Rising Temperatures Are Causing California’s Coast Redwoods to ‘Starve’

A new study from the University of California, Davis, indicates that coast redwoods are significantly reducing their photosynthetic activity during periods of extreme heat, a physiological response that could jeopardize the longevity of the species and the ecosystems they support.

Coast redwoods have evolved in a cool, fog-draped climate, relying on stable environmental conditions for millennia. However, researchers found that when temperatures spike, the trees cannot cool themselves sufficiently and are forced to close the pores in their leaves, known as stomata. While this conserves water, it simultaneously blocks the intake of carbon dioxide, effectively stopping the trees from “eating” and building the energy reserves necessary for growth.

“Because leaves are a physical, light-absorbing object, they heat up just like asphalt would in the sun,” said lead author Lily Klinek, a PhD student at UC Davis. “They’re heating up faster than the air around them… But they can only really do that if they have enough water to lose.”

The study, which involved biweekly measurements over two years in Mendocino County, highlighted a dangerous feedback loop. When trees cannot store enough carbon, they may lack the resources to replenish their leaves in subsequent years. This “carbon starvation” can lead to reduced photosynthetic efficiency and eventual death, a pattern previously observed in tropical trees but now documented in redwoods for the first time.

Chris Still, a forest ecologist at Oregon State University who was not involved in the research, noted that leaves are energetically expensive for plants to produce. The threat is compounded by the fact that many current redwood forests are second-growth stands planted within the last few centuries. These younger trees are in a rapid growth phase, similar to adolescents, and require substantial carbon intake to reach their full height and bulk.

Redwoods are among the most efficient carbon storage systems on the planet, sequestering more carbon dioxide per acre than any other forest type. Anthony Ambrose, a plant physiological ecologist and co-founder of the Ancient Forest Society, warned that reduced sequestration not only exacerbates climate change but also limits the physical development of the trees. Smaller trees provide less canopy space for the complex elevated ecosystems—mats of soil and ferns—that host species like the wandering salamander and the marbled murrelet.

Compounding the heat stress is a decline in the fog that traditionally acts as a natural air conditioner for these forests. Todd Dawson, a plant ecophysiologist at the University of California, Berkeley, described the situation as a “double whammy,” noting that fog coverage has dropped by 30 to 40 percent. With warmer oceans and drier air making fog more sporadic, the trees are losing a crucial source of hydration and shade.

While the redwoods studied appeared to recover after heat waves subsided, Klinek cautioned that this resilience may not persist as climate change drives temperatures higher and heat events more frequent. “In five or 10 years, if temperatures are even hotter and heat waves are even more frequent, we don’t really know if there would be a recovery,” she said. As the climate shifts, the delicate balance of these ancient giants and the diverse life they harbor hangs in the balance.