine density decreased, the researchers found.
California has a Mediterranean climate, McIntyre said. Almost all of its precipitation comes in the winter, and in the summer it dries out. A lack of year-round water availability limits the amount of water stored in the soil and can hinder tree growth, he said.
Higher temperatures, which cause earlier snowmelt and more moisture loss, compound the water scarcity problem for larger trees.
Fire and timber management practices also play a major role in forest structure changes, the researchers said. The common reaction to wildfires is to quickly suppress them. But the study raises questions about how to adjust intervention techniques to allow healthy burns without jeopardizing human safety.
Mark Schwartz, professor of environmental science and policy at UC Davis and director of the John Muir Institute of the Environment, studies the effect of climate change on wildfires.
A denser forest allows fire to travel faster, causing more devastation, he said. After a fire, new, smaller trees grow that are more likely to catch fire, and the cycle continues.
“These are historically fire-maintained ecosystems,” Schwartz said. “The firemen are faced with this notion of when a fire is reported and started, do they go out and bring out helicopters, trucks and people and put the fire out or do they let it burn?”
Just how much the change in forest structure is due to fire suppression and how much results from climate change is hard to tell because the two are interrelated, Schwartz said.
“This is an active and controversial topic in California -- should we have more prescribed burns? Should we be out there cutting down these small trees?” said David Ackerly, a researcher involved in the study and professor in the department of Integrative Biology at UC Berkeley.
But as California temperatures are expected to increase, the researchers said, addressing forest management questions becomes more important. Average annual temperature is predicted to increase by up to 5 degrees Celcius, or 9 degrees Fahrenheit, by 2100, according to the study. That increase would mean projected increases in a water deficit of 30% or more in many areas.