Tree Decline

Drought stress has a serious negative impact on trees

By: Sandy Feather ©2013
Penn State Extension


I have seen a number of small trees and shrubs severely declining, and even dying this year. Many are newly planted, but some were well established. Despite our plentiful – sometimes too plentiful – rain this summer, woody plants carry the insults of previous growing seasons with them for life, particularly the dry years. The damage caused by drought often does not show up until the following growing season. Some of the declining and dead plants were well established and had lived through more than one drought.

 

To their owners, they seemed to die overnight, but in reality they had been slowly declining over time. Such symptoms can be subtle and may not be noticeable unless you know what you are looking for. Some died from insect or disease problems that they were susceptible to because of drought stress; others because of difficult site conditions made worse by drought.

And although we did have more than enough rain in July, much of it came as driving rain that does not soak into the ground as well as more gentle, prolonged rains do. And the dense foliage of mature trees often does not let the rain reach the ground. Amazingly, I am seeing drought-stressed plants.

early fall color
Early fall color is a trouble sign

If you can remember back to last year’s weather, it got very dry in mid-May and stayed that way until July. We had above average rain in July, only to go back into hot, dry weather from August into September. Fall was pretty average until we got to November – last November was the third driest since the weather service began keeping records. By November, hoses and irrigation systems are drained for winter, and no one is thinking about watering. Except for those thirsty trees and shrubs, that is.


Drought is most likely to have severe impacts on trees and shrubs that are:

  • newly planted, especially container-grown plants. Those in containers are often grown in an artificial soil mix that dries out much faster than the surrounding soil.
  • planted on stressful sites – those with compacted soil, heavy clay, restricted root zones, or reflected heat from buildings and/or pavement.
  • planted and/or mulched too deeply. The root flare of trees should be visible at ground level, and crowns of shrubs should not be buried by mulch.
  • suffering from mechanical damage. Wounds to the trunk from lawn mowers, weed whackers, deer, rodent activity – anything that damages the bark can impact a tree’s vascular system.
  • small species such as flowering dogwood, redbud or crabapple. They have smaller root systems than large shade trees, and therefore less drought tolerance.
  • those with typically shallow root systems, such as conifers, maples and honeylocusts. Their shallow root systems are less drought-tolerant than species with deeper root systems.
  • those with girdling roots.

girdling root
Girdling roots grow across the side of a tree
trunk causing constriction

All of a plant’s physiological functions depend on water – food production and storage, manufacture of defensive compounds, and growth and reproduction. Drought itself can severely damage plants, but it also makes them more susceptible to insect and disease problems.


Impact of Drought on Trees

Drought has several impacts on plants. One is that their roots develop a waxy layer to help prevent moisture loss. But when the rain returns, that works against plants since the waxy layer limits their ability to take up water. Furthermore, the fine root hairs that are most responsible for absorbing water and nutrients are the most susceptible to damage and death from drought because they tend to grow in the upper layers of soil. This double whammy to the root system, especially for plants growing on difficult sites, can be all it takes to push them into decline. When such a drought is followed by a period of wet weather, plants with compromised root systems may decline steeply because the saturated soil does not allow enough oxygen to reach the roots.

Drought also limits photosynthesis, the process plants use to manufacture carbohydrates from carbon dioxide and water in the presence of sunlight thanks to the chlorophyll molecule. When plants suffer from drought stress, the pores in their leaves (stomata) stay closed to conserve moisture, which reduces the amount of carbon dioxide they can absorb. When both water and carbon dioxide are limited, so is photosynthesis. Woody plants that cannot build up life-sustaining carbohydrate reserves are in trouble. That is why trees and shrubs that are defoliated by insects for several years in a row often die – they use up their carbohydrate reserves when they have to send out new leaves to replace the damaged ones. Drought often causes trees and shrubs to drop their leaves prematurely, which also limits photosynthesis.


Late Frost

We also had several frosts after plants started leafing out. If the tender new leaves were killed, affected deciduous trees put out a second flush of growth. But that second flush of growth forced them to deplete their carbohydrate reserves more than would have been necessary otherwise. That is enough to send already-stressed plants over the edge.

Frost on a Laceleaf Japanese Maple is brutal!
Frost on a laceleaf Japanese maple


T.L.C.

If you have trees and shrubs that appear stressed, a little TLC can help. If they have more than two or three inches of mulch over their root systems, pull it back. Deep mulch becomes water-repellant once it dries out thoroughly and can prevent rain from reaching the plant’s roots. This also allows very saturated soil to air out more efficiently, in situations where drainage is not good. If the soil is dry, remove the nozzle from your hose and allow water to trickle slowly around the base of the plant, moving it every twenty to thirty minutes, until the plant is watered deeply around the entire circumference of its root system. Make sure trees and shrubs do not go into winter under drought stress.

Avoid the temptation to fertilize stressed plants until they recover a bit, especially if insect or disease problems are involved. Fertilization can make some pest problems worse. Also, if their roots systems have been compromised, they might not be able to keep up with the new growth forced by fertilization.

Dead branches and stems should be pruned out over the dormant season when you are less likely to spread disease-causing organisms. As added insurance, disinfest pruning tools between cuts by dipping them into 70 percent alcohol and allowing them to air dry.

If you have very large trees that need attention, it is wise to hire a certified arborist for help diagnosing any problems and developing a management plan to get them healthy and keep them that way.

MORE

Starting Spruce trees from seeds

Splotches on tree trunks

Girdling tree roots

      


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