Let’s face it: trees are better off in the woods. Although forest trees – like their more urban counterparts – face many threats to growth and survival, at least in the woods they have the benefit of native soil and face almost no human-built infrastructure. Indeed, while healthy, intact forests contain some struggling and unhealthy trees, many forest-grown trees live and grow quite well for hundreds of years. Most urban trees are lucky to see 20.
But as much as trees are better off growing in forests, urban areas are undoubtedly better off with trees growing in them. Trees are proven, positive difference makers, providing everything from cool shade, clean air, and stormwater attenuation to bird habitat and increased real estate values. As the saying goes, life is a shade better in a treed city. Plus, urban trees function as gateways, connecting people and trees along the forest continuum, from town green to mountaintop. Many benefits come, to both downtowns and more distant woods, when people find something in a street tree that takes them to the back 40 and beyond. It is all good.
Alas, the valuable benefits package that accrues with community trees does not come easily. It turns out humans are very hard on trees: as we develop roads, utilities, and buildings, we make it ever harder for trees to succeed. Proof abounds. Street and yard trees everywhere exhibit ghastly evidence, ranging from wilted leaves and stunted growth to rot.
While the symptoms are readily observable above-ground, about four-fifths of all urban tree health problems stem from less obvious, below-ground conditions.
Soil compaction is by far the biggest problem for trees in the built environment. Though not often measured – or even much noticed – it is a pervasive and chronic stressor of trees. Compaction destroys a soil’s natural structure by compressing the pore spaces needed for air, water, and roots. Remember the roots? Any limitation on the roots will result, directly or indirectly, in degraded tree health above ground. Even if the soil under a tree is watered and fertilized, the tree won’t benefit one bit if soil compaction is limiting growth of its roots and the infiltration and uptake of water.
No tree is immune to the negative consequences of the compacted soil conditions typical of most non-forest planting sites. But tree species do vary in their tolerance to compaction. Accordingly, much emphasis should be placed on picking the right tree for the site. Examples of urban or “street” trees that exhibit some tolerance of compacted soils include red maple, hackberry, hawthorns, red oak, and red cedar. Sugar maple, on the other hand, is particularly intolerant of compacted soils.
Unfortunately, while the causes of compaction may be many, solutions are few. The best chance for success comes when tree plantings – or construction projects around existing trees – are planned to minimize damage to the native soil in advance of site work.
If you did not plan ahead, hope remains. Homeowners, planners, foresters, and arborists have some tools for soil renovation to improve tree health and longevity in yards, parks, and downtowns.
Most renovation strategies begin by limiting vehicle access with strategic placement of fences and benches, for example. This is to minimize pressure and prevent additional compaction to a tree’s rooting area. In some cases, piercing the soil with hydraulic core aerators throughout the rooting volume can significantly enhance aeration and water infiltration. Special tools using high-pressure air can also be used to break up compaction. In cases of extreme compaction in highly space-restricted conditions, breaking, aerating, and mixing of soil with added organic matter may be necessary. And, in instances where foot or vehicle traffic is present, specially constructed soils can be used to combat compaction. Cornell University’s patented CUStructural Soil, for example, uses a mix of gravel and soil (about 80 percent stone to 20 percent soil) to support heavy loads above while allowing tree roots to grow below.
Urban tree planting conditions and contexts vary, so – just as in the woods – good guidance from a qualified local professional is recommended. You might also check out the Vermont Department of Forests, Parks and Recreation’s online “tree selector” tool that helps users find a tree species that will be a good match for their urban site.
Michael Snyder, a forester, is Commissioner of the Vermont Department of Forests, Parks and Recreation.