Trees add significant value to a property. They provide financial benefit for resale, aesthetic value, climate control, wildlife habitat, water and soil protection and air pollution mitigation. In a drought, trees have a few adaptive defense mechanisms that help to reduce water loss. However, over extended periods of drought, tree growth and appearance can suffer and trees may die.
Water is lost through leaves, which have pores called stomata that open and close to allow a plant to absorb and then retain the carbon dioxide it needs for photosynthesis. However, when the stomata are open, water vapor is also released. A tree stressed for water will close its stomata to reduce water loss. While this saves water, it reduces the amount of carbon dioxide absorbed, which decreases photosynthesis and the production of energy a plant needs to thrive. Less photosynthesis means fewer root hairs and feeder roots resulting in less water absorption. It also means new leaves may be smaller than normal and shoots can die back. The tree may wilt and lose leaves. Growth in height and girth can decline. All these responses help the tree survive with less water.
Other non-water-saving factors you might notice with a drought-stressed tree are heavier-than-normal seed production and increased susceptibility to insects such as spider mites, aphids or woodborers.
There are things you can do to help trees survive a drought. Remove all lawn under trees from the trunk to the drip line. Mulch this area with compost or other organic material three to four inches deep, leaving an eight to 12 inch gap between the trunk and the mulch. Tree roots grow more densely underneath mulch than underneath grass. The extra roots that are produced make the tree more resilient to drought. This technique can also save you money. A 3,000-square-foot lawn requires approximately 3,700 gallons of water per week during summer, while a medium to large tree requires around 120 gallons per week.
Proper watering will also help your trees. Water to a depth of 12 to 15 inches with each irrigation. This is where tree roots grow. Lawn watering rarely reaches this depth, which is why trees should be irrigated separately from the lawn. Placing soaker hoses in concentric rings under the drip line work well. Multiple emitters on a drip system can also work, if their number is increased as the tree grows.
Take care of your trees. In our desert environment, they are a valuable asset.
JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at email@example.com or 887-2252.