Some effects of drought on trees
Some effects of drought on trees are apparent, and other effects are unseen, though just as damaging. Here are three major damages that drought can do.
1. Dead feeder roots
The fine feeder roots that collect moisture and nutrients for most trees are located within the first 15-18 inches of topsoil. If that soil dries out for a long time, the feeder roots will die. Without feeder roots, the tree cannot effectively absorb nutrients and water, even when it rains again.
2. Reduced photosynthesis
Some trees apparently drop leaves as a drought-survival mechanism. Without leaves (or with fewer leaves,) they lose less moisture through evaporation. However, photosynthesis is reduced when leaves fall prematurely, so there are risks as well as benefits to this drought response.
Photosynthesis produces glucose. The cells need glucose as they manufacture a wide range of organic compounds that sustain and regulate every aspect of the tree's life. (Many of these chemicals can be grouped under the term "secondary metabolites.")
Some of the most obvious effects of reduced photosynthesis are a slower growth rate and reduced seed production. In the long term, the tree's lifespan may be shortened.
3. Reduced resistance
In addition, trees are susceptible to invasion by pests or disease, when they lack the normal flow of water, nutrients and secondary metabolites. For example, a tree may be unable to compartmentalize a disease as effectively as normal. Though the disease may not become apparent immediately -- such as root rot -- it can kill the tree over the next decade or two.
Much information about drought damage to trees is available on the internet. Here are some readable and informative articles for non-scientists like me.
The Long-term Effects of Drought
The Effects of Drought on California Oaks
Long-term Drought Effects on Trees and Shrubs
How Drought Affects Trees and Shrubs
Kentucky Forests Respond to the Drought
The Damaging Effects of Drought (pdf)