California’s Tallest and Most Iconic Trees Are Getting Heatstroke

Coast redwood forests may sequester less carbon and support fewer arboreal species due to heat stress.

A new study from UC Davis reveals that coast redwood leaves reduce photosynthesis during extreme heat events. Researchers measured redwood leaves over two years and found that when temperatures rise, the trees close their stomata, pores on their leaves, to conserve water. This action, while saving water, also slows down photosynthesis, which is how trees "eat" and store energy for growth. This phenomenon, termed "carbon starvation," could create a cycle where trees store less carbon, leading to less efficient photosynthesis in the future and potential decline. This is the first study to observe this heat stress response in coast redwoods, which are crucial for carbon sequestration and harbor unique ecosystems. The study also notes that declining fog cover, a natural cooling and hydration source for these trees, exacerbates the problem by reducing their ability to cope with rising temperatures.

The study highlights a critical vulnerability in coast redwoods: their inability to maintain photosynthesis under prolonged heatwaves. This "carbon starvation" mechanism, previously observed in tropical trees, poses a novel threat to these iconic giants, which are vital for global carbon storage. As new-growth forests are more "hungry" for carbon, this heat-induced slowdown could hinder their growth and long-term health, potentially impacting their capacity to store carbon as effectively as their older counterparts.

Compounding the issue, the decline in coastal fog, a natural cooling and hydration mechanism for redwoods, leaves them more exposed to escalating temperatures. This reduction in fog diminishes the trees' ability to regulate their temperature and access water, creating a "double whammy" effect that stresses these ecosystems. The loss of fog also threatens the unique microclimates within redwood forests, potentially jeopardizing the habitat for species like the wandering salamander and marbled murrelet that depend on these cool, moist conditions.