The main driver of tree canopy conductance across different environments is the vapour pressure deficit (VPD). This means that how much water trees release through their leaves is mostly influenced by how dry the air is. Other factors like soil water content and sunlight intensity also play a role, but VPD is the most important. The study looked at data from over 120 sites worldwide and found that VPD consistently had the strongest impact on tree canopy conductance. This information is crucial for predicting how forests will respond to climate change in the future.