Big Trees Are Disappearing, and Here’s Why
Big trees play an outsize role in the ecosystem, spreading seeds, feeding and housing animals, and absorbing carbon from the atmosphere.
But a new study finds that California’s once towering forests are shrinking.
Researchers compared data on tree sizes collected in the 1930s with surveys conducted in the 2000s. Over that time, the state lost half of its large trees—those more than two feet in diameter—in the 46,000 square miles studied.
While logging, habitat loss, and wildfire suppression are factors in the decline, the main culprit appears to be climate change, according to scientists. They found an increase in smaller trees like oak, which can withstand warmer temperatures. The study, published Tuesday in the journal Proceedings of the National Academy of Sciences, is the first to link smaller trees to a drier and hotter climate and has implications outside California as the world experiences more prolonged droughts.
“Forest composition in California in the last century has shifted toward increased dominance by oaks relative to pines, a pattern consistent with warming and increased water stress,” the study authors wrote.
The biggest loss of big trees came from areas where water was most scarce or had the largest “climatic water deficit.” Researchers determined “water stress” with computer models that traced how much water trees were getting compared with how much water a tree needs. Rainfall, air temperature, soil moisture, and snowpack can all determine a tree’s climatic water deficit.
Patrick McIntyre, an ecologist with the University of California, Davis, said rising temperatures are having the biggest impact on tree size by affecting the time water stays in the soil and decreasing snowpack.
“Our findings suggest that understanding the dynamics of water availability and drought stress may help understanding changes in forest structure,” McIntyre said. “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. Forests and woodlands cover a third of California, so this has important implications.”