The biosphere can exert strong controls on the hydrologic system through plant regulation of water and energy fluxes. Plant physiological responses to rising CO₂ have been shown to influence the water cycle through changes in the surface water budget as well as large-scale precipitation patterns, with implications for drought and flooding. However, significant uncertainty remains in our understanding of the magnitude and form of these plant responses, which can vary greatly across different Earth system models. In this project, we quantify the role of plant processes in regulating hydrological (e.g., evapotranspiration, precipitation, and runoff) and biogeochemical (e.g., photosynthesis) cycling, as well as climate extremes (e.g., heatwaves).  We analyze a broad array of simulations and leverage observations from tree rings. The simulations include CMIP5/6 Coupled Climate–Carbon Cycle Model Intercomparison Project, coupled/uncoupled Community Earth System Model perturbed parameter ensembles, and controlled leaf-area experiments. Simulation results are compared to novel observation-based estimates that we developed from tree ring records over the historical period, including water use efficiency and leaf area. Our results demonstrate that the influence of plant processes on hydrological and biogeochemical cycles is strongly sensitive to the representation of leaf-level (e.g., stomatal conductance) and organism-level (e.g., leaf area) processes, as well as the degree of land-atmosphere coupling.