California’s Tallest and Most Iconic Trees Are Getting Heatstroke

Researchers from the University of California, Davis, measured leaf‑level gas exchange on coast redwoods twice monthly over a two‑year period and observed that, during heat waves, the trees close their stomata to conserve water. This physiological response cuts carbon intake, effectively “starving” the trees of the energy needed to replace foliage and sustain growth. The study notes that redwoods normally operate within an 8‑27 °C (46‑80 °F) window; recent temperature spikes push them beyond this range, and the trees cannot evaporatively cool fast enough. Compounding the problem, fog—once a reliable source of moisture and shade—has dropped by 30‑40 % in the region, stripping the canopy of its natural air‑conditioning.

The findings dovetail with a growing body of evidence that climate‑driven stress is eroding the performance of the world’s most efficient carbon sinks. Coast redwoods store more CO₂ per acre than any other forest type, and the majority of California’s redwood stands are now second‑generation growth that relies heavily on rapid carbon uptake to reach mature stature. If heat‑induced stomatal closure becomes chronic, these forests could sequester significantly less carbon, undermining both state climate goals and the global carbon budget. Moreover, the loss of fog and the attendant rise in leaf temperature echo similar stress patterns documented in tropical rainforests, suggesting a broader shift in forest physiology that may outpace the adaptive capacity of long‑lived tree species.

Looking ahead, forest managers and policymakers must monitor redwood water balance and temperature trends more closely, perhaps integrating supplemental fog‑capture systems or selective thinning to reduce canopy heat load. The study also raises the specter of cascading biodiversity loss: ferns, soil mats, salamanders, and marbled murrelets all depend on the cool, moist microclimate that redwoods create. If photosynthetic rates decline, reduced growth will shrink the three‑dimensional habitat that sustains these species. Tracking fog frequency, implementing watershed protection, and investing in climate‑resilient planting strategies will be critical to preserving both carbon storage and ecosystem complexity.