By: Liz Baessler
When you think about soil, your eyes probably drift down.Soil belongs in the ground, underfoot, right? Not necessarily. There’s a wholedifferent class of soil that exists high above your head, up in the treetops.They’re called canopy soils, and they’re an odd but essential part of theforest ecosystem. Keep reading to learn more canopy soil info.
A canopy is the name given to the space made up of the collectedtreetops in a dense forest. These canopies are home to some of the greatestbiodiversity on earth, but they are also some of the least studied. While someelements of these canopies remain a mystery, there is one we’re activelylearning more about: soil in trees that develops far above the ground.
Canopy soil isn’t found everywhere, but it has beendocumented in forests in North, Central, and South America, East Asia, and NewZealand. Canopy soil isn’t something to buy for your own garden – it’s animportant part of the forest ecosystem that helps regulate temperature andmoisture and spread nutrients. But it is a fascinating quirk of nature that’sgreat to admire from afar.
Canopy soil comes from epiphytes– non-parasitic plants that grow on trees. When these plants die, they tend todecompose where they grew, breaking down into soil in the nooks and crannies ofthe tree. This soil, in turn, provides nutrients and water for other epiphytesthat grow on the tree. It even feeds the tree itself, as often the tree willput out roots directly into its canopy soil.
Because the environment is different from that on the forestfloor, canopy soil makeup isn’t quite the same as that of other soils. Canopysoils tend to have higher amounts of nitrogen and fiber, and are subject tomore extreme changes in moisture and temperature. They also have distinct kindsof bacteria.
They are not completely separate, however, as heavyrainfalls will often wash these nutrients and organisms down to the forestfloor, making the composition of the two kinds of soil more similar. They arean important part of the canopy ecosystem, serving an essential role that weare still learning about.
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Whether you are planting small plants in pots, ornamentals in your yard or a food forest, you need plants that will provide an upper canopy for others. If you have small plants, then you will have a short canopy. Maybe your canopy is a tomato plant. Maybe its an oak. Whatever it is, canopy has many functions.
Upper canopy provides shade so that other plants can grow. It drops leaves, bark, flowers and seeds and/or fruit to provide compost and food for all levels of animals down to soil microbes. Canopy provides protective shelter for many kinds of mammals, birds, reptiles and insects as they hide under the leaves. A mature oak is home to over 300 species. Old scarred canopy full of holes is the natural home for honeybees, and many types of bird and other animal. It is a storage unit for acorns gathered by woodpeckers. Where you have animals, you have droppings. All the poo, feathers, regurgitated pellets, fur, scales and other organic waste that falls from canopy is vitally important for the health of the soil below.
Canopy provides a perch for raptors and larger birds that help with rodent control.
Canopy helps slow the wind the fewer trees we have the harder the winds. Canopy also filters the wind, blocking dust and other debris. Canopy helps cool and moisturize the wind. The leaves of canopy trees help buffer the rain. Rain on bare ground is as compacting as driving over the dirt with a tractor. If rain hits leaves it bounces, rolls or shatters. Rain can then hit other layers below the canopy, finally rolling through leaf mulch to percolate into the soil without compacting it.
Canopy catches moisture as well. Here in Southern California we may not receive a lot of rain, but we do have moisture during the night. Often I’ve walked through Finch Frolic Garden of a morning to feed the hens, and the garden sounded as if it had its own special rain cloud over it. That is because moisture condenses on the leaves and rolls off. The more canopy and the higher the canopy, the more water we can collect. In that same way, canopy begins to hold humidity on the property, which the rest of the guild contributes to. Pollen dries out. With longer, hotter, drier summers there is worse pollination even if the pollinators are active, because the pollen isn’t viable. Less humidity equals fewer fruits, nuts and vegetables. Therefore, the more canopy, and other parts of a guild, the moister the air and the better the harvest.
Canopy is in connection with all other plants in its community, linked via webs called mycorrhizal fungi. Through these webs the canopy sends chemical messages and nutrients to other plants. Every plant in the community benefits from the strong communications from the canopy trees.
Canopy builds soil. Canopy trees are large on top and equally large underground. Tree root growth can mirror the height and width of the above-ground part, and it can be larger. Therefore canopy trees and plants break through hard soil with their roots, opening oxygen, nutrient and moisture pathways that allow the roots of other plants passage, as well as for worms and other decomposers. As the roots die they become organic material deep in the soil – effortless hugelkultur canopy is composting above and below the ground. Plants produce exudates through their roots – sugars, proteins and carbohydrates that attract and feed microbes. Plants change their exudates to attract and repel specific microbes, which make available different nutrients for the plant to take up. A soil sample taken in the same spot within a month’s time may be different due to the plant manipulating the microbes with exudates. Not only are these sticky substances organic materials that improve the soil, but they also help to bind loose soil together, repairing sandy soils or those of decomposed granite. The taller the canopy, the deeper and more extensive are the roots working to build break open or pull together dirt, add nutrients, feed and manage microbes, open oxygen and water channels, provide access for worms and other creatures that love to live near roots.
Canopy roots have different needs and therefore behave differently depending upon the species. Riparian plants search for water. If you have a standing water issue on your property, plant thirsty plants such as willow, fig, sycamore, elderberry or cottonwood. In nature, riparian trees help hold the rain in place, storing it in their massive trunks, blocking the current to slow flooding and erosion, spreading the water out across fields to slowly percolate into the ground, and turning the water into humidity through transpiration. The roots of thirsty plants are often invasive, so be sure they aren’t near structures, water lines, wells, septic systems or hardscape. Some canopy trees can’t survive with a lot of water, so the roots of those species won’t be destructive they will flourish in dry and/or well-draining areas building soil and allowing water to collect underground.
In large agricultural tracts such as the Midwest and California’s Central Valley, the land is dropping dramatically as the aquifers are pumped dry. Right now in California the drop is about 2 inches a month. If the soil is sandy, it will again be able to hold rainwater, but without organic materials in the soil to keep it there the water will quickly flow away. If the soil is clay, those spaces that collapse are gone and no longer will act as aquifers… unless canopy trees are grown and allowed to age. Their root systems will again open up the ground and allow the soil to be receptive to water storage. Again, roots produce exudates, and roots swell up and die underground leaving wonderful food for beneficial fungi, microbes, worms and all those soil builders. The solution is the same for both clay and sandy soils – any soil, for that matter. Organic material needs to be established deep underground, and how best to do that than by growing trees?
In permaculture design, the largest canopy is often found in Zone 5, which is the native strip. In Zone 5 you can study what canopy provides, and use that information in the design of your garden.
How do you achieve canopy in your garden? If your canopy is something that grows slowly, then you will need to nursery it in with a fast-growing, shorter-lived tree that can be cut and used as mulch when the desired canopy tree becomes well established. Some trees need to be sacrificial to insure the success of your target trees. For instance, we have a flame tree that was part of the original plantings of the garden. It is being shaded out by other trees and plants, and all things considered it doesn’t do enough for the garden to be occupying that space (everything in your garden should have at least three purposes). However a loquat seeded itself behind the flame tree, and the flame tree helped nursery it in. We love loquats, so the flame tree may come down and become buried mulch (hugelkultur), allowing that sunlight and nutrient load to become available for the loquat which is showing signs of stress due to lack of light. With our hotter, drier, longer summers, many fruit trees need canopy and nurse trees to help filter that intense heat and scorching sunlight. Plan your garden with canopy as the mainstay of your guild.
Therefore a canopy plant isn’t in stasis. It is working above and below ground constantly repairing and improving. By planting canopy – especially canopy that is native to your area – you are installing a worker that is improving the earth, the air, the water, the diversity of wildlife and the success of your harvest.
What makes up a plant guild.
Canopy is improving the water storage of the soil and increasing potential for aquifers. The more site-appropriate, native canopy we can provide in Zone 5, and the more useful a canopy tree as the center of a food guild, the better off everything is. All canopy asks for in payment is mulch to get it started.
Figs. 1-6. No plants like salty soil but some will tolerate it. Salt-tolerant shrubs include (shown top to bottom, left to right) caragana, buffaloberry, common lilac, golden currant, skunkbush sumac and juniper. North Dakota has vast areas of salty land. This includes thousands of home landscapes. Saline soils become gray and crusty, and are associated with high water tables and low rainfall. It’s a harsh environment.
Plants hate salty soil and landscaping in these soils is a challenge. The salts will burn plant roots and prevent them from absorbing water needed for healthy growth. Salts lead to compacted ground with poor aeration and poor drainage.
Young plants are especially sensitive. Scorched leaf margins and needle tips are the initial symptoms of distress.
The best way to cope with a salty soil is to grow plants that tolerate it. Among leafy shrubs, the most tolerant plants include caragana, buffaloberry, silverberry, sea buckthorn, common lilac, golden currant, ‘Freedom’ honeysuckle and skunkbush sumac (Figs. 1–5). The best evergreen shrubs are junipers these include Rocky Mountain juniper and Eastern red cedar (Fig. 6).
The most tolerant leafy trees include green ash and Russian olive. These trees are found in abundance across the state but are rarely used in landscapes today. Leafy trees with moderate tolerance to salty soil include honeylocust, catalpa, coffeetree, corktree and hawthorn. Ponderosa pine is the most tolerant evergreen tree used in North Dakota landscapes, but spruces and other pines show some tolerance.
We can take some steps in our landscape practices to minimize the impacts of saline soil:
Irrigate deeply and infrequently, rather than shallowly and frequently. In most cases, a single irrigation of one inch of water per week is sufficient for healthy growth. Deep watering promotes a deep root system and flushes harmful salts away from roots.
Mulch your plants. This will reduce evaporation, which leads to the accumulation of salts.
Fertilize plants only when needed. Fertilizers contain salts.
Add organic matter (compost or sphagnum peat moss). In new landscapes, you can an inch of organic matter and till it into the soil. In established landscapes, one strategy is to core aerate the soil, filling the holes with organic matter.
Flush out salts when they appear on the surface. Apply 2 inches of water over a 2–3 hour period, stopping if runoff occurs. Repeat again in 3 days if the salts reappear.
Appleton, B., V. Greene, A. Smith, S. French, B. Kane, L. Fox, A. Downing and T. Gilland. 2005. Trees and shrubs that tolerate saline soils and salt spray drift. Virginia Tech:Blacksburg.
Written by Tom Kalb, Extension Horticulturist, North Dakota State University. Published in the NDSU Yard & Garden Report, August 21, 2016. Photos were made available under Creative Commons licenses specified by the photographers: pverdonk, Julia Adamson, Martin LaBar, amy_buthod, Leah Grunzke, Peter Gorman.
The size of an urban forest can be seen from above.
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.
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.
In biology, even shrubs or grass can create a canopy: when talking about trees, we generally mean those who give shade.
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.)
Trees make bicycle routes more cyclist friendly by slowing traffic and freshening the air.
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.
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.
Insufficient soil volume makes a tree vulnerable to disease, impact, and wind.
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.
Tree roots often extend much farther than the tree’s crown and can seek water even where there’s asphalt.
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.
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 are generally used with other plants or bioswales
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.
This R-8754-1 Greenwich Tree Grate is easy to expand as the tree grows.
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.
A tree guard is installed in a R-8708 Boulevard Tree Grate.
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.
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.