The US Forest Service published an interesting article this week on how trees respond to drought by looking at the mechanics of water movement within the tree and through the leaves.  Different species of trees respond in different ways and at different rates.  You might enjoy reading about this at https://www.srs.fs.usda.gov/compass/2016/04/26/open-or-shut-how-trees-respond-to-drought-at-the-leaf-level/.

Source: Lord of Konrad, Commons Wikimedia
Source: Lord of Konrad, Commons Wikimedia