Suffering Pines

The findings mesh with previous work done on a smaller scale. In 2009, scientists reported that the number of big trees in Yosemite National Park had fallen by nearly a quarter since the 1930s. Some of the most severe declines in Yosemite were among pine trees, including ponderosa, sugar, and Jeffrey pines.

The new study shows that a similar decline is happening among pines throughout the state. But the data are too sparse to say whether the state’s trademark giants—sequoias and redwoods—are also losing some of their stature, McIntyre said.

Large trees in general appear to be more vulnerable to a water shortfall, he said. Though it’s not clear why, one reason may be that in large, tall trees the internal hydraulic system that pumps water from roots to leaves is more susceptible to failing when water is short. Another factor could be that many of those trees sprouted centuries ago, when California’s climate was colder, said Jim Lutz, a Utah State University forest ecologist and lead author of the Yosemite study.

“Those individuals were very well adapted for the climate when they were growing rapidly. And that’s different from the climate now,” Lutz said.

While the number of big trees has declined in California, the number of trees smaller than a foot across has surged across the state, and trees between one and two feet in diameter have fared well in a number of areas. Aggressive wildfire control is thought to have abetted the trend, by letting forests fill with small trees that might have succumbed to more frequent fires.

But in another sign that climate is changing California’s forests, McIntyre and his colleagues found that oaks are replacing pine trees in large swaths of the state. Oaks are an increasing presence in the Sierras and along the north coast, while declining slightly in southern California. Pines meanwhile have declined everywhere.

That’s what one would expect in a warming climate, McIntyre said: Pollen records going back 150,000 years show that oaks have thrived in hotter, drier eras, while pines were more plentiful during cooler ones.