Brandon M. Gallagher Watson, Rainbow Treecare Scientific Advancements

Urban Tree Stress

Facts that contribute to Urban Tree Stress

  • Drought
  • Over watering
  • Soil compaction/poor aeration
  • Freezing/temp fluctuations
  • Defoliation
  • Nutrient deficiencies
  • Chemical injury
  • Mechanical damage
  • Transplant shock
  • Improper planting depth
  • Lack of root space
  • Competing vegetation

The Stress of City Living

Urban tree stress is a widely accepted term that covers the factors that lead to urban trees typically living 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 brought on by a number of different causal factors. Unlike, for instance, 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 not as easy to put into a box and thus its management almost always involves 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 associated organisms (soil micro-organisms, beneficial insects, etc) to reach their full genetic potential. Needless to say, when a tree is taken from an environment that took millions of years to adapt to and planted in, say, a downtown sidewalk box one or more of these requirements will inevitably be compromised. This places stress on the tree and, subsequently, it 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 tree population by ‘thinning the herd’ of weakened individuals and often cannot attack a healthy tree. Many tree pests that are minor issues in a native setting become major pests for city trees as urban tree stress makes them susceptible.

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. 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 turf grass, compacted and nutrient-poor soils, under and over watering, extreme temperatures, pollution, improper planting, and eventually being too large for the planting site. Age also plays a contributing role in the susceptibility to stress as newly planted trees and older, mature individuals are the most likely trees to be negatively affected. Root loss from either 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.

Urban Tree Stress Symptoms

Typical Symptoms of Urban Tree Stress

  • Sprouts
  • Scorch
  • Stunted growth
  • Increased fruit production
  • Frost cracks
  • Cankers
  • Susceptibility to infectious diseases
  • Susceptibility to insects, especially borers and bark beetles
  • Chlorosis
  • Declining root systems

Symptoms a tree may express when suffering from urban tree stress will differ by species, the 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 presence of insect pests, especially borers and bark beetles. An increase in fruit production can also be a signal a tree is under stress as trees sensing they may be on the decline will often increase their reproductive structures as a last ditch 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 sources of stress is significantly easier and cheaper than trying to remediate them after a tree has begun to show symptoms of decline. Certainly getting a tree properly established 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 replacing turf under trees with a few inches of organic mulch significantly improves tree roots by increasing soil aeration, nutrient availability, and, most importantly, removing the grass that is a fierce competitor for the same resources as the tree. The closer you can get to recreating the natural environment of a particular tree, the better chance that tree has to thrive in an urban environment. If the tree was adapted to wetter soils, such as a willow, its planting site must be kept moist. If a tree was adapted to acidic soils, such as a pin oak, the site must not be too alkaline, and so on. The better understood the tree’s native habitat the better an arborist can advise the urban site it will perform best in.

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 tell-tale 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 trees 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 have been looking to treat 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.

Urban Tree Stress treatment

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 tree growth regulators (TGR) can improve injured roots on trees by redirecting energy from canopy growth into other structures, including fibrous roots. TGRs should not be seen as a stand-alone treatment for a tree declining due to significant root damage but when combined with other practices such as air tools, mulching, and proper irrigation the results can be quite favorable. Insect and disease pressures are often acute problems and can be addressed using available plant health care products, yet as stated before, if the pest problem was a secondary issue the primary cause must also be addressed. Of course, not all urban tree stress issues can be helped 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 tree growth regulators and being able to offer a combination of these services will better position your company as a resource for treating stress on trees. Early diagnosis and, better yet, prevention of urban tree stress offers a far greater success rate of thriving trees compared to trying to turn around a tree in decline.  Educating your clients on the conditions, symptoms, and management options associated with urban tree stress is a key to maintaining a healthy urban forest.

Brandon Gallagher Watson

Brandon Gallagher Watson is an ISA Certified Arborist and the Creative Director of Rainbow Tree Company. Rainbow consists of a full-service tree care company, a lawn care and pest control division, and Rainbow Treecare Scientific Advancements, a research & development branch dedicated to developing tools and protocols for science-based tree care.