Abiotic Urban Tree Stressors

Stressful conditions can reduce a tree's ability to produce energy as well as force the tree to divert energy from growth, flowering, and fruiting to defense. Reducing energy production results in less storage of energy (starch in woody plants) and can exhaust a tree's defenses. Some stressful conditions are acute and may not last very long, like a seasonal drought, while others are more chronic, such as compacted soil conditions. It is often when trees are under stress from abiotic (non-living) causes and energy and defenses become limited that biotic stressors like insect pests and disease pathogens are more easily able to establish, exacerbating tree decline and death.

The struggles trees face when growing in many urban or suburban settings can mean the difference between healthy trees that provide shade, boost property values, and improve air quality, or expensive removals. When we think about tree stressors, we often think about insects and diseases that can certainly impact trees, but there are numerous abiotic (non-living) stressors that cause the demise of thousands of urban trees. Those abiotic tree stressors can be grouped into several categories, such as environmental factors, site factors, and human activities.

Environmental Stressors

Environmental stresses limit a tree's ability to acquire the resources it needs from its growing space or environment. The first environmental stress that comes to most minds is drought. Water stress may be caused by a lack of rain, or it may be caused by inadequate soil space from which to draw water.  Drought is a primary contributing factor to tree decline. Extended drought can lead to a cascade of stress responses in a tree, starting with the loss of fine absorbing roots. To manage the stress of inadequate water, a tree will shift from a growth mode to a survival mode, similar to how a tree enters a dormant mode in winter. Leaves, where most water is lost from a tree, will be shed, typically starting from the outermost branches. As drought continues, the tree will continue to move towards a more dormant state in an effort to survive until adequate water is available again. We may see signs or symptoms of a pest or disease appear in a drought-stressed tree as it has less energy available to defend itself. While we may see the pest or disease as the cause of the decline of tree health, environmental stressors like insufficient water are often the primary cause.  Even with adequate soil space and precipitation, drought stress can occur from root injuries that short-circuit the tree’s ability to take up water and nutrients that may be available.

Another environmental stress can be a late spring frost or freeze that damages newly emerging leaves. Water expands as it freezes in plant tissues, bursting cell walls. When the tissue thaws again, the ruptured cells die and darken soon after. Tender tissues are prone to such injury in late spring frosts. If leaves are damaged in the spring, the tree will need to dip into reserve (or stored) energy to create new leaves needed for photosynthesis that growing season.

Extreme heat can be a major stress on some species of trees, causing moisture loss, leaf scorch, and premature leaf drop. In many cases, heat stress includes a component of drought stress, as the tree moves more water via evapotranspiration to combat the effects of heat, often exceeding the available water in the ground. This stressor can be exacerbated by site conditions found in parking lots or downtown streetscapes with limited soil space and lots of reflected heat.

Snow and ice loading during the winter can cause major limb breakage, which results in a reduction in a tree's canopy. That lost leafy canopy reduces the tree's ability to produce energy for itself, leading to stress on the tree. Ice can increase the weight of a branch by 30 times or greater, depending on the thickness of the ice, making it more susceptible to breakage.  High winds can dry out evergreens during the winter when frozen ground can reduce the availability of water, causing increased moisture loss.  High winds also lead to leaf tater (ripped and damaged leaves) on some deciduous trees, and limb breakage.

Site Related Stressors

Soils

Soils are the growing medium for trees and their roots, providing water, oxygen, essential nutrients, support or anchorage, and space to explore as the tree grows larger. Urban and suburban soils are usually disturbed, degraded, and contaminated. They may have been graded, excavated, and placed back with normal layers inverted. The organic layer and topsoil may have been removed during development, leaving poorly aerated and infertile subsoil at the surface for root growth. Urban and suburban soils tend to be highly compacted by heavy construction equipment and contaminated with discarded building materials such as concrete, bricks, or drywall buried on site. Roots tend to struggle to move through compacted soils and will resort to growing closer to the surface.  Contaminates like concrete and drywall affect soil chemistry, elevating soil pH beyond what some tree species can tolerate.

Soils that are compacted or consist of large amounts of clay tend not to drain very well and lack pore space for air. Trees growing in these oxygen-deprived environments resort to anaerobic respiration, which is much less efficient and produces alcohols, aldehydes, and other compounds that build up to toxic levels. Some trees will slowly decline and die, while others are more tolerant of low oxygen levels. These species tend to out-survive competitors in flood-prone soils and compacted urban soils. Other trees will adapt to those soil conditions by growing most of their roots at the surface, where they can obtain water and oxygen.  Urban trees often cause sidewalk lifting because the soils are so compacted (for sidewalk construction) and the best place for them to grow is in the gravel subbase below the concrete sidewalks.

De-icing Salts

During winter months, large amounts of de-icing salts are put down on roadways and sidewalks to prevent auto accidents and pedestrian slip and fall accidents. Trees are impacted by salts in several ways. First, it impacts soils and roots, damaging soil chemistry and potentially reversing the normal direction of osmosis, causing roots to dehydrate. Saline-rich waters taken up by trees and moved to the leaves can cause leaf scorch later in the growing season as temperatures rise. Windy and high volume and speed road salt application can create salt spray, damaging buds on trees, causing witches brooming on tree branches as the terminal buds are killed off during the winter. Salt damage to trees is heaviest on street and parking lot trees, but a study at the Morton Arboretum showed that wind-driven salt spray moved up to 1000 feet into the arboretum, impacting some trees.

Competition for Space

Trees need room above and below ground to grow and obtain the resources they need to survive. In urban and suburban environments, trees are competing for space underground with utilities, roads, curbs, and sidewalks. Above ground, tree canopies will compete with buildings, roadways, streetlights, surveillance cameras, road and business signs, and overhead electric utilities. In most cases, an urban tree will lose the battle for space if it grows into a sign, or building, and especially utilities that are critical infrastructure in urban environments.

Human-Caused Tree Stress

Trees growing in a forested setting might have to deal with competition for light, hungry deer, and invasive species, but urban and suburban trees must deal with us humans. And we can be brutal.

It begins with improper planting and a lack of aftercare during the establishment period. Trees planted too deep, bury the trunk flare and stress roots needing oxygen and water. Burlap left above ground wicks water away from a reduced root system that was severed during the transplant process in balled and burlap trees. Pot-bound, circling roots in container-grown trees not shaved or manipulated to prevent future girdling roots that will kill the tree. Mulch high on the trunk, leading to future root and trunk issues, including decay and death. Newly planted trees transplanted during the heat of summer or left to fend for themselves without regular watering during their establishment.  Stakes and guy wires left on for years after planting can girdle and kill a tree. If we want a newly planted tree to provide future benefits, we must properly examine and evaluate the planting site, select a tree species for those conditions, purchase quality nursery stock, protect the tree during transportation, plant it properly with the trunk flare visible, and water it weekly to reduce transplant shock that often kills young trees. 

Trees growing in and around homes, buildings, turfgrass, and roads are often subjected to injury from mowers, string trimmers, construction activities, and even everyday activities. Not only does turfgrass compete with tree roots for water and nutrients, the maintenance of grass around the trunk of a tree with mowers and string trimmers can repeatedly wound the trunk and roots, opening the tree to decay. A properly placed ring of mulch around a tree will help protect the trunk and roots from mower damage. During construction, compacted fill soils can suffocate a root system, causing a slow decline in health and leading to death. Even driving heavy construction equipment under or around existing trees will impact their root systems and health. Severing roots for new sidewalks or utility installation will greatly impact a tree's health, survival, and stability. Everyday activities can also cause injury to trees. Chaining a bike or attaching objects to a tree, as well as hits from car doors or bumpers can all create wounds on a tree.

Improper horticultural practices such as tree topping, heading back branches, volcano mulching, over-fertilization, or misuse of herbicides around trees can cause tremendous damage and shorten the life of a tree. Whenever we prune trees, we are removing food-producing foliage and creating wounds that can lead to internal decay. If we work with the tree’s biology and natural defenses, we can reduce stress as well as promote good wound closure and compartmentalization of decay.

When trees are topped, heading cuts are made without considering lateral branches or natural defenses at branch collars. The tree has no defense against decay organisms establishing at heading cuts on these wounded trees. Prolific new growth may sprout from adventitious buds in the branch or stem. This new growth is often fast-growing and weakly attached to the limb as the shocked plant works to replace photosynthetic capacity. As these sprouts grow, decay organisms often establish and digest the older inactive xylem tissue, or wood that holds them aloft.

At some point in time, all trees will experience some stress because they can't pick up and move away from a site with poor conditions or seek out water during a hot, dry summer, but repeated or multiple stressors can lead to a tree having decreased energy, impacting its ability to grow, reproduce (flowering and fruiting), and defend itself. Reduced energy and defenses leave a tree more susceptible to insect pests and diseases and less able to recover. 

Preventing or reducing some of the abiotic stressors that urban and suburban trees experience can mean the difference between an unhealthy and declining tree that will become an expensive removal or a healthy tree that continues to provide multiple benefits for many years to come.