Declining hydraulic safety in a drier world.

Forests worldwide are increasingly exposed to soil drought under climate change, with their fates depending on the ability to maintain essential functions like water transport (hydraulics) and photosynthesis. Acclimation is expected to mitigate drought impacts, but the extent to which trees acclimate remains largely untested, which limits our predictive confidence. Here, we examined 24 physiological attributes from 40 globally distributed forest throughfall reduction (TFE) experiments and found no evidence that trees adjusted their hydraulic or photosynthetic systems in response to drought. Key attributes related to embolism resistance, hydraulic efficiency, leaf nutrients, and Rubisco carboxylation capacity remained unchanged, regardless of coniferous or broadleaf trees, local precipitation levels, or the duration and severity of drought treatments. However, drought-induced declines in tissue water potentials, combined with unchanged embolism resistance, led to narrower hydraulic safety margins and thus an increased risk of hydraulic failure. Although net photosynthetic rate declined significantly due to stomatal closure, nonstructural carbohydrates (starch and sugars) remained stable, suggesting a shift in carbohydrate allocation toward storage. These findings indicate that trees maintain their hydraulic and photosynthetic capacities under drier conditions, enabling them to maximize carbon assimilation on favorable periods following rainfall, while facing an increased risk of hydraulic failure during drought. Physiological acclimation is unlikely to mitigate future drought impacts, while tree mortality from hydraulic failure is likely to increase.