Stress-Related Tree Decline and Death

I receive numerous calls every year about trees losing leaves and dying. There can be many contributing causes. Once a significant decline has begun, the prognosis is not very promising, and there isn’t a simple “spray-on” cure. I’ve compiled the following information to help you take steps to prevent tree problems before they occur and to alleviate problems as soon as they are detected.

Causes of Tree Loss

The causes of tree death are numerous and often interrelated. Problems in the root system are the primary cause of tree death. Remember that 80-90% of a tree’s root system is in the upper 12 inches of soil and can extend outward from the trunk distances of several times the height of the tree! Anything that damages the roots in even the upper 4-8 inches of soil may have serious consequences for tree health.

Trees indicate root loss in several ways. Leaves may begin to dry out, beginning with the leaf margins and tip. Leaves on branches at the top and outer periphery of trees are often the last to receive water and nutrients and therefore the first to show dieback.

Construction can be devastating to trees. Physical injuries to trunks and limbs create wounds where diseases may enter. Trenches dug for utility lines sever roots. Compaction from vehicles and heavy equipment reduces soil oxygen levels and slows water infiltration, resulting in root stress and death.

Driveways, parking lots, sidewalks, and home foundations damage or kill tree roots. Raising the soil level by placing even a few inches of soil over most of the area beneath the branch spread of a tree also reduces oxygen to the existing root system, severely affecting root health.

Building near a tree can change the tree’s environment significantly. Trees that were in a protected semi-shade location may now be in the blazing sun. Areas that were well-drained may now tend to be waterlogged during rainy seasons as roof drains and changes in the surface slope redirect water runoff. Moist areas may now lack moisture during dry spells. A tree’s roots must adjust to these changes if it is to survive, but such adjustments are too slow to address the tree’s immediate needs.

Droughts also take their toll. Trees on shallow and/or sandy soils, in crowded conditions, and under other stresses often lack the soil moisture needed to withstand an extended dry spell. Grasses are strong competitors for soil water. Therefore, trees in pastures and grassy areas can suffer increased drought stress.

Stresses work to compromise a tree’s natural ability to fend off insect and disease problems. Trees under stress are more susceptible to insect and disease attack. For many tree species, root rots, borers, and canker diseases pose little threat until a tree becomes weakened by other stresses. Problems down the line with such pests may have been initiated by past drought, physical damage, and other root stresses.  

When it comes to stress-related problems, not all trees are created equal. For example, post oaks are among the more susceptible to chronic, stress-related decline, while live oaks are much more resilient.

Preventing Tree Loss

Select Adapted Species

Much can be done prior to planting to prevent problems down the line. Don’t waste time and money on a poorly adapted species! If you are planning on planting a tree, select a species that is adapted to your area and specifically to your soil conditions and drainage.

Tree species vary in their growth rate and longevity. In general, a species that grows very rapidly will not be long-lived. Slower-growing species tend to make better, long-lived landscape trees. Proper planting, fertilizing, and watering can “speed them up” during their early years. Every species has its good and bad points. Know them before you make your purchase, and you’ll be much happier in the long run.

Avoid Root Injury

Since most stresses are root-related, you should avoid damaging the root system whenever possible. Trenching severs the roots on the side of the trench away from the trunk. The closer a trench runs to the tree trunk, the more roots will be lost. Route trenches as far away from the tree trunk as possible.

Sometimes people locate their homes very close to or even partially surrounding a beautiful, majestic tree on their lot. Often these trees end up dying within a few years, necessitating expensive removal. Consult a qualified arborist before building around a prized tree! They can advise you on steps to take prior to building and care during and after the construction is completed.

Adding soil to the surface can significantly change the aeration in the root zone, resulting in root loss. Just how much can safely be applied at one time depends on the tree species, how much of the root zone is being covered, and the type of soil or soil mix being added. As a rule of thumb, apply no more than 2 inches of soil over a tree’s root zone. More may be applied to small areas to fill low spots. Removing soil to level the ground also can have a negative effect, as subsurface roots are exposed or cut.

Water Well During Periodic Droughts

Although we receive on average a good supply of annual rainfall, we usually experience an extended dry spell in the summer months. Sandy soil cannot hold much water, and reserves are quickly depleted. Soils with a layer of impervious rock near the surface are likewise unable to hold a large volume of water in the shallow root zone.

During hot summer weather, if it hasn’t rained at least an inch in the past 14 days, consider giving your trees a good, deep soaking to wet the soil about 8” deep. While tree roots reach far beyond the branch spread, focus your watering on the area beneath and a little beyond the branch spread of the tree. On rocky or sloped sites, it may be necessary to water for a while, turn the water off and let it soak in, and then water again. Set out a coffee can or other straight-sided container to catch sprinkler water and determine how long it takes to apply an inch. Repeat applications every 10-14 days in the absence of at least an inch of rain.

Trees planted in the past year should be watered once or twice per week in an area about twice the width of their branch spread around the trunk. Newly planted trees have a very limited root spread and may require watering every 2 days in hot weather to keep the root cylinder moist. A berm of soil around the tree about twice the diameter of the tree’s original container will prevent runoff and allow the water to soak in deeply where the roots are.

The standard advice for a cold is, “Get plenty of rest and drink lots of fluids.” This gives your body the opportunity to “heal itself” by avoiding stresses that weaken it further. The same may be said for stress-plagued trees, as many stress-related problems lack a simple, effective cure. The best remedy is to create a favorable environment for the tree (especially the roots), so it has the best chance of recovering.

If construction or other damage has occurred, there is little you can do except wait and see, except to keep the tree’s remaining root area adequately watered, so the roots do not lack what they need. Avoid applications of salt-based fertilizer during drought stress. In fact, any fertilizer is of minimal value until the tree comes out of the root/drought stresses and can use the nutrients to support growth.

Some Final Thoughts

Until the tree recovers and gets “back on its feet” it is in a very precarious position. Its chances of survival depend on the species affected, the tree’s general health going into the stress period, and your ability to alleviate the stress and prevent further stresses.

Landscape trees are too valuable to lose, especially when something could have been done to prevent it. An ounce of prevention is worth a pound of cure when it comes to avoiding stress and saving these valuable plants in our landscape!