Trees, where are they going & why (Site Factors)?

No matter the route, when you enter the Borough of New Cumberland, a noticeable canopy of trees is appreciated.  However, age, disease, and strong storms to name a few, are taking a toll on our shade trees. What can you do to maintain and increase our shade trees canopy? Plant a tree – the right tree in the right spot is the key to decades of shade, songbirds, energy savings and beauty. Recommendations for the variety of trees to plant can be secured from a reputable arborist or a community forester from the PA Department of Conservation and Natural Resources.

Why are trees disappearing in the Borough and across the region? 

This article is the second in a series looking at factors that contribute to the loss of trees and our tree canopy. 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 trees’ 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, boosts property values, and improves 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. The second topic in this series is Site Related Stressors.

Environmental Stressors – Find out more about environmental stressors here.

Site Related Factors/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 effect soil chemistry, elevating soil pH beyond what some tree species can tolerate.

Soils that are compacted or consist of large amounts of clay tend to not 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.

The trees with limited rooting space, compacted soils, and paving are declining while the trees set back in open soils are growing well.

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.

Witches broom from salt spray injury

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.

This content was adapted from a Penn State Extension article.