Drought impact on trees
The effects of drought are different for individual species of trees. Certain species of trees have adapted to an environment with little rainfall and can handle long periods without rain. These trees develop particular characteristics that make them more suitable to dry environments like deep roots, small thick leaves and a shorter stature. The symptoms of drought stress on trees can vary from the immediately obvious, like wilted or dead leaves to the less obvious symptoms that may not show up until later in the life of the tree.
A tree may decline due to adaptations that the tree is making to the drought circumstances. When a tree focuses on conserving resources (namely water in this case), it cannot use this energy to create more energy, so photosynthesis will slow. The tree also cannot put energy into growth. Even with a return of rain, it takes time for a tree to return to the normal photosynthetic capacity, thus limiting future growth under normal conditions as well.
Northeast Georgia experienced a wetter than normal early Spring of 2019. This period of heavy rainfall does not negate the effects of the current Stage 1 drought. In fact, periods of heavy rainfall can exacerbate later drought conditions.
The Middle Oconee River provides a good visual indicator of the level of drought we are currently in.
In early 2018, the state of Georgia removed hundreds of trees along the “Loop” of Highway 10 in Athens. This was seen by the Department of Transportation as an effort to minimize future maintenance needs and prevent potential failures into the roadway. This removed the trees that could potentially fall into the roadway, but also had a long term effect on the remaining stands. Some of the symptoms of this stress on the remaining forest are beginning to show as we move into the Stage One drought in Northeast Georgia area.
The trees along the loop provide a particularly visible example of how drought can affect already stressed trees. These remaining stands are beginning to succumb to the stress and are starting to die off. The drought is a secondary and in some cases tertiary stress on the trees. The initial removal damaged surrounding trees in many cases. Precautionary care was not taken as an effort to preserve the standing trees, so saw blades and equipment frequently struck remaining trees. These inadvertent strikes likely are not enough to directly kill the trees, but could be an indirect cause of their ultimate failure.
The damage to the trees as well as the removal of certain trees attracts several species of damaging beetles. The beetles and other insects are attracted to ethanol that the tree produces in response to the initial stress. Then, beetles attack the tree and send out pheromones to attract other beetles. If the tree is healthy enough, it will produce sap that can defend against these beetle attacks and prevent it from killing the tree. However, in times of drought, the defenses are limited due to conserving resources. If the tree is focused on survival due to limited water intake, it cannot devote any energy into defense and it will ultimately succumb to the damage caused by insects.
In this case, none of the individual factors will kill the tree. The removal of surrounding trees is a stress, it increases wind load, changes the surrounding environment and could have potentially caused damage from saws and machinery. The attraction of beetles and other insects are stressful on the tree, but ultimately in healthy trees, would not necessarily be enough to cause the demise. The drought alone can be stressful, but with rain in the forecast could be only temporary. But with these combined factors, it is likely that many of the remaining trees will fail.
There are several ways to reduce drought stress to the trees on your property. First and foremost, if we experience long periods of dry conditions, particularly in times following extremely wet conditions, it is important to water your trees. This is one stressor that we can control, particularly when water restrictions are still low. Watering at night can also help minimize pest problems. Unlike turfgrass, which is sometimes watered daily, trees prefer infrequent, deep watering. Irrigating with one inch of water once or twice a week is ideal. And an easy way to determine when you’ve watered with the equivalent of rainfall is to place an empty tuna can in the watering area, when it fills to the top you’ve watered one inch.
Installation of mulch is also an excellent method of retaining water while also providing beneficial nutrients to the soil. Mulch should be installed 3 to 4 inches deep from near (but not touching) the trunk to the edge of the dripline (or as far as is reasonable for the site). Mulch can also serve as a visual reminder of where not to take the lawn mower, and mulch helps prevent soil compaction from machinery, vehicles, or foot traffic. Finally, since tree roots do a poor job of competing with turfgrass for water and nutrients, using mulch below trees reduces the stress caused by turf.
It is also important to make sure that your trees are not impacted by machinery. Make sure to keep lawn mowers and weed eaters far away from the base of the tree. Try to limit foot traffic near the base of the tree. Soil compaction can make it more difficult for trees to reach the limited water supply and does not allow the soil to hold water due to limited pore space.
As you are watering your trees, take note of any insects that you might see. This can be a sign that your tree is stressed and the insects are taking advantage of the weakened state. A trained arborist can help identify the pest and prescribe particular treatment. If noted and treated early in the infestation, often the tree can overcome this stress
Keep in mind that removing a large number of trees can negatively impact the other trees in the area. If you feel that it is necessary to remove the trees on your property, it is essential to consult with an arborist to come up with a plan that will help preserve your urban forest and avoid impact to the trees you hope to keep.