Climate-dependent coupling and decoupling between canopy height and root-zone water storage in global forests

Forests govern exchanges of water, energy, and carbon between the land surface and atmosphere through interactions between aboveground structure and subsurface access to stored water. Canopy height and root-zone water storage capacity are therefore key controls on ecosystem function, yet they have rarely been assessed together at the global scale. Classical ecohydrological theory posits coordinated investment above and below ground, whereby enhanced access to deep water supports taller and more structurally complex canopies. However, it remains unclear whether such coordination holds across diverse climatic conditions. Here, we integrate global GEDI-derived canopy height observations, independent estimates of root-zone water storage capacity, and an integrated climatic gradient capturing water availability, atmospheric demand, and seasonality to evaluate this long-standing hypothesis at the global scale. By quantifying how above- and belowground traits co-vary across hydroclimatic regimes, we assess how access to deep water influences forest structure and identify where empirical patterns diverge from theoretical expectations. Our results reveal that the relationship between canopy height and root-zone water storage capacity is far more variable than classical theory suggests, with clear decoupling in both humid and strongly seasonal regions. These findings advance our understanding of vegetation–water interactions, highlight limitations of simplified assumptions about hydraulic constraints and structural investment, and provide a data-driven foundation for improving the representation of vegetation processes in land–atmosphere and Earth system models.