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Early fall color is a symptom of drought. Photosynthesis requires a watery matrix in which to take place, but it also requires carbon dioxide. CO2 enters the leaves through the stomates, which are little openings in the leaf that allow for gas exchange.This is where a plant is often in a quandary — it needs the stomates open to receive CO2, but if the stomates are open too long on a hot day, the plant loses too much water and becomes drought stressed. If the stomates stay closed, water stays in but the reduced CO2 input results in photorespiration, a much less efficient form of food production for the plant. Photorespiration can reduce a plant's energy production by up to 25 percent, so the plants are constantly trying to avoid this. Some desert plants have evolved a means to open their stomates at night, store the CO2, and utilize it for photosynthesis when the sun is shining the next day. The vast majority of plants, however, cannot photosynthesize this way and must open and close their stomates throughout the day, enough to let in CO2 but minimize the loss of water.Abscisic acid is a hormone produced throughout the plant that can be utilized to open and close stomates responsively as the conditions change. Commonly known as "the stress hormone," abscisic acid is one of the plant's first lines of defense against drought. www.arborage.com A drought-stressed ash. Trees respond a few different ways to drought. Mostly we think about wilting as a drought symptom, but usually by the time a plant is showing wilt it has been stressed for a while. Preclinical symptoms (those symptoms that occur before we can see them) include an increase in abscisic acid, and, in certain trees, a decrease in defense compounds. In order to compensate for the reduced photosynthesis, trees will often liquidate sugars stored in the roots. By doing this, they are run the risk of attracting opportunistic pathogens such as Armillaria.Trees also become more susceptible to secondary pests, such as boring insects and bark beetles.A healthy tree can fight off these invaders by pushing them out with sap and producing defense compounds to make them less tasty. When water supplies are low, the trees are unable to defend themselves and they become susceptible. In the natural forest, these insects play a key role in weeding out the weakened trees and strengthening the population. In an urban forest, an arborist may be called upon to help bolster the tree's defenses against these pests. Fortunately,drought is a tree stress that has several management options in the toolbox.The obvious remedy for lack of water is to simply add water.As an arborist, you will not likely be around to water your client's Arbor Age / July/August 2013 13