California's forests are losing their biggest trees, and researchers say warmer, drier climate is driving a change to forests of smaller trees more susceptible to wildfires.

Using historical tree surveys done between 1929 and 1936 and comparing them to similar surveys from 2001 to 2010, researchers determined that large tree density fell everywhere in the state, with declines reaching as much as 50 percent in some areas.

The reduction in large trees -- those with trunk diameters of 2 feet or more -- was more than matched by a dramatic increase in smaller trees, scientists from the University of California campuses in Berkeley and Davis and state and federal agencies report.

"Older, larger trees are declining because of disease, drought, logging and other factors, but what stands out is that this decline is statewide," says study lead author Patrick McIntyre of 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."

In drought conditions, less water is available for tress to grow to large size, while those same conditions increase the numbers of wildfires and the speed with which they can spread, the researchers say.

"Based on our data, water stress helps to explain the decline of large trees," says McIntyre, who began the research while a postdoctoral fellow at UC Berkeley. "Areas experiencing declines in large-tree density also experienced increased water stress since the 1930s."

Large trees are vital as carbon sinks taking up and storing fossil fuel emissions, and their loss could making the global carbon situation worse, says study co-author David Ackerly, an integrative biology professor at UC Berkeley.

"There's no question that if you are losing large trees, you are losing the standing carbon in the forest," he said.

Loss of big trees could have consequences for the carbon cycle, he says, turning a valuable carbon sink into another source of carbon being released to the atmosphere.

A forest's larger trees "contribute disproportionately to forest structure and function, carbon stocks, and the cultural values of forests" compared to smaller trees, the researchers write in the Proceedings of the National Academy of Sciences.

What is happening in California could be a harbinger of forest responses to climate change throughout western North America, they say, with large-scale changes resulting from rising temperatures and declining availability of water.

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