A new study reveals that extreme heat can reduce photosynthesis rates in coast redwood needles by roughly 50 percent.

A new study published September 22 in the Journal of Geophysical Research: Biogeosciences reveals that towering coast redwoods may be vulnerable to modern heat waves. Researchers found that when temperatures spike, the needles of these iconic trees can cut their net photosynthesis roughly in half.

Lab Experiments Show Photosynthesis Slowdown

Researchers conducted lab experiments showing that the rate of photosynthesis in coast redwood needles roughly halves when temperatures rise from 30° to 40° Celsius. Lily Klinek, an ecophysiologist at the University of California, Davis, notes that such a change usually indicates severe levels of stress, which can cause tree deaths and jeopardize California's carbon storage. From July 2022 to October 2024, Klinek, Troy Magney, and colleagues repeatedly measured needle performance from redwoods in California's Mendocino County, a period that included two heat waves. When subjected to increasing heat in the lab, photosynthesis rates declined even when samples were submerged in water with open stomata, indicating the slowdown was not caused by carbon starvation.

Stomata Closure and Extreme Needle Temperatures

Trees typically stay cool when porelike openings called stomata allow water to evaporate, much like human sweat. However, under extreme heat, the research team found that dark redwood needles absorbed ample sunlight and reached up to 10 degrees Celsius hotter than the surrounding air, suggesting stomata were at least partially closed. These pores are crucial for photosynthesis because they allow trees to take in carbon dioxide and release oxygen. Klinek notes that trees could essentially starve if they lack access to enough carbon. Because sampling older growth giants over 100 meters tall is impractical without a jetpack, the study focused on younger coast redwoods.