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Why do urban trees live shorter lives than trees growing in natural settings? Because of urban tree stress. As the name implies, urban tree stress affects trees growing in urban environments where conditions are not necessarily favorable to natural tree growth.

Identifying Urban Tree Stress

Urban tree stress can be diagnosed by recognizing a number of different symptoms. It is not as easy to put into a box. As a result, its management almost always requires a multifaceted approach.

Why Are Trees in Urban Landscapes at Risk?

To understand how trees become susceptible to stress, we need to understand how trees grow in their native environments. Like all living things, trees have adapted themselves over millions of years to thrive in certain conditions. Every tree requires a specific soil texture, nutrient complex, stand density, moisture regimen, temperature range, photo period, and associated organisms (soil microorganisms, beneficial insects, etc.) to reach its full potential. Needless to say, when a tree is taken from an environment that is the result of millions of years of adaptation and is planted in, say, a downtown sidewalk box, one or more of its requirements will be compromised. This places stress on the tree, and it may not thrive.

Causes of Urban Tree Stress

Urban tree stress can be caused by many factors and each tree species is able to resist these factors in its own way. The factors include:
  • competing turf grass
  • compacted and nutrient-poor soils
  • too little or too much water
  • temperatures that are too hot or too cold
  • pollution
  • improper planting
  • being too large for the planting site
Age also plays a role in urban stress. Newly planted trees and older, mature individuals are the most likely trees to be negatively affected. Root loss from construction damage, grade changes, or transplanting can be a significant source of stress for all trees. Trees such as ash, ginkgo, Chinese pistache, Bradford pear, and hackberry are well known for their hardiness in urban landscapes because they thrive even in difficult growing situations. But the more negative factors a tree faces, the more difficult it will be for that tree to survive. This is true even for the hardiest of species.

Urban Tree Stress Symptoms and Indicators

Typical symptoms of tree stress include: Other urban tree stress indicators include frost cracks, cankers, decay, and the presence of insect pests, especially borers and bark beetles. An increase in fruit production can also be a signal. Trees that sense their own decline will often increase reproduction as a last-ditch survival effort. Other symptoms that may not be as readily seen are root decline and infection by root-rot diseases such as Armillaria.

Can Urban Tree Stress Be Managed?

When it comes to a managing urban tree stress, an ounce of prevention is worth a ton of cure. Avoiding sources of stress is easier and cheaper than trying to remediate them after a tree has begun to show symptoms of decline. Making sure a tree has the right soil, light, and space to grow are the keys to long-term vitality. Urban tree stress for established trees can be avoided with irrigation, pruning, and adding mulch around the base. Research has shown that replacing turf under trees with a few inches of organic mulch significantly improves tree roots by increasing soil aeration and nutrients. Most importantly, by removing the grass the tree does not have to compete for the same resources.

Managing Tree Stress

For trees already suffering and showing symptoms of urban tree stress, action must be taken if the trees are to survive. A tree showing any of the telltale signs is usually entering advanced stages of decline. It will continue in a downward spiral if the source of the stress is not addressed. For this reason, it is extremely important that the cause of the stress is properly identified before a management plan is implemented.

Identifying Underlying Issues

If a tree is treated for boring insects, but the underlying issue was drought, the symptom is only temporarily fixed. The borers will likely return when the insecticide wears off. A full management plan for this tree would be to treat the boring insects to alleviate the immediate stress and then to establish a watering schedule so the tree receives adequate moisture during periods of low rainfall. Once the source of the stress is identified, a plan can be implemented to address it. Trees in compacted soils can benefit from the Air-Spade to reduce compaction, incorporate organic matter, and improve aeration. Trees suffering from nutrient deficiencies can be supplemented either through soil application or tree injection, depending on the nutrient necessary. Research has shown that tree growth regulators (TGRs) can improve injured roots on trees by redirecting energy from canopy growth into other structures, including fibrous roots. A TGR should not be seen as a stand-alone treatment for a tree in decline, but when it is combined with other practices, such as air tools, mulching, and proper irrigation, the results promising.

Back to Nature

Remember that the tree spent millions of years adapting to a specific environment, so the more closely those specific conditions can be recreated for an urban tree, the better chance it has to thrive. A willow is adapted to wetter soils, so its planting site must be kept moist; a pin oak is adapted to acidic soils, so its site must not be too alkaline, and so on. The better the tree’s native habitat is understood, the better an arborist can advise on the urban site in which it will perform best. If your trees are experiencing urban tree stress, contact a certified arborist at Growing Earth Tree Care today.