*shared from Tree Services Magazine article by Sharon Lilly from 1/1/13
There may be a battle brewing on your property between your trees grass. Trees and turf tend to be mutually exclusive in nature; you don’t see many trees growing in the prairies or grasslands as you may have noticed that grass is not common on the forest floor.
Our urban landscapes represent an unnatural ecosystem in which we force two somewhat incompatible plant types together and expect optimum performance from each. Trees and turf compete for sunlight, water, mineral nutrients and growing space below ground. Turf roots typically outcompete tree roots and win the belowground battle. However, the dense shade of a tree’s crown can be too much competition for turf, and trees win the aerial war.
Shade leads to reduced grass density, increased root competition and increased weed invasion. There are some varieties of turf that are somewhat shade tolerant, but this may be a partial solution, because shade-tolerant grasses tend to be less tolerant of wear.
Pruning for light penetration
Pruning to increase light penetration should be considered, keeping in mind that it is not a permanent solution. An important axiom to remember is that trees will grow into the voids created by pruning. Keep in mind the old rule of thumb not to remove more than one-fourth of the tree’s foliage-bearing crown in a single pruning. If a tree is topped or thinned too much, it will be stressed and will probably produce many water sprouts (suckers) along its branches to compensate for lost foliage. This defeats the purpose of pruning to allow more light penetration.
It may help to “raise” a tree’s crown to improve light penetration. Crown raising involves the removal of lower branches, and most tree species are tolerant of this pruning practice. Crown raising, however, does not significantly increase sunlight to the turf in most cases.
Some trees have a tendency to form surface roots, which can be a major problem in lawns. Besides ruining the appearance of the turf, they can interfere with mowing equipment, and can even become a safety hazard. Homeowners always want to know to what extent they can prune or remove tree roots without bringing about the demise of a tree. Since cut roots tend to develop more roots, root pruning is usually not a good solution.
The most simple maintenance recommendation is perhaps also the most important: mulch. Mulching the root areas of trees is probably the least expensive but most beneficial thing you can do to enhance tree health and minimize competition with turf. Mulch helps retain soil moisture, moderates soil temperature, and reduces competition from weeds. Organic mulch can help condition the soil and improve microbial activity.
Apply mulch about 3 to 4 inches deep, but do not pile it against the trunk of the tree. As far as the trees are concerned, the bigger the mulched area the better. Group trees together in mulch beds and extend the mulched areas as far out as practical.
There is a long-standing, but inaccurate, belief that trees must be “deep-root” fertilized. This belief is associated with the myth that a tree’s root system is an underground mirror of the crown. Because most of the absorbing roots are actually in the upper few inches of soil, it makes little sense to place the fertilizer deeper.
If the lawn is being fertilized and trees are occupying the same area, the trees might not require supplemental fertilization. The key to any fertilization program is to base the application on the plant’s needs. Soil and foliar analyses can provide the information required to make an educated decision about nutrient needs.
Mowing equipment and string trimmers can damage trees. Most people don’t realize the degree of damage that can be caused by the bumping of a mower or the whipping action of the nylon string in a trimmer. A tree’s bark can provide only so much protection against these devices. Young, thin-barked trees can be damaged almost immediately. In the worst-case scenario, the trees are eventually girdled and die. Those that are not killed will be stressed. The wounds may serve as entry points for diseases, borers or other insects. Many canker rot and root decay fungi have entered trees from wounds created by lawn and landscape maintenance workers.
Herbicides, especially broadleaf weed killers, are often used on lawns. Since most trees are broadleaved plants they can be injured or killed if high enough doses reach them. Homeowners must keep in mind that “weed and feed” fertilizers contain herbicides that can damage trees.
Achieving a balance
Trees and turf can peacefully coexist and even thrive together in a landscape. Armed with an understanding of how each affects the other, you can modify your landscapes and adjust your maintenance procedures to optimize the growing conditions for both.
One of the best things you can do to ensure a beautiful, healthy and thriving spring landscape is to properly put your yard and plants to bed before the harsh weather arrives.
Whether you’re mourning summer’s end or unpacking your winter boots in anticipation of cooler days, these tips will help you prioritize what needs to be done before the snow flies.
In The Yard
- Prune back shrubs in late fall (after a couple of frosts) to keep them protected from winds and heavy snow and encourage healthy spring growth.
- Wait to prune and shape your trees until after they have gone dormant.
- Harvest all fruit from trees and the ground to prevent them from rotting in the winter, attracting pests and diseases.
- Mulch around your trees to help moderate the soil temperature and prevent the loss of moisture. Adding a good fertilizer will help promote root growth for robust new growth in the spring.
- Wrapping your trees from top to bottom can help prevent sunscald and damage from deer and other animals throughout the winter.
- Fall is the perfect time to plant new trees and shrubs. Young trees have a chance to become established before the heat of summer, and the cooler temperatures help promote new root growth.
- Remove all diseased or dead branches and trees from the yard before they become a problem next year. If they’re too large or hard to reach, hire a certified tree removal company to do it for you.
- Don’t forget to water your lawn, trees and perennials during dry spells. Even while dormant, plants need water to survive.
- Rake leaves and dead grass periodically throughout the fall and winter. A buildup of debris on your lawn can harbor pests and fungal disease, prevent proper drainage and suffocate the grass below.
- Aerating and fertilizing your lawn before winter sets in is a good way to stimulate root growth and encourage a healthy yard next spring.
In The Garden
- Pull out vegetables and annuals that are done for the season and discard or compost them. Leaving them in the ground during the winter encourages pests and disease.
- Remove any lingering weeds now; some weeds can overwinter and go to seed early the next spring.
- Layer mulch on your garden beds and ornamental shrubs. Leaves or straw work well as a winter cover, or you can use burlap sacks, which should be removed in early spring. This helps the soil retain heat and moisture, while suppressing weed germination the following season.
- Cut back on the amount of work next spring and prepare your garden beds now. Adding compost and manure in the fall allows the freezing and thawing cycle to work it into the soil for you.
- Tilling garden soil before the ground freezes helps to prevent dormant insects from taking residence in your garden during the winter (and at the same time, provides a nice snack for the birds).
In The Tool Shed
- Now’s the time to clean and disinfect garden tools and pots by removing caked on soil, rinsing them with bleach (to prevent the spread of disease), and storing them in a sheltered place.
- Take note of which garden supplies need to be replenished and purchase them now at a discount!
When it comes to your yard, a little work now will make for a beautiful spring. When your landscape is properly “put to bed” for the winter, it will be healthier and require less maintenance next year, allowing you to sit back and enjoy spring’s show.
Think of a tree and what comes to mind probably has some leaves, some roots, and a trunk. The Quaking Aspen regularly reproduces via a process called suckering. An individual stem can send out lateral roots that, under the right conditions, send up other erect stems; from all above-ground appearances the new stems look just like individual trees. The process is repeated until a whole stand, of what appear to be individual trees, forms. This collection of multiple stems, called ramets, all form one, single, genetic individual, usually termed a clone.
But not all trees follow that formula. Some form what are called clonal groves: swaths of forest connected underground by a single network of roots, with each trunk genetically identical to the others.
The most famous example of a clonal grove is Pando, a grove of quaking aspen in Utah’s Fishlake National Forest. This entire forest, possibly over 80,000 years old, in Utah that is made up of ONE single tree with ONE massive underground root system is called a Quaking Aspen. It’s also the heaviest known organism, weighing over 6600/tons.
Its name means “I spread” in Latin, and for good reason. Pando is among the largest and oldest organisms on Earth. Its 47,000 stems cover more than 100 acres. It’s tricky to tell exactly how long Pando has been around, since the individual trunks only live up to 100 or 150 years.
Scientists have sequenced the genome of a couple dozen shoots of Pando and confirmed the main swath really is a clone, with very closely related but not quite identical trees surrounding Pando proper.
Clonal aspens like Pando can reproduce the traditional way, but they aren’t as good at it. That’s because they have three sets of chromosomes instead of the usual two. So when conditions aren’t perfect, they stick to sending up new shoots from the mass of roots under the grove.
Pando has survived this way for a long time, but scientists are worried it may not be able to clone itself quickly enough to stay alive in the future. They’ve noticed that most of the aspen trunks are relatively old and when new shoots do develop, they don’t last long enough to become full-fledged trees.
Scientists have a couple of theories why that is, but one of the most common is that deer and other animals nibble away at the shoots before they grow old and thick enough to protect themselves. In response, sections of Pando have been fenced off. Deer still sometimes sneak in, but in places where the fence is well-maintained, young shoots inside the fence are doing better.
The Rockefeller Center Christmas arrived around 8am Saturday morning from State College, Pennsylvania stated CBS New York reports.
The beautiful Norway spruce stands 75’ tall and roughly 50’ in diameter. It is set to be decorated with more than 50,000 LED lights on around 5 miles of wire and topped with a 25,000-crystal Swarovski star.
The tree was discovered when Rockefeller Center’s gardener Eric Jose attended a football game in State College seven years ago. The 80-year-old softwood towered over school superintendent Jason Perrin’s backyard until it was cut down on Thursday.
“I have to be honest, when Eric first knocked on my door, I didn’t believe him,” Perrin said of the moment he met Pauze in 2010. “After I figured out it wasn’t a prank, Erik told me my tree looked like it was tall and wide enough, and full enough, and he thought that in a few years it might be ready to be the Rockefeller Center Christmas tree.” “Close to a million people are going to see it on a daily basis,” Perrin said. “It kind of blows my mind in a way.”
The 12.5-ton tree, which grew 10 feet since Pauze first saw it, is nearly 50 feet wide.
The 85th Rockefeller Center Christmas tree lighting ceremony will take place on Nov. 29, and the tree will light up the plaza until Jan. 7, 2018.
At the end of the season, the spruce will serve another important purpose as NBC will donate the tree to Habitat for Humanity so its lumber can be used to build homes.
In November we’re starting to think of oyster dressing, pumpkin pie and turkey dinners, despite the fairly mild temperatures and presence of still-green leaves on many Maryland trees. While the leaves are past peak and have even started to drop in western Maryland, the brilliant orange, scarlet and yellow foliage is coming on strong in central Maryland and the mid-Eastern Shore.
In the central region, reports from Patapsco Valley State Park indicate a colorful scene. For fabulous views of fall foliage, stop by the Valley Overlook in the Hollofield Area near Ellicott City in Howard County. It’s also a perfect time to take in the two-mile scenic drive along Orange Grove Road in the Avalon Area, near Elkridge. With the weather expected to be cool and clear over the next few days, why not enjoy a hike in the park’s McKeldin Area in Marriottsville.
Equestrians can enjoy riding at Morgan Run Natural Environment Area, which reports fantastic conditions right now. Located in Carroll County, this relatively undisturbed area has 7 miles of trails running through old farm fields and forests.
Baltimore, Carroll and Harford counties are nearing peak, with most understory trees (dogwood, spicebush and sumac) as well as black gum, cherry, hickory, red maple, sassafras, sweet gum and yellow-polar near peak. Sugar maples changed a little and then lost their leaves, which is becoming typical for this season.
Although the recent storm accelerated leaf dropping, the Route 1 corridor from Forest Hill to Fallston can compete with anything New England has to offer. The burning bush is flaming and the maples are quite brilliant. Grass is still green making for a colorful contrast.
At the top of the Chesapeake Bay, Elk Neck State Park in Cecil County is reporting some beautiful conditions right now, with foliage about 50 percent peak. The park’s sandy beaches, marshlands and heavily-wooded bluffs comprise the peninsula formed by the North East and Elk rivers and the Chesapeake Bay, and several trails meander through the diversified topography, revealing a great variety of plant and animal life. An easy walking trail to the Turkey Point Lighthouse provides a fantastic view.
On the upper shore in Kent and Queen Anne’s counties, red maples are still turning orange/red in most areas. Willow oaks are still green as are most of the oaks although some have turned brown. Dogwoods are deep burgundy and starting to lose leaves. Woodland areas are showing some signs of yellow.
Maryland’s mid-shore is about mid-peak, with oaks just beginning to turn. Colors are lovely, in a water-color sort of way, as seen in the photo of Wye Mills Lake.
The leaves are just beginning to turn in Wicomico and Worcester counties; however, rain and high wind brought down some of the early color.
Montgomery County reports a lot of color change during the past week. Oaks and maples are quite colorful in Prince George’s County as well.
Tulip poplars and gums have completely turned color. In Southern Maryland leaves are now starting to turn in Calvert and Charles counties, while St. Mary’s is mostly yellow with some red appearing on the maples.
Even after a tree is selected and installed based on the site conditions of sun, shade, soil drainage, proximity to other trees and shrubs, nutrient availability, desired size, slope, surroundings, adjacent activity and more, it can fail to thrive.
Sometimes that’s because the tree wasn’t chosen well and sometimes it’s because it wasn’t planted well. But even more critical to the tree’s success is where it was planted. A tree’s proper location usually will determine whether it becomes an asset or detriment to the landscape.
Tried and true: RPRP
The gold standard of “Right Plant, Right Place” still reigns supreme.
Most arborists would say failure to mind this guiding principle begins with a lack of consideration for the tree’s size. Many reasons exist for this, including lack of foresight and denial that a certain species will actually get that big or wide. Many property owners are too lazy or self-centered to consider the future of their land and the potential ramifications associated with it.
The next consideration is the square footage of available rooting space required for adequate growth, nutrient absorption and structural and support. Although it’s a gross generalization, the average shade tree requires about 1,000 square feet of unimpeded surface – either covered by turf, mulch or groundcovers – in which to grow. In many locations, this much space isn’t provided.
The desired shape of tree – vase, cylindrical, rounded – also can play a role in RPRP. Some sites allow these forms to grow and develop well, and others don’t. The ones that require the largest available space are often limited on site.
The availability of sunlight is a key consideration when choosing tree species, as many require full sun exposure, while others are tolerant of, or actually prefer, shade. Species such as serviceberry, pagoda dogwood and redbud are often poorly sited as a result of this factor.
Tough sites and why
Unfortunately, many sites are compromised in one way or another. These are the most common tough sites.
Slopes: Gentle slopes are desirable; a 2 to 3 percent drop-off facilitates water movement away from buildings, yet generally allows for water movement downward through the water profile. When the degree of slope is 5 to 10 percent or greater, problems commonly arise in tree performance and landscape maintenance. At least four undesirable outcomes are associated with a severe slope:
- Decreased infiltration rate: On slopes, natural rainfall and irrigation water isn’t absorbed as quickly as at the top or bottom. Trees growing on the face of the slope often suffer from inadequate moisture in the root zone.
- Difficulty in mowing: When turf grass is grown on the slope, an increased chance exists for the mower to slip and slide, potentially striking the tree trunk or lower branches, causing damage.
- Difficulty in application of fertilizers and pesticides: After landscape maintenance products are applied on slopes, there’s a tendency for them to move downward, especially when granular products are used and/or moderate to severe rainfall occurs after application.
- Difficulty in mulch retention: Over time, mulch pieces tend to drift downward or sideways on slopes, moving away from installed landscape plants.
Hell strips: The thin, narrow and sometimes oddly shaped portions of the landscape often provide an inhospitable location for trees to grow well. The lack of available rooting and absorptive space is the main limiting influence. In northern climes, hell strips (also called tree lawns or devil strips, depending on your region) are often the space between the sidewalk and a street; in these situations, applied salt and sand for winter traction control causes damage and adds to the lack of rooting. In general, the siting of woody plants in these locations should be carefully considered.
Parking lots: Parking lots offer many of the same negative influences as hell strips. Salt, sand, radiating heat – along with the added occasional misfortune of cars, juvenile delinquents and shopping carts running into the trunk – are the major ones. The main differences between the two are that parking lots generally offer a bit more rooting space and a whole lot more interaction with pedestrians.
Middle of turf: Generally, trees and turf don’t mix. In most scenarios, turf requires more water and fertilizer than trees; in a mixed planting, if moderate amounts are applied to keep the turf thriving, excessive amounts of both are received by the trees. As well, in the midst of an island of turf, tree trunks are prone to mower blight, especially by youngsters and turf maintenance professionals who are in a hurry. The key message to deliver to them is to stop the movement of the mower before it reaches the tree, not after.
Next to concrete and rock: These materials have their place in the landscape, but it’s hard to overlook their negative impact on trees. They have a warming effect on the soil, don’t facilitate horizontal root growth as well as organic materials, provide no soil replenishment and are just so-so on moisture retention and weed suppression as compared with organic mulches and materials.
Compacted sites, high traffic spots: Settings where the soil particles are routinely compressed are tough locations for trees. Common locations for compaction of the soil are those that receive high traffic such as parks, campus grounds and shopping malls.
Adjacent to tough sites: Locations that are adjacent to the tough sites of compaction, hell strips, parking lots and other concrete surfaces may appear to be in good shape, or at least have the potential to produce healthy shoots and roots, however they are still adjoining and share a root zone. At best, sites adjacent to poor locations are half and half – half compromised and, hopefully, half conducive in terms of healthy soil, adequate space and overall growing conditions.
Close spacing: Often a scenario where the original property owner didn’t take size into account and planted way too many trees in way too small of a space. Close spacing is really an issue of trees competing for sun, nutrients and water.
Extremes of sunlight reflection: Commonly noticed when one side of a tree – the side that faces an office building – becomes blighted by excessive sunlight that can cause desiccation of the bark, stems and leaves.
Extreme shading: Opposite of sunlight reflection, absence of sunlight can cause etiolation, or a stretching for adequate light to support sturdy shoot growth. Growth that occurs in a heavily shaded location is usually thinner and weaker than when grown under ideal conditions.
So, what to do?
Identification and understanding is the first step in dealing with difficult sites; four other actions are next as making a difference in your landscape:
*First – it’s important to evaluate the status of the tree in question. Inspect for tree hazards and document defects that will influence future actions.
*Second – decide whether to keep the tree in the landscape or remove it. Consider the number of issues that weaken its structural integrity and limit the aesthetic value. If the tree doesn’t contribute toward the goals for the property, perhaps it shouldn’t remain on site.
*Third – if removal is chosen, possible replacement choices for each site can be contemplated. This is especially true for specimens that are not performing well due to an incompatibility with the size, sun, shade, soil and slope specifics of the site. For example, if a large tree is growing where a small one is called for, the potential replacements should be chosen from that group of options. In terms of possible selections for new specimens, local botanic gardens and arboretums are good places to gather information on suitability.
*Fourth – avoid planting in tough locations in the future. Seek advise on the microclimates where trees simply aren’t a good choice. In many scenarios, tall shrubs, groundcovers, perennials, native grasses and other landscape ornamentals are better options.
The Inside Story
- The outer bark is the tree’s protection from the outside world. Continually renewed from within, it helps keep out moisture in the rain, and prevents the tree from losing moisture when the air is dry. It insulates against cold and heat and wards off insect enemies.
- The inner bark, or “phloem”, is pipeline through which food is passed to the rest of the tree. It lives for only a short time, then dies and turns to cork to become part of the protective outer bark.
- The cambium cell layer is the growing part of the trunk. It annually produces new bark and new wood in response to hormones that pass down through the phloem with food from the leaves. These hormones, called “auxins”, stimulate growth in cells. Auxins are produced by leaf buds at the ends of branches as soon as they start growing in spring.
- Sapwood is the tree’s pipeline for water moving up to the leaves. Sapwood is new wood. As newer rings of sapwood are laid down, inner cells lose their vitality and turn to heartwood.
- Heartwood is the central, supporting pillar of the tree. Although dead, it will not decay or lose strength while the outer layers are intact. A composite of hollow, needlelike cellulose fibers bound together by a chemical glue called lignin, it is in many ways as strong as steel. A piece 12″ long and 1″ by 2″ in cross section set vertically can support a weight of twenty tons!
Leaves Make Food for the Tree
And this tells us much about their shapes. For example, the narrow needles of a Douglas fir can expose as much as three acres of chlorophyll surface to the sun.
The lobes, leaflets and jagged edges of many broad leaves have their uses, too. They help evaporate the water used in food-building, reduce wind resistance—even provide “drip tips” to shed rain that, left standing, could decay the leaf.
Often times, urban trees are vulnerable to pests and diseases because they are highly stressed. Stress can result from a variety of factors including the following:
-paved surfaces that interfere with water absorption
-restricted root spaces
-competition from dense layers of turf that surround a tree – Note: your trees should have layer of mulch surrounding diameter – call our Hometown Landscape division @ 301-490-5577 to assist with this recommendation.
Trees are also often times planted where they shouldn’t be or can be damaged from construction or heavy equipment. There is no single solution for managing all these issues; however, Hometown Tree Experts could assess this issue to ensure improving the overall health of your trees. Research has shown that proper care can increase the fine root density, helps to reduce water loss during dry and hot periods, and could increase the protective barrier of leaves. The trees are also greener and are better able to deal with stress. Call Hometown Tree Experts for solutions to your possible stress management @ 301-250-1033.
Slime spotted on trees is known as bacterial ooze. There are different types of bacterial ooze, and they’re not very well studied. Bacterial ooze can easily go unnoticed. At its most basic they form when a tree gets damaged and subsequently infected with bacteria. In certain circumstances if the bacteria is able to feed on the tree sap and nothing prevents it from multiplying it will eventually form this slime.
Trees, like all plants, have an immune system which should protect them from severe infections like this. Bacterial ooze happens when the tree is unable to heal a wound and prevent the bacteria from feeding on the sap. Bacterial oozes are often fatal; the ooze that forms will rot the tree as the bacteria ‘eats’ it, ultimately leading to the tree’s death.
Without knowing what bacteria is causing the problem, it’s difficult to know how contagious an ooze might be, but in most cases the ooze itself only forms when specific conditions occur on a tree so shouldn’t spread in a woodland. The bacteria involved are often present in a woodland anyway without causing any problems – the ooze forms when something goes wrong and the bacteria breeds out of control.
Bacterial oozes may be accompanied by other pathogens that further harm the tree. For example, slime flux is a type of bacterial ooze that is a mix of bacteria and yeast. It has quite a distinctive orange/yellow appearance. The yeast and bacteria ferment the tree sap, leading to an unpleasant smell and attracting insects to the ooze.
How we could help
If you spot a tree with bacterial ooze on it, we recommend that you call Hometown Tree Experts to assess. Hometown Tree Experts would assess your tree to ensure that it is indeed safe as the rot may be weakening your tree. This issue may be spreading bacterial oozes to other trees on property so would advise assessment to prevent. If you are able to provide good photos we would appreciate this photo being emailed to us as we’d love to see them!
Sulfur dioxide, nitrogen oxides, particulate matter, fluorides, carbon dioxide, ozone. What do all of these hard-to-pronounce things have in common? They are all making their way into your body when you breathe. That’s right, these air pollutants are everywhere, even when you can’t see them. In cities, there’s a mouthful in every breath.
There are two types of air pollutants: primary and secondary. Primary pollutants are toxic as soon as they are released into the air and typically have a source that can be pinpointed. The biggest threats in this category in cities are particulate matter (PM), sulfur dioxide (SO2) and fluorides. Secondary pollutants, on the other hand, form in the air from interactions whose components might not have been toxic on their own. The major secondary pollutant we find in cities is ozone (O3).
When we talk about ozone as an air pollutant, we’re referring to ground-level ozone (which we don’t like) as opposed to stratospheric ozone (which we do like) that creates a layer in the atmosphere protecting us from UV rays. Ground-level ozone is common in areas with dense populations and traffic because ozone forms when hydrocarbons and nitrogen oxides (NOx) from industry and automobile emissions interact with sunlight.
Particulate matter consists of microscopic particles from car exhaust, road dust, industry and other emissions. It is usually measured in two categories according to size: PM10, the larger kinds, and PM2.5, the smaller and more dangerous. The smaller the particle, the deeper into your lungs it can travel, and once it’s down there, it stays there. This leads to respiratory illnesses like asthma and lung cancer — outdoor PM causes 3.2 million deaths every year worldwide. SO2 and fluorides are produced by fossil fuel combustion, which of course there’s a lot of in cities.
With the known health consequences of respiratory illness, cardiovascular disease, and lung cancer, it is clear that we should avoid exposure to these toxic air pollutants, but we don’t always have a choice. People in cities are especially vulnerable, since they have such frequent exposure to high concentrations of them. According to the World Health Organization, concentrations of PM exceed safe levels on the streets of more than 600 U.S. cities. Thankfully, city trees and greenery offer the beginnings of a solution to urban air pollution.
A study in London linked the annual removal of 90.4 tons of PM10 by urban trees to a decrease in 2 deaths and 2 hospitalizations per year. And according to a study in the U.S., the amount of PM2.5 removed annually by trees in 10 cities across the country in 2010 ranged from 4.7 tons in Syracuse to 64.5 tons in Atlanta. In the same cities, estimates of the annual monetary value of human health effects associated with PM2.5 removal, such as hospital admissions, respiratory symptoms and related deaths, ranged from $1.1 million in Syracuse to $60.1 million in New York City. That’s right: trees save lives and money.
Strategic placement of grass, ivy and other plants in cities can reduce the street level concentrations of NO2 and PM by 40 and 60 percent, respectively. There are multiple ways trees help to make urban air cleaner by filtering out pollutants:
Lowering temperatures reduces the movement of harmful ambient particles and prevents more pollutants from evaporating into the air. Trees create a great cooling effect by shading homes and streets, breaking up urban heat islands, and releasing water vapor into the air through their leaves. More tree crowns mean less dark surfaces like parking lots and paved streets being exposed to sunlight and emitting heat. Tree canopy cover in Los Angeles has decreased over the last 50 years, and a corresponding 6°F increase has been measured. Depending on the tree placement, trees can cool a city by up to 10°F, reducing the concentration of PM and other air pollutants with each degree.
Removal of pollutants
The first way trees remove air pollution is by particle interception: trapping pollution particles on their leaves and bark. Once the particle has been removed from the air, it is usually washed off the tree by rain or falls onto the ground with leaves and twigs. Studies have shown that in one urban park, tree cover removed 48 pounds of PM, 9 pounds of NO2, 6 pounds of SO2 and 100 pounds of carbon — daily. Silver birch trees in particular have been studied for their particle interception abilities: They have been found to reduce concentrations of PM by more than 50 percent.
A more complex way that trees filter the air is through gas uptake by leaf stomata. The stomata are tiny pores on tree leaves, and they absorb air to collect CO2 in order to perform photosynthesis. During that uptake of air, they also absorb gaseous pollutants in the air. Once inside the leaf, the gas diffuses throughout the leaf’s pores. It is then absorbed by films of water inside the leaf where it will either form acids or react with inner-leaf surfaces to become less toxic. It is estimated that one tree can absorb almost 10 pounds of polluted air through its leaf stomata every year.
Energy effects on buildings
Now that we understand trees’ chemical abilities, we can factor in their physical ones. Trees shade buildings in the summer and block winds in the winter, so it makes sense that they reduce building energy use for both heating and cooling purposes. Minimizing energy needs lowers the amount of fuel combustion necessary and therefore reduces the amount of pollution from power plants entering the air in the first place.
Once seen as an aesthetic window dressing, trees have never seemed as important in cities as they do now that we know their full potential. We can’t decide to stop breathing when we walk down a city street, but we can decide to plant and maintain healthy trees and hedges in cities to support the cause for greener cities. The air we breathe is a little bit cleaner thanks to each and every tree. Let’s keep it that way.