Ground-level ozone pollution affects how trees use water and carbon, but it's not clear how exactly. By studying four tree species exposed to different ozone levels, researchers found that as ozone levels increased, the trees' water-use efficiency decreased. This was because the trees were losing the balance between gaining carbon and losing water. In normal air, the efficiency drop was due to lower photosynthesis and stomatal conductance, while in high ozone levels, it was mainly because of reduced photosynthesis. This study shows that ozone pollution can significantly reduce trees' water-use efficiency by disrupting how they regulate photosynthesis and water loss.