They’ve gone to logging and housing—but especially to climate change, says a new study

By Warren Cornwall, National Geographic

PUBLISHED January 19, 2015

Photograph by Raul Touzon, National Geographic Creative

There are a lot fewer big trees in California than there were in the 1930s California has lost half its big trees since the 1930s, according to a study to be published Tuesday in the Proceedings of the National Academy of Sciences —and climate change seems to be a major factor.

The number of trees larger than two feet in diameter has declined by 50 percent on more than 46,000 square miles of California forests, the new study finds. No area was immune, from the foggy northern coast to the Sierra Nevada Mountains to the San Gabriels above Los Angeles. In the Sierra high country, the number of big trees has fallen by more than 55 percent; in parts of southern California the decline was nearly 75 percent.

Many factors contributed to the decline, said Patrick McIntyre, an ecologist at the state Department of Fish and Wildlife who was the lead author of the study. Loggers targeted big trees. Housing development pushed into the woods. Zealous fire suppression has left California forests crowded with small trees that compete with big trees for resources.

But in comparing a census of California forests done in the 1920s and 1930s with another survey between 2001 and 2010, McIntyre and his colleagues documented a widespread demise of big trees that was evident even in wildlands protected from logging or development.

The loss of big trees was greatest in areas where trees had suffered the greatest water deficit. The researchers estimated water stress with a computer model that calculated how much water trees were getting versus how much they needed, taking into account such things as precipitation, air temperature, soil moisture, and the timing of snowmelt.

The current drought, now entering its fourth year, does not figure in the new study—even though it’s taking a toll on California’s trees—because the most recent forest census ended before the drought began. (Read “When the Snows Fail.”)

Since the 1930s, McIntyre said, the biggest factors driving up water stress in the state have been rising temperatures, which cause trees to lose more water to the air, and earlier melting of snowpacks, which reduces the water supply available to trees during the dry season.