By Shawn Bernick and Brandon Gallagher Watson
This mature bur oak is being treated from urban tree stress using an Air-Spade and TGRs.
All photos provided by Rainbow Treecare Scientific Advancements
A: Urban tree stress is a widely accepted term that encompasses the factors that cause urban trees to live significantly shorter lives than trees growing in natural settings. As the name implies, urban tree stress affects trees growing in urban environments where conditions are not necessarily conducive to proper tree growth. Urban tree stress can be diagnosed by a number of different symptoms caused by a number of different factors. Unlike a vascular wilt disease where one can pinpoint a susceptible host, a specific pathogen, a suitable environment for infection, and, often, a single treatment option, urban tree stress is difficult to put into a box, and thus its management is almost always a multifaceted approach.
Why are trees in urban landscapes stressed?
To understand the factors that lead to a tree suffering from urban tree stress we must first understand how trees grow in their native environments. Trees, like all living things, have adapted themselves over millions of years to thrive in certain conditions. Every tree requires a specific soil texture, nutrient complex, stand density, moisture regime, temperature range, photo period, and associate organisms (soil microorganisms, beneficial insects, etc.) to reach its full genetic potential. When a tree is taken from an environment to which it has adapted over millions of years and is placed in, say, a downtown sidewalk box, one or more of these requirements will inevitably be compromised. This places stress on the tree and, subsequently, the tree will not be able to thrive. A second key to understanding urban tree stress is the role of fungal diseases and insects such as bark beetles and borers in a native ecosystem. These organisms play a vital role in maintaining the health of a tree
Factors that contribute to Urban Tree Stress
Soil compaction/poor aeration
Improper planting depth
Lack of root space
Improper pruningpopulation by “thinning the herd” of weakened individuals and they often cannot attack a healthy tree. Many tree pests that are minor issues in a native setting become major issues for city trees as urban tree stress makes them more susceptible to pests.
What causes urban tree stress?
Urban tree stress can be caused by one or more contributing factors, and each species of tree varies in its inherent ability to resist these factors. For example, trees such as ash, ginkgo, Chinese Pistache, Bradford pear, and hackberry are well-known for their hardiness in urban landscapes because they are still able to thrive in difficult growing situations. The factors that create these difficult growing situations include competition with turfgrass; compact, nutrient-poor soils; under- and over-watering; temperatures too hot or too cold; pollution; improper planting; and eventually growing too large for the planting site. Age also plays a contributing role in the susceptibility to stress: newly planted trees as well as older, mature trees are most likely to be negatively affected. Root loss from construction damage, grade changes, or transplanting can be a significant source of stress for trees of all sizes and ages. The more of these factors a tree is faced with, the more difficult it will be for that tree to survive, even for the hardiest of species.
Typical Symptoms of Urban Tree Stress
Increased fruit production
Susceptibility to infectious diseases
Susceptibility to insects, especially borers and bark beetles
Declining root systemsSymptoms a tree may express when suffering from urban tree stress will differ by species, age of the tree, and source of the stress. Typical symptoms of tree stress include stunted growth, epicormic sprouts, scorched leaves, and chlorosis. Other visual indicators may 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 that a tree is under stress, as trees sensing they may be on the decline will often increase their reproductive structures as a final survival effort before they die. Other symptoms which 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 management plan for confronting urban tree stress, an ounce of prevention is worth a metric ton of cure. Avoiding these sources of stress is significantly easier and cheaper than trying to remediate them after a tree has begun to show symptoms of decline. Properly establishing a tree in its environment is paramount to long-term vitality by ensuring the tree has the right soil, light and space to grow. Urban tree stress can be avoided on established trees with proper irrigation, pruning and the addition of mulch around the base. Research has shown that replacing turf under trees with 2 to 4 inches of organic mulch significantly improves tree roots by increasing soil aeration, nutrient availability and removing the grass that competes for the same resources as the tree. Going back to the concept that trees spent millions of years adapting to a specific environment, the closer you can recreate these conditions for an urban tree, the better chance it has to thrive. If a tree, such as a willow, was adapted to wetter soils, its planting site must be kept moist. If a tree, such as a pin oak, was adapted to acidic soils, the site must not be too alkaline, and so on.
Small planter boxes, surrounding pavement, and competing vegetation are all sources of stress for these pear trees.For trees already suffering and showing symptoms of urban tree stress, action must be taken if the tree is to survive. A tree showing any of the telltale signs indicated above is usually entering advanced stages of decline, and 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 and subsequent decline is properly identified before a management plan is implemented. If a tree is treated for boring insects but the underlying issue was drought, we may temporarily remediate the symptom but have not provided a long-term, sustainable solution for the tree and the borers will likely return when the insecticide begins to wear off. A full management plan for this tree should include treating the boring insects to alleviate the immediate stress, then establishing a supplemental irrigation regime to ensure the tree is getting 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 use of tools like an 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. 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. TGRs should not be viewed as a stand-alone treatment for a tree declining due to significant root damage; but when combined with other practices such as aeration, mulching, and proper irrigation, the results can be favorable. Insects and diseases are often acute problems that can be addressed using available plant health care products. However, as stated before, if the pest problem is a secondary issue, the primary cause must also be addressed. Of course, not all urban tree stress issues can be corrected by an arborist. If a tree is only borderline hardy for an area it may survive a few mild winters but no intervention by an arborist can save it from a severe cold snap.
From a business perspective, there are opportunities with air tools, plant health care products, and TGRs for treating stress on trees. Being able to offer a combination of these services will better position your company as an urban tree stress treatment resource. Early diagnosis and, better yet, prevention of urban tree stress offers a far greater success rate than trying to save a tree already in decline. Educating your clients on the conditions, symptoms and management options associated with urban tree stress is key to maintaining a healthy urban forest.
Shawn Bernick is director of research and technical support at Rainbow Treecare Scientific Advancements, Minnetonka, Minn.
Brandon Gallagher Watson is communication director at Rainbow Treecare Scientific Advancements, and is an ISA Certified Arborist (#MN-4086A).