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2013-08-15 / General Stories

Drought and Environmental Effects on Trees


Lee County Extension Agent Keeton Ehrig Lee County Extension Agent Keeton Ehrig Does it appear as though your trees are finally succumbing to a series of environmental and past life history stress factors? This is a very common occurrence that coordinates with this time of year. Daniel Lewis, Staff Forester with the Texas Forestry Service, offers this advice about stress factors.

Some of the stress factors may include:

* Drought - repeated drought cycles and sporadic rain events over the last decade. Drought causes many problems including loss of fine feeder roots that pull in moisture and nutrients, dieback of the crown, and increased susceptibility to diseases and insect attack.

* Compaction - compacted soil damages and prevents root development (post oaks are particularly intolerant).

* Root rots - Armillaria and Ganoderma, among other root rots, cause loss of roots and may cause the tree to fall over even when the tree is still carrying green leaves. “Phytophthera cactorum” is also a very common root rot that causes root rot, twig dieback, and bleeding cankers on the trunks.

* Grade changes - Adding or removing soil from around a tree changes that tree’s ability to undergo gas exchange of its roots and causes root death.

* Trunk and root damage (construction, trenching, hitting the tree or exposed roots with string trimmers and mowers, etc.) - this type of damage opens the tree for infection from air or soil borne pathogens and causes the death of plant tissue.

* Insect defoliation - repeated defoliation by canker worms, leaf rollers, grasshoppers, katydids, and many more cause the tree to have to put on more leaves which can overly stress the resources of an already stressed tree.

* Oak wilt - while oak wilt is not commonly seen in post oaks, these trees are susceptible. Oak wilt is a vascular fungal pathogen that can spread through open wounds and root grafts between trees.

* Hypoxylon canker - Hypoxylon canker is an opportunistic secondary fungal pathogen that may remain dormant for many years within the bark of healthy trees until such time as environmental stressors, or other factors, reduce a tree’s water content below a certain threshold. At this point, this fungus will begin to penetrate into the xylem of the tree, finishing it off. This pathogen can usually be seen as strips or patches of bark that slough off the tree revealing fungal material that may be greenish, brownish, black or silvery gray. These trees rapidly decay once this fungus establishes and the tree may fail structurally very quickly. More information is available at: http:// plantclinic.tamu.edu/helpfulfactsheets/. Scroll down and click on the Hypoxylon Canker pages.

Lewis said, “All of these conditions are naturally occurring responses to tree stress except for oak wilt. In most instances, within a yard setting, tree loss can be linked to the activities of man on the landscape; such as trunk damage, over-watering to keep grass alive, inadequate or no watering, grade changes and construction, weed and feed (and other herbicides and chemicals, compaction, etc.)”

“The best course of action is to try to identify and reduce as many stressors as possible from the tree’s environment. You obviously cannot control drought, but many of these factors are within your control. Improper watering within a yard setting is likely one of the most common causes of tree stress. In most yard situations within the Post Oak Savannah, homeowners try to establish a sod forming grass that has high water demands. In order to keep the grass healthy, it is watered frequently and shallowly. Since post oaks in particular are upland species, accustomed to low rainfall and poor soil fertility, the trees are constantly in unfavorable conditions within a yard setting,” Lewis continued.

Trees, in general, need a thorough and widespread soaking of the entire root system (which may extend two to three times the crown spread) about once a month. The roots need to dry out some between soakings or root rots may develop in the anaerobic conditions caused by too much water.

Keeton Ehrig is the Extension Agent, Agriculture and Natural Resources, in Lee County. He can be reached at (979)542-2753 or visit the Lee County office at 310 S. Grimes Street in Giddings.

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