Global dependency of canopy height on vapour pressure deficit and its projections under climate change.

Canopy height is an important aspect of forest structure and functioning. Although water availability is important for canopy height growth, the climatic niche for tall trees remains poorly understood. Here we use global spaceborne lidar-derived canopy height to study its dependence on climate variables. We find that vapour pressure deficit (VPD) strongly controls geographical patterns of canopy height, observing a negative association also in tropical regions where water limitations are modest. Taller trees are prevalent in humid tropical regions, but canopy height decreases sharply as mean annual VPD surpasses 0.68 kPa. By 2100, projected increases in VPD under a warming climate could enhance limitations to canopy height growth, resulting in height losses in 87% of the humid tropical regions. Conversely, we project a widespread increase in canopy height across drylands, linked primarily to changing precipitation regimes. These results suggest that limitations on height growth driven by shifts in atmospheric dryness could lead to reduced future forest carbon sequestration.