Hydraulic Safety Mechanisms Override Traditional Wood Economics in Hyper-Arid Environments.

Environmental stress drives plant communities toward conservative ecological strategies through increased wood density (WD) within the Wood Economics Spectrum (WES). However, hyper-arid regions like the Coastal Atacama Desert (CAD) challenge this pattern, where woody plants exhibit acquisitive traits and decreased WD with increasing aridity. The underlying anatomical mechanisms remain poorly understood. This study examined how extreme aridity in the CAD shapes wood economics strategies, testing whether anatomical changes prioritize hydraulic safety over efficiency within the WES. Six shrub communities along an aridity gradient were sampled, measuring composition and abundance of 29 species across 20 plots per site. Community-weighted means of eight wood traits-including WD, vessel density/diameter, parenchyma fraction, fiber fraction, and lumen fractions-were analyzed. PCA, triangular and linear models assessed trait variation along the aridity gradient. Unexpectedly, WD increased at both gradient extremes but through different tissue compositions, with no clear shift from acquisitive to conservative strategies. Instead, vessel traits (density and diameter) were key, reflecting an independent hydraulic safety-efficiency trade-off. PC2 strongly correlated with aridity, showing reduced vessel size and increased density under greater aridity. Findings reveal that hyper-aridity disrupts traditional wood economics, with hydraulic adaptation-not WD-mediated resource use-driving community assembly. This highlights the dominance of safety-efficiency trade-offs in structuring shrub communities in extreme deserts, emphasizing hydraulic traits over conventional resource strategies.