I just saw a talk by Nate McDowell on climate change and the ecosystem. I learned that when the air is dry, leaves close little openings in their outer leaves (stomata, not to be confused with stigmata) to prevent moisture from escaping. During a drought, the stomata stay closed, which causes processes of defense, growth, and the intake of nutrients stagnate. The leaf becomes weak and dehydrated and bark beetles can have their way with the hapless plant.
In this way, drought leads to closed, unhappy plants.