The Effects of Drought on Trees
Lisa Olson, Master Gardener
I have heard it said tongue in cheek that Minnesota’s average weather is the midpoint between two ridiculous extremes, because when we talk about averages, it can be misleading. Sometimes it happens that one huge deluge of rain will offset a long dry spell, at least on paper, where the average rainfall for the month or year may appear as normal in the record books. But those long drought periods are not so easily erased in the lives of the trees.
According to the US Forest Service, healthy trees mean…
Healthy people: 100 trees remove 53 tons of carbon dioxide and 430 pounds of other air pollutants per year.
Healthy communities: Tree-filled neighborhoods lower levels of domestic violence and are safer and more sociable.
Healthy environment: One hundred mature trees catch about 139,000 gallons of rainwater per year.
Homeowner savings: Strategically placed trees save up to 56 percent on annual air-conditioning costs. Evergreens that block winter winds can save 3 percent on heating.
Better business: Consumers shop more frequently and longer in tree-lined commercial areas and are willing to spend more.
Higher property values: Each large front yard tree adds to a home’s sale price.
One thing that helps to keep trees healthy is a consistent, sufficient amount of rainfall. Extended dry periods, even if followed by enough rain to offset the deficit in the record books, will still have a detrimental effect on trees. During an extended dry spell, young trees will be the first to show the effects, while older, mature trees with large root systems may take three to five years to show outward signs of drought effects and at that point it may be too late for them to recover.
Additionally, droughts are getting worse. Due to global warming, the increase in average temperature amplifies the effects of drought. Hot and dry is much worse than cool and dry. Higher temperatures mean evaporation increases which means soil dries out faster and trees transpire more. As a result, trees have evolved strategies to cope with drought conditions.
One strategy a tree may employ is to close its stomata, or leaf pores, to minimize transpiration. Transpiration is the evaporation of water in plant leaves. A large oak tree can give transpire a whopping 40,000 gallons of water per year. 10% of the moisture in the atmosphere comes from transpiration. With the pores closed, besides preventing transpiration, the tree is unable to take in carbon dioxide which is critical to the process of photosynthesis. Without the ability to make food, it becomes dependent on the energy it has stored to keep itself alive. Symptoms can include scorched, wilted, or rolled leaves, or early fall color. A tree may drop some or all of its leaves to further prevent transpiration. As a tree uses up its reserve energy, growth will stop and twigs and branches begin to die back. Symptoms of drought stressed trees will occur from the top of the tree down, and from the outside toward the center. In other words, the extremities are the first areas to show damage.
Damage caused by drought may not show up until the winter when a normally resilient tree becomes affected by sunscald, frost-splitting, or its weakened branches break from heavy snow. Another secondary effect of drought is that a weakened tree is more susceptible to disease and insect damage. Widespread areas of damage from emerald ash borer or diseases like oak wilt or other diseases are more prevalent when trees are stressed.
One area of damage on a tree cannot be easily observed for damage. Most tree roots are in the top six to 24 inches of soil making them extremely vulnerable to dry periods.
This leads us to what we can do to minimize the effects of drought on our trees.
Water using good practices. Slow, deep watering is critical. Water every five to seven days or more often if the temperature soars above 95 degrees. Place a shallow can where you are sprinkling to be sure at least two inches of water are slowly applied. Be sure to water the entire root area. Roots spread at least two to three times farther than the dripline of the tree. Another factor regarding watering is that any plants growing above the tree roots are also competing for water, so increase the amount of water if other plants are present and be sure to remove any weeds. A couple inches of mulch, but not touching the base of the tree, is a good alternative to grass under a tree. The best way to see if water is getting to the roots is to dig a hole approximately 10 inches deep to see if a sufficient amount of water is getting down to that depth from your watering. An area of heavy clay soil may seem saturated if the water is puddling, but in fact the water may just be sitting on the surface as it takes a long time for water to infiltrate clay. Young, newly planted trees will need more frequent watering until their roots are developed. The Minnesota Department of Natural Resources put together this video How to Water a Tree for tips on watering newly planted trees. While it may be tempting to water a little bit every day instead of deep watering once a week, shallow watering will cause the roots all to move closer to the surface where they are more likely to be affected by the extreme weather conditions and unable to grow deeper where more water may be available. For best results, water in the morning and consider using a soaker hose.
Avoid pruning stressed trees. Healing the wounds uses up the energy the tree is trying to conserve just to survive.
If you are planting new trees, choose your plant wisely. Always consider the site where the tree will be living and select a plant that can tolerate the conditions.
If you are interested in learning more about the effects of drought on trees, the links below include helpful information!
Photo Credit: National Drought Mitigation Center (1), University of Minnesota Extension (2), Lisa Olson (3), University of Florida (4)