Drought Stress — What's Really Going On

The main strategy trees employ in dealing with our hot dry summers is the ability to close off their stomata. If you remember from biology class, the stomata are the little openings on the underside of the leaves that allow for gas exchange into the tissues of the leaves. CO2 drifts into the stomata where it is critical for photosynthesis. The stomata also happen to be the exit site for water as it leaves the leaf and drives photosynthesis. This is important, as the water evaporating from the leaf cools the leaf much like a swamp cooler cools a house. Unfortunately, in drought conditions, the tree cannot afford to lose extra water. It is estimated that 95 percent of the water absorbed through the roots into the tree is lost to transpiration. Therefore, trees must find a balance between CO2 absorption and water loss as the regulate their stomata.

Another consideration is the strong UV radiation in our high desert environment. UV damage from our intense sunshine can lead to tissue damage. The tree will respond by triggering the production of secondary metabolites like flavonoids and anthocyanins that help combat the free radicals. Some trees are not as well adapted to this such as the Horsechestnut. On these trees, you will find leaf scorch around the margins of the leaf. They experience this every year, however, and don’t seem to skip a beat. 

A horse chestnut with scorched leaves

Other acclimations that you may be observing with the visible eye are the curling of leaves. Certain species will curl their foliage in order to limit direct sunshine to the surface of the leaf. Both leaf surface temperatures and UV exposure are mitigated by this technique. 

Under the ground, trees are able to drink up more water by increasing the water potential in their roots. What does that mean? It means they create more solutes in their roots, thereby attracting more water to their roots like salt wicking moisture out of a brining chicken. Furthermore, drought conditions trigger the production of abscisic acid, which stimulates fine root growth, thereby increasing their root to shoot ratio. In summary, trees can increase water uptake by developing more fine root hairs, and increasing their absorptive factor by adding more solutes. Combining this with minimizing transpirational loss through stomatal closures and adjusting leaf orientation will help your trees during these dog days of summer.