Hazelnut trees in Willamette valley can get stressed from high temperatures and low humidity, affecting their ability to use water efficiently. Scientists found that measuring how much water the leaves release (stomatal conductance) can show stress early on. They studied hazelnut trees in an orchard and saw that when the air was dry, the trees closed their stomata to save water. This happened even when the trees had enough water in the soil. The researchers also found that measuring stomatal conductance on leaves facing west and north gave the best results. Overall, measuring stomatal conductance was a better way to spot stress in hazelnut trees than measuring water potential. This information can help plan future experiments to study plant stress.