The overlooked role of root water content in the root economics space

Root traits are fundamental to plant survival, growth and adaptation to environmental changes. Despite increasing attention to the root economics space, a quantitative understanding of global patterns and key drivers of root trait variation remains elusive. By combining metabolic theory with global trait datasets, we reveal universal nonlinear relationships of five key root traits with root water content regardless of plant growth form or climate zone. Root water content emerges as a stronger predictor of growth-related root traits and shows a closer association with the conservation gradient than the widely considered root nitrogen, thereby better defining ‘fast’ resource acquisition strategies. Moreover, replacing nitrogen with tissue water content in analyses reveals a closer alignment of leaf and fine-root traits than expected. Our findings highlight general quantitative biotic and abiotic controls on plant trait variation, offering broader insights into plant economics strategies, community dynamics and ecosystem functioning under changing climate and resource availability.