Leaf hydraulic and photosynthetic traits mediate forest-scale resource use efficiencies across the continental United States

Temperate forests play a critical role in global carbon cycling and other ecosystem services, yet their resilience to climatic stresses remains uncertain. We combined physiology, community composition, flux data, and modeling to test a functional trait-based approach to predict forest ecosystem processes. For eight U.S. forests spanning broad precipitation and temperature gradients, we scaled up key leaf functional traits—maximum carboxylation velocity of Rubisco ( V c , m a x ), maximum leaf hydraulic conductance ( K l , m a x ), and turgor loss point (that is, wilting point, π t l p ) to predict gross primary productivity, light-use efficiency, standard and intrinsic water-use efficiency (WUE and iWUE), and their sensitivities to soil drought. In multiple regressions, ecosystem processes were strongly associated with community-weighted mean (CWM) trait values: enhanced productivity was linked with high V c , m a x ¯ , K l , m a x ¯ and π t l p ¯ , whereas high WUE and iWUE were linked with low K l , m a x ¯ and high π t l p ¯ . The CWM functional traits also influenced the sensitivity of ecosystem processes to soil moisture. A causal role of functional traits in driving ecosystem processes alongside climate was supported by parameterizing the LM4.1 vegetation model with CWM functional traits. This study highlights the important role of functional traits in determining forest responses to climate-driven water stress at a continental scale.