Planting and protecting street trees for a healthy urban forest
Urban tree canopy is of increasing importance in cities all over the world. As climate events become more common, trees help moderate their effect. They clean and cool air, manage water, prevent erosion, and manage the urban heat sink. And those are just some of the benefits of trees. Creating these benefits relies on a healthy canopy—one that is supported by street trees. Since street trees live in an unusual environment, surrounded by concrete and cars rather than other plants and animals, they need extra care and support.
What is a tree canopy?
A tree canopy is the area covered by the uppermost branches and leaves of a community of trees. Sometimes it is used to describe the total crown area of a single specimen. “Canopy” can be used in biology to describe the upmost layer of any group of plants or crops, but “tree canopy” is generally used to describe trees that provide shade at human scale.
What is an urban tree canopy?
Urban tree canopy describes the total shade canopy over a city. To capture the sense of scale, this can be imagined as the percent of a city’s ground covered by tree branches and leaves when the city is seen from above. What foliage would someone see from an airplane or satellite? The urban forest is comprised of trees on public and private lands. Parks, street trees, and trees around houses all contribute.
In some cities, trees on big single family lots comprised a lot of a city’s urban tree canopy. As single lots are given to laneway houses and triplexes, the total building footprint increases. There is less room for trees. (On the other hand, tree canopy around tall multi-family dwellings is often denser than people generally expect.)
Why are street trees more important than they used to be?
As land use in cities turns means less green space in residential areas, healthy street trees make up more of the potential urban canopy. Street trees are often stunted by insufficient water or low-quality soils. The hardy, adaptive ones will seek whatever nourishment they can find, with their roots digging into water mains or heaving the pavement. In either circumstance, whether inability to thrive or causing infrastructure problems, street trees get removed and replaced before full maturity.
Yet fully mature street trees can provide substantial canopy in the urban forest. It’s just a matter of getting them to full height.
Planting street trees
Some of problems cities have with street trees can be addressed at the time of planting. Poor soil profiles and compaction around tree roots lead to challenges for both the tree and the infrastructure around it. Trees need air, water, and nutrients. In poor soils under impermeable surfaces like concrete and asphalt, they will send curious roots to find what they need. Cracks in the sidewalk, imperfections in the surface, or the tiny area right around the tree (called the tree pit or tree basin) end up being the target. Sidewalks and roadways are heaved up from below.
If the tree is not successful in its search, it will also be more vulnerable. Bugs, illness, and failure to thrive may mean a badly planted tree is replaced over and over. Many downtown treescapes are full of saplings that get replaced every decade or so. It’s no wonder these locations never get a flourishing urban tree canopy. Lack of tree canopy is part of why cities feel hotter and grittier in the summer than surrounding suburban and rural areas.
Roots are also both feeding system and anchor to the above ground weight of the tree. A healthy root system reaches out farther than the tree’s canopy. Trees need soil volumes that will support such expansiveness: the only way to get a lush urban tree canopy. Increasingly, regions are setting minimum allowable soil volumes. Planning requirements are being built to offer the best chance at success. (Check out an updating list here for your state or province.)
Needed soil volume depends on region, soil, and tree, so it is hard to give universal rules of thumb. However, the range is generally between 400 and 1200 cubic feet of soil depending on projected tree size. This rule of thumb is many times larger than the small spaces sometimes given to trees.
Even with sufficient soil, important infrastructure may need protection from questing roots. And for all soils, water and air must be able to get in, in sufficient quantities. Increasingly there are ways to plant trees to ensure good outcomes for both the tree and the structure around it.
A soil vault, or modular soil support system, is an underground structural lattice that helps prevent soil compaction. A cubic or honeycomb matrix stops the weight of traffic above from crushing the substrate below. This vault is installed under ground and then filled with a rich, aerated soil which gives roots room to grow. Additionally, good soils will absorb and hold moisture—excellent both for the tree and for storm water capture.
Cobblestones, pavers, permeable concrete, bioswales, and other porous surfaces support trees—and urban stormwater management—by allowing greater volumes of water to permeate soils rather than running off to sewers. Trees and tree leaves have evolved to channel water down their trunks and into their roots, but this may not be enough! Roots on trees often reach out past the tree’s dripline, or the widest point in the tree canopy. There, it might drink from areas where there is no evaporation loss, like there is from leaf surfaces.
Giving the tree more room than its own surface area can therefore help the tree grow larger. Creating strips of porous surfaces that extend between trees, such as a cobblestone strip or a line of grates, can help a tree get sufficient water to grow quickly and to stretch a wider canopy.
Even with lots of room, air, and water, a tree’s seeking roots can become a problem for utilities and surfaces. When the tree is given sufficient room, a root barrier along one or two sides can help protect road surfaces and water mains by encouraging roots to grow down, rather than up. If there is a large enough root zone and the barrier is deep enough this strategy can forestall problems.
It is generally not advised to build a root barrier all around a tree: the root barrier encourages growth into mutually beneficial areas. If the root barrier encircles the tree, the adaptive tree will grow down and out eventually. If the root barrier instead directs roots to an appropriate strip of soil, there will be less incursion into inappropriate places.
Root barriers used to be built of concrete. However, moisture, soil load, and movement would eventually crack and crumble this underground concrete. Roots—in many species evolved to twist their way through rocky soils—would slowly work their way through. Current materials are made from plastics or meshes. These are more flexible yet still prevent root spread.
When a tree root zone is created, roots of nearby trees can grow into contact. Research around the world has been pursuing the theory that there’s communication between trees. It appears that trees can alert each other to things like pests or illness and so mount a biological response. Although research on tree communication among street trees is still limited, tree communities do seem to support each other in parks and forests.
Perhaps underground, mature tree root zones are best if they echo a mature urban tree canopy, with roots and crowns both intertwined.
Protecting street trees
One of the greatest dangers to street trees is the footfall of pedestrians on the soil around their trunk. Soil compaction caused by regular use can create surfaces almost as hard as concrete. With badly compacted soils, neither air nor water can percolate through the soil surface.
Another issue for young saplings is their instability as they root. Wind or impact can tear away the fragile reach of new, tendril roots and topple the tree.
Sidewalk trees are therefore usually protected with fences, grates, and sometimes guards, as well as being anchored as they become established. Any successful addition of a sidewalk tree to the urban tree canopy comes with a strategy to help keep it safe from the traffic around it. (When the tree is large enough, it helps protect itself: cars drive more slowly on tree lined streets, lowering the likelihood of crashes that can take a tree over!)
Tree pit guards
Tree pit guards (sometimes called border or landscape fences) often surround a tree pit that contains a planted swale or garden. The guard is a small fence installed on at least three sides. Three sides are always near the sidewalk to prevent pedestrian incursion. However, if the swale is immediately next to the street, may not have a fence on that side.
In order not to be a trip hazard, fences should be at least 18 inches tall. When installed these tree pit guards keep feet, pet waste, and garbage out of the soil.
A small ecosystem, complete with the developing humus of the garden, often flourishes in such a tree pit. This can be especially true if surface run-off flows into those spaces.
Choosing plants that grow well with the tree is important for success. Bare soil will attract weeds and garbage. Additionally, windy weather will pick up layers of soil and blow it away.
One note about trees and swales: if the planting’s successful, over time a large urban tree canopy will develop as intended. And, as intended, it will be wonderful for local climate, property values, traffic calming, and curb appeal. However, such success is often hard on the other plants in the swale. Even in a forest or park, plants at ground level beneath spreading shade trees are often starved for water and light. This issue can be made worse in the city by the hardscape and sewers that surround sidewalk trees. Carefully choosing shade-tolerant, low-water plants can make these swales more successful in the long term, as well as capturing and directing rainwater to the tree pit. The swale will need maintaining by the city over time, and may need watering as well.
Tree grates cover tree pits with grating that lets water and air through while preventing soil compaction. It’s wise to have tree and soil planted right beneath the grate to prevent it becoming a capture for litter. Sometimes, little volunteer plants will show up around these trees. In general, these are weeds that come and go, but if they are aggressive, they can compete with the tree for nutrients and water, especially when the tree is quite young. Stony mulch and/or landscape fabric installed beneath the grate will keep volunteers at bay.
Grates should be expandable, with easily removable inner rings. As the tree grows, the municipality must widen the tree aperture, so that the tree does not become strangled by its grate. Most issues with tree grates come when the tree trunk needs more space than the center ring allows.
Tree grates do not have the bioswale’s planting to help with stormwater processing. However, with root zones and porous hardscape, mature trees make a significant impact on their own, and run-through finds its way back to groundwater.
When trees are young, tree guards are often installed along with a tree grate. These guards affix to the grate itself and protect the sapling from early misadventure as it grows. As with tree grates, guards left too long can strangle a growing tree. They are generally removed the first time the tree grate is expanded outward.
Supporting the urban forest, one tree at a time
One of the adages of foresters is that trees are successful when you put “the right tree in the right place.” Trees chosen for a city sidewalk should be tolerant of the light and weather in that location. In the middle of the city, fruit trees may not be tolerated—lots of fruit on the ground may be slippery or attract rats—yet choosing only male fruit trees can lead to high pollen counts and aggravate allergies. Large trees are beautiful and provide the most benefits to an urban tree canopy, but they can also block light, get in the way of utilities lines, or have too many branches at pedestrian level. Drought tolerance and species vulnerability to disease and insects is worth considering as well.
Part of picking a tree, therefore, is picking the right species. Another important part is choosing proper planting infrastructure. At grade, a city will also be maintaining the growth of the tree: in different areas, maintenance strategies might be different. Will a swale’s water management be useful in an area and the ongoing maintenance of those gardens feasible? Will the city commit to expanding tree grates as saplings grow? What are the needs of the tree for nutrients over time? A well-placed tree will pay off many times over in benefits as it grows. However, reaching that payoff means choosing the right strategy for planting and maintenance at the outset.