A group of California scientists published a study this week comparing forest surveys from the 1920s and ’30s to recent U.S. Forest Service data. What they found was not encouraging for the future of the state’s renowned large trees. Published on Tuesday in the journal Proceedings of the National Academy of Sciences, the study found that drought, changes in land use, and fire suppression efforts have caused the number of trees larger than two feet in diameter to decline by 50 percent in a 46,000 square mile area of the state’s forest they surveyed.
“Older, larger trees are declining because of disease, drought, logging and other factors, but what stands out is that this decline is statewide,” said study leader Patrick McIntyre, who manages biodiversity data for the California Department of Fish and Wildlife. “Forests are becoming dominated by smaller, more densely packed trees, and oaks are becoming more dominant as pines decline.”
With California currently locked in a serious drought, the role of water stress in determining tree size does not bode well for the future. The study does not factor in the four-year drought now underway as the latest census was taken just before it began. Scientists have determined that the impacts of climate change are exacerbating the state’s current drought. Climate models predict that the state and much of the Southwest will continue to get hotter and drier.
“Based on our data, water stress helps to explain the decline of large trees,” McIntyre said. “Areas experiencing declines in large-tree density also experienced increased water stress since the 1930s.”
According to the study, both large tree declines and increased oak dominance are associated with increases in water deficit, “suggesting that water stress may be contributing to changes in forest structure and function across large areas.”
Declines in large trees were found in all the regions of California that were surveyed. An increase in small trees was found in every region aside from the south and central coast. Small tree density around the interior Sierra Nevadas had increased dramatically, more than doubling in the Sierra Nevada highlands.
Co-author David Ackerly, a professor of integrative biology at UC Berkeley, said that the loss of large trees is more than just symbolic or aesthetic; it could actually have a negative impact on carbon dioxide emissions. Forests full of big, healthy trees are able to soak up more carbon dioxide.
“There’s no question that if you are losing large trees, you are losing the standing carbon in the forest,” he said. “Loss of these big trees and the impact of drought stress become a big concern going forward in terms of its impact on the carbon cycle; they can turn a carbon sink into a source of carbon released to the atmosphere.”
Dense forests packed with small trees also represent an increased fire hazard, a threat that California is already struggling to keep pace with. Mark Schwartz, professor of environmental science and policy at UC Davis and director of the John Muir Institute of the Environment, told the Los Angeles Times that denser forests allow fires to travel faster.
Princeton forest ecologist Willian Anderegg told National Geographic that this study is another sign of the “shrubbification” of western forests.
“The loss of these majestic largest trees is a pretty emotionally powerful thing to think about,” he said. “These are often the trees that have been around for thousands of years. It’s kind of a less magical future having lost those trees.”