“Drought is the primary contributor to tree kill, but it may not be exactly the way you might be thinking,” Taylor said. “You may find this hard to believe, but relatively few trees likely died directly from dehydration in 2011. Water, particularly soil moisture, is critical for all a tree’s physiological processes. Trees require water to make and transport food, take in and release carbon dioxide, conduct biochemical reactions, build tissue and more.”
According to Taylor, trees can get heat stroke.
“Much of the recent tree deaths and general decline might also be attributed to the extreme and prolonged heat of 2011,” he said. “Extreme temperatures, not only during the day but also in the early evenings and night, have negative impacts to tree physiological processes.”
Taylor said although it is the lack of water that’s at the root of tree death by heat stroke, there’s more to it than the tree being thirsty. As do humans, trees sweat to cool themselves off. Only with trees, the process is called “transpiration,” and it’s water evaporating primarily from leaves that dissipates heat.
Inadequate soil moisture coupled with hot air temperatures means a tree’s ability to transpire is limited.
“As a result, the cells in leaves and small branches can ‘cook’ to death,” Taylor said.
This “cooking” result in cell and protein breakdown, the generation and/or buildup of toxins, lesions and eventually death of the tree, he said.
As for tree deaths from macro fungi on hardwoods such as hypoxylon canker, it’s stress brought on by drought and heat that creates opportunity for the disease, not the direct effects of moisture shortage. Hypoxylon is a white-rot fungus that is usually considered a weak pathogen - not aggressive enough to take over healthy trees. It is only of consequence when the trees are under severe stress and wood moisture drops significantly.
“Often, the first symptom that may be observed is the dying back or thinning of the crown,” Taylor said.
If the drought continues or intensifies, homeowners can reduce tree stress by paying attention to over-crowding, proper pruning, minimizing damage to the stem and roots, and proper watering. For existing landscapes, proper watering during a drought is the best way to reduce water stress.
A rule of thumb is to begin supplemental watering if significant rainfall has not occurred in the past seven to 10 days. You can use a soaker hose or by trickle or drip irrigation, and water just outside the drip line of the tree’s crown. (The drip line is the area on the ground directly under the farthest-reaching branches.)
It is not necessary to encircle the entire tree, especially if a very large tree. A good watering on half or one quarter of the root system can be very beneficial. Do not concentrate the water at the base of the tree. Doing so can lead to root diseases.
The water should soak into the soil without run off. If the water runs off or puddles, reduce the flow rate. Water until the moisture has soaked in to the soil to a depth of at least 8 to 10 inches.
The best time to water is during the early evening and at night. This is the time when trees normally catch up and replenish the water they loose during daytime activities like photosynthesis.
During the drought times, plan on watering trees once a week.