Hydraulic diversity strengthens stem growth in temperate forests

Temperate forests are increasingly exposed to drought stress, leading to elevated risks of tree mortality, canopy die-off and forest carbon sink decline. Forest adaptation and resistance to drought stress largely rely on hydraulic diversity – the variety and range of hydraulic traits that regulate water use under drought conditions. Yet, how hydraulic diversity emerges within temperate forests and links to forest growth remains poorly resolved. To fill these gaps, we use eight hydraulic traits relating to stomatal closure, structural demand management, water storage, hydraulic resistance and rooting depth to quantify hydraulic diversity within forests, explore how it varies across temperate regions and explore its relationship with forest stem growth using the USA forest inventory data. A total of 31,304 forest plots were aggregated at a 1° grid-cell resolution and hydraulic diversity (721 metacommunities; those with fewer than three tree species are excluded) was quantified as the hypervolume size along the first two axes of principal component analysis (PCA). We found that higher diversity values were observed in the regions of the eastern USA while lower diversity values were found in the western and central USA and boreal regions. The variation in strategy diversity in temperate forests aligns mostly with changes along the acquisitive – conservative axis, spatial hydraulic diversity within temperate forest metacommunities indicates summer precipitation is more crucial than other climate variables. Interestingly, forests with low diversity are widely distributed across the full range of summer precipitation, suggesting that factors beyond water availability – such as temperature – may play an important role, particularly in the Pacific coast of Northern America. Moreover, we observed there is a positive relationship between hydraulic diversity and stem growth across the USA forest metacommunities. Our results provide a foundation for understanding forest hydraulic diversity and improving the accuracy in predicting forest carbon sink potential under a warmer and drier conditions.