Why cities need trees: The science behind a 30 percent canopy

 

 

THE NEWS TRIBUNE

January 2, 2016

 

Highlights:

One tree can produce the same cooling effect as 10 room-size air conditioners

Studies show that tree-lined streets increase shoppers’ willingness to pay for goods by 12 percent

Trees slow traffic and shade sidewalks, making walking safer and more pleasant

 

By Rosemary Ponnekanti

rponnekanti@thenewstribune.com

 

When Tacoma took on the goal of creating a 30 percent tree canopy in the city by 2030, officials relied on a study by the conservation organization American Forests to answer the question: How do you quantify a city’s need for trees?

 

WATER AND AIR

Trees are vital in keeping a city’s air and water healthy for people.

The American Forests study, called “Calculating the Value of Nature,” analyzed the Tacoma-Everett-Redmond triangle using images from 1972 to 1996.

Dollar values were placed on the ability of trees to control stormwater flooding ($2.4 billion annually) and filter air pollution ($95 million annually.)

The loss, over that period, of 600,000 acres of highly canopied land meant a 35 percent increase in stormwater runoff, which affects salmon populations, pollution in Puget Sound, flooding on city streets, soil erosion and more.

“Each tree absorbs hundreds of gallons of water, filtering it through the root system so clean water goes back to the ground,” said Pierce Conservation District’s Melissa Buckingham.

And by filtering greenhouse gases such as carbon dioxide out of the atmosphere, trees contribute over time to cooling the air, as well as cooling it in real time with shade during summer.

According to California nonprofit Canopy.org, evaporation from one tree can produce the same cooling effect as 10 room-size air conditioners. That lowers the average summer temperature in highly canopied cities such as Palo Alto, California (which has 37.6 percent canopy), by up to 8 degrees compared with neighboring areas.

This effect will be increasingly important, officials say, during the hotter, drier summers forecast for Western Washington in climate change models.

 

MONEY

As the city of Tacoma points out in its Urban Forestry Policy, numerous studies have shown that trees — the higher the number, the greater the benefit — raise property values by up to 15 percent.

Trees also increase profitability of business districts: Studies by University of Washington professor Kathleen Wolf show that tree-lined shopping streets increase shoppers willingness to pay for goods by 12 percent.

Properties rent more quickly, tenants are happier, and workers report higher productivity, Buckingham said.

That benefit is especially important in business districts such as McKinley, on Tacoma’s East Side, where a recent “depave” project replaced 7,000 square feet of sidewalk concrete with trees, shrubs and ground covers.

Because the trees are in the right-of-way, and thus the responsibility of property owners, the project was a partnership, lining up the city and Conservation District with business owners committed to caring for the young trees.

Now, even in winter, the shopping block in one of Tacoma’s low-income areas has gone from gray and bleak to more inviting, the pinks and reds of the sedums contrasting with the evergreen of pine and hemlock.

“It’s a good start to changing the feel of our neighborhood to warm and caring,” said resident and neighborhood council member José Chavez.

 

 

SAFETY, HEALTH, COMMUNITY

Canopy advocates such as Chavez and Buckingham also point out social advantages of trees: they slow traffic and shade sidewalks, making walking safer and more pleasant. They reduce noise and encourage folks to congregate.

“Adding trees gives a more welcome feeling,” Chavez said. “It shows that there are hardworking, caring people no matter what kind of neighborhood it is. … It means pride, too. That’s a good investment.”

Recent studies by the Pacific Northwest Research Station of the U.S. Forest Service go deeper into the social benefits of trees.

A 2011 Portland study showed greater tree canopy had positive effects on babies’ birth weights and thus lifelong health: Greater canopy cover within 50 meters of a mother’s house and the proximity to open space were associated with a reduced risk of having an underweight baby.

“It may be that trees improve birth outcomes through stress reduction,” said study author Geoffrey Donovan in the November 2011 issue of “Science Findings.”

His findings were backed up by a subsequent Spanish study showing that proximity to green space made positive birth weight differences in poor or low-educated socioeconomic groups.

Donovan’s study also found more tall street trees correlated with lower residential crime rates.

One explanation, he said, might be that mature trees create a pleasant setting for folks to congregate while not blocking views of houses, thus increasing the risk of exposure for criminals.

In addition, Donovan agrees with Chavez that trees indicate a cared-for neighborhood, which can be a disincentive for criminals.

Past studies have shown that views of trees speed healing in hospitals and reduce stress rates for residents.

In a 1984 study in the journal Science, R.S. Ulrich showed that surgery patients with views of trees spent fewer days in the hospital, used fewer narcotic drugs, had fewer complications and registered fewer complaints with nurses than patients with views of concrete walls.

Later studies also showed that patients responded better to brighter green trees and those shaped with a broad, spreading crown. One theory suggests this comes from our evolutionary ancestry on the African savanna, where trees are shaped like this.

Studies also have shown that views of nature can reduce stress in workers and university students.

As environmental scientist, author and academic Nalini Nadkarni points out, trees are also used to heal from loss, such as in the Living Memorials Project after the 9/11 attacks.

“For centuries, humans have looked to the symbolic resonance of nature ... to heal our sense of loss from death,” she said.

For Harvard biologist E.O. Wilson, the reason trees can heal goes far deeper. His “biophilia hypothesis” argues that humans have an innate emotional affiliation to other living organisms and that the health of the planet, trees included, is inextricably intertwined with our own.

“Trees aren’t just trees,” Chavez points out. “They’re many things.”

 

EMOTIONAL HEALTH

Trees don’t just contribute in just practical ways, says Nadkarni.

In her 2008 book “Between Earth and Sky,” Nadkarni, a former environmental studies professor at Olympia’s The Evergreen State College and founder of the International Canopy Network, also explores their spiritual benefits.

Citing her own personal and scientific experiences and those of other scientists and writers, she asserts that trees teach us about stillness, mindfulness, the interconnectedness of all life and what lies beyond the mundane.

“Trees at once enrich and instruct our lives in ways at once simple and complex,” Nadkarni writes. “Opening up to something as simple and pleasurable as climbing a tree — or sitting silently beneath it — can make humans feel at home with the world and with themselves.”

Read more here: http://www.thenewstribune.com/news/local/article52357620.html#storylink=cpy

 

 

 

 

 

Trees Prove Effective At Removing Stormwater From Bioswales

By Bryant C. Scharenbroch, Justin Morgenroth and Brian Maule

Chicago, IL (October 14, 2015) — In a study published in the Journal of Environmental Quality, tree species proved most effective at removing stormwater from bioswales and back into the atmosphere—a process known as water cycling. These heroic trees were part of an upgraded parking lot and visitor center built at the Morton Arboretum, just outside Chicago.

The upgrades took place in the 1990s. As an education and research facility, the team at Morton wanted to use best management practices in their design. This was especially important because the new center and parking lot would be immediately adjacent to Meadow Lake and the east branch of the DuPage River. The center wanted to avoid polluting these important waterbodies.

Parking lots create water pollution in a few ways. By their nature, parking lots interrupt the natural flow of water into soil. Most parking lots are made of asphalt, and are impermeable. Cars naturally drip some hydrocarbons onto the pavement in the form of oil and other fluids. In northern climates, salts must be used to keep the parking lots safe for cars and pedestrians. All these chemicals become runoff during rainstorms and snow melts, and can pollute neighboring water bodies.

Partially funded by a grant from the Illinois Environmental Protection Agency, the team designed a permeable-paver parking lot that included bioswales to clean and capture the water, and reduce the environmental effects of the parking lot and building. Bioswales are depressions in the ground meant to capture water.

Why include trees? According to Bryant Scharenbroch, corresponding author of the study, “A key component of biofilter design is the inclusion of vegetation. The vegetation helps absorb water and maybe even pollutants. Trees may be best suited to improving the biofilter performance because of their large water demand associated with their large above– and below-ground biomass.”

The study showed that the trees within the bioswales returned a lot of water back to the atmosphere. A primary way is through the leaves. The trees move water up from the roots and out through microscopic holes in the leaves, called stomata, back to the air.

In addition to moving water into the canopy, the tree has roots that keep the soil loose and healthy. The channels created by roots also help water move down through the soil and into the groundwater system. Tree roots also help promote an active biological system of organisms functioning to store and clean water.

The research shows that about ½ to ¾ of the total water removed from the parking lot at the Morton Arboretum was through the bioswale system. This reduced the runoff and discharge from the parking lot into the neighboring water bodies.

“Trees are very active in the water cycling of this parking lot,” says Scharenbroch. “The research clearly shows that trees are an integral part of the green infrastructure system and function to reduce runoff and discharge from the parking lot.”

Learn more: Susan Fisk, “Doubly green trees,” American Society of Agronomy

Read the research: “Tree Species Suitability to Bioswales and Impact on the Urban Water Budget,” Journal of Environmental Quality

http://actrees.org/news/trees-in-the-news/research/trees-prove-effective-at-removing-stormwater-from-bioswales/

 

 

 

 

7 Urban Tree Health Benefits You’d Hardly Believe

Posted by Shane Carpani on Oct 28, 2015

 

 

City life isn't always easy, but trees are proven to have surprising effects on the physical and psychological well-being of urban populations. Scientific research outlined in this article continues to prove overwhelming evidence of the numerous health benefits and advantages that mature trees offer. In this article, we discuss the ways that urban trees improve human health, and techniques that landscape architects and other related professionals can use to increase our urban tree populations. 

The fact that trees can improve the quality of life for city residents and make a positive contribution to large-scale planning and infrastructure requirements in now beyond question. In fact, the establishment of healthy urban tree populations is quickly becoming a central component in strategies set out by urban planners and local authorities, due to the many proven urban tree health benefits - some of which we cover here. 

1 – Carbon Reduction

One area of international concern in which trees are already playing a vital role to control is the creation of carbon offsets and the regulation of greenhouse gas emissions. Planting trees remains one of the most cost-effective means of drawing excess CO2 from the atmosphere, in light of global efforts to reduce climate change. A single tree can absorb as much as 48 lbs of carbon dioxide per year and can sequester 1 ton of carbon dioxide by the time it reaches 40 years old (Department of Horticultural Science, North Carolina State University). Although our tree populations currently absorb approximately 2% of total emissions, this could potentially be a lot more.

2 – Oxygen Creation

A single mature tree can release enough oxygen back into the atmosphere to support 2 human beings (Mike McAliney, 1993). Research showing that over a 50-year lifespan, a tree generates almost $32,000 worth of oxygen and provides $62,000 worth of air pollution control (Urban Forestry Network).

3 – Pollutant Removal

As one of their most important impacts, trees are capable of improving air quality by removing and storing a surprising amount of harmful pollutants such as:

• Sulphur Dioxide
• Nitrogen Oxides & Particulates
• Carbon Monoxide
• Cadmium
• Nickel
• Lead

4 – Youth Development

A study by Andrea Faber Taylor and Frances Kuo from the University of Illinois suggests that trees and other greenspace can have a therapeutic effect for children suffering from Attention Deficit & Hyperactivity Disorder (ADHD), improving both their attention levels and social functioning. The same research also showed that students with regular access the trees and greenspace performed better in tests.

5 – Illness Moderation

Due to the removal of harmful chemicals from our local ecosystems, trees decrease smog formation and have been shown to help reduce incidences of skin cancer, asthma, and stress-related hypertension illnesses (Tree Design Action Group). 

A past study performed in the Netherlands indicates that every 10% increase in greenspace can postpone health complaints in communities by five years, while a separate US study suggests that hospital patients with a view of nature through hospital windows recover better after surgery. This type of data has prompted health authorities around the world to increase tree plantings on hospital properties.

6 – Cardio-Metabolic Health Improvement

While many studies over the years have revealed that exposure to greenspace can improve mental health, reduce blood pressure and stress levels, and increase physical activity; few studies have quantified the impacts that individual trees have on human health.

A new joint research project piloted by the Institute for Clinical Evaluative Sciences, the University of Chicago, and the Rotman Research Institute (amongst others), was conducted earlier this year in Toronto, Ontario. The study probed into the health benefits affiliated with street trees based on the assumption that trees are the most consistent green components in urban areas. The study focused on greenspace consisting of tree canopy only and not of bushes, grassland, or other "natural" settings. The research also went as far as to distinguish between trees along roads and streets versus those in private yards, parks, and other open areas; in hopes that such as a distinction would be helpful for future urban planning policies. The researchers hypothesized that street trees could have stronger beneficial associations with an individual's health than park trees and other greenspace because they are usually more accessible to all residents in a given neighborhood or inner-city location, as people are exposed to street trees in their daily activities and through views from windows when indoors.

7 – Health Perception

Released earlier this year, the study suggests that people who live in neighborhoods with a higher density of trees on their streets report significantly higher health perception and considerably less cardio-metabolic conditions, even when controlling for socio-economic and demographic factors such as age and income. This analysis shows that having an average of 10 more trees in a city block improves health perception in ways comparable to an increase in annual personal income of $10,000 and moving to a neighborhood with $10,000 higher median income, or being 7 years younger.

Results from multiple regressions and multivariate canonical correlation analyses indicate that people who live in areas that have more (and/or larger) trees on the streets have significantly fewer cardio-metabolic conditions, having recorded decreases of 0.04 units of cardio-metabolic conditions for every increase of 408 cm² / m² in tree density.

The study continues to annotate that 10 more trees in every block is about a 4% increase in street tree density in a dissemination area in Toronto. With most Toronto areas having between a 0.2% to 20.5% range of street tree density, this increase seems to be logistically feasible. According to the research findings, the resulting improvement in health perception and decrease of cardio-metabolic conditions by planting 10 more trees per city block is equivalent to increasing the income of every household in that city block by more than $10,000 – which far outweighs the cost of planting the additional 10 trees.

Additional Urban Tree Plantings Needed

The Toronto research project went on to explain that additional street tree plantings to reach the defined canopy density can be incorporated into various areas along roads in both residential suburban areas and downtown streets. The difficulty with street tree planting of course, is the fundamental conflict between maximizing uncompacted soil volume for root growth vs providing structural integrity required for roads and sidewalks.

Tree growth and fertility are strongly influenced by soil structure, as it affects the movement of air, water, and nutrients required for trees to flourish. Structural soil that has been compacted for load-bearing stability is resistant to root penetration and significantly reduces root growth. This stunts the maturity of a tree and in many cases leads to death of the tree.

 A well-constructed soil functions like a reservoir, enabling a tree to accept, store and transmit water, nutrients, and energy; while providing room for roots to propagate. Good uncompacted soil allows the space required for life and the necessary biochemical exchanges for growth. 

There is a way however to maintain the structural integrity of paved surfaces without compromising the soil requirements that urban trees need to thrive. Structural support modules (or soil cells) are modular units that assemble to form a skeletal matrix situated below paved surfaces. They support pavement loads, including vehicular traffic, while providing a large volume of uncompacted soil within the matrix for healthy tree root growth. 

http://blog.greenblue.com/urban-tree-health-benefits-youd-hardly-believe?utm_source=hs_email&utm_medium=email&utm_content=23353558&_hsenc=p2ANqtz-8NwI_6l18USbKvhGk4ena4ol5xLbwnhF7CxwwPdw-Zcc6QtejQptFht8DO2XVB1R9Tkwp7Z4SEt9bnk-hgViPYBIrSxsOgEkMGXLMVr7k-eMgqa5w&_hsmi=23353558

 

 

 

 

The (Pretty Much Totally) Complete Health Case for Urban Nature (Part 2 of 2)

An annotated, chart-filled look at the scientific evidence.

• Eric Jaffe
• @e_jaffe
• Oct 20, 2015

 

m01229 / Flickr

 

I’m not a doctor, but I do sit near one in The Atlantic’s New York office. So you can trust me to know that MD-in-residence James Hamblin is on to something when he writes in the magazine’s October issue about the rising appreciation among physicians for the health benefits of parks and green space. Hamblin writes of “a small but growing group of health-care professionals who are essentially medicalizing nature”:

At his office in Washington, D.C., Robert Zarr, a pediatrician, writes prescriptions for parks. He pulls out a prescription pad and scribbles instructions—which park his obese or diabetic or anxious or depressed patient should visit, on which days, and for how long—just as though he were prescribing medication.

 

PART 2

Attention

In more recent years, a lot of research has focused on how urban nature helps people … focus.

A highly cited study by Marc Berman, John Jonides, and Stephen Kaplan gave some test participants a tough attention-related task that involved remembering numbers. Thus cognitively spent, some participants then took a walk through the famed Nichols Arboretum in Ann Arbor, Michigan, while others walked through the downtown area. When the participants returned to the lab and took the test again, the refreshed nature group scored significantly higher, the researchers reported in a 2008 issue of Psychological Science.

Other work is mixed on just how ensconced in leaves you need to be to get the cognitive boost. A study from 2012 found that the denser a park’s vegetation—meaning, less sight of the city through the trees—the better. But other work has found attention benefits from a mere 40-second micro-glimpse of a green roof, or even looking up from your screen to see a desk plant.

A walk through Nichols Arboretum in Ann Arbor, Michigan (above), gave a boost to attention compared with a walk downtown, according to a 2008 study. (Sean Munson / Flickr)

Child attention

Even tots get a mental bump from grass and bark. In a 2009 study, kids aged 7 to 12 with diagnosed attention-deficit disorder showed better concentration after a 20-minute walk in the park, compared with children who walked downtown or in a neighborhood. “‘Doses of nature’ might serve as a safe, inexpensive, widely accessible new tool in the tool kit for managing ADHD symptoms,” the researchers concluded in a 2009 issue of the Journal of Attention Disorders. More recent work extended the cognitive benefits of urban nature to children without ADHD, too.

Kids 7-12 who walked in a park showed better concentration than those who took a stroll in a more urban environment. (Journal of Attention Disorders, 2009)

Aggression and restraint

The power of nature seems capable at times of transcending particular vulnerable environments. In a 2001 study, Illinois scholars Frances Kuo and William Sullivan found reduced levels of aggression in Chicago public housing residents whose view overlooked some trees, compared with others in the same complex who looked onto an empty common area. Kuo and Sullivan report in the journal Environment and Behavior that the additional mental fatigue that comes with not having visual access to nature might play a role in the diverging outcomes.

A study by Kuo and Sullivan as well as Andrea Faber Taylor, published the following year in the Journal of Environmental Psychology, extended that research to discipline among girls in the same housing complex. Young women with tree views performed better on tests of concentration, delayed gratification, and inhibited impulsivity compared to those with barren views. “Perhaps when housing managers and city officials decide to cut budgets for landscaping in inner city areas, they deprive children of more than just an attractive view,” the researchers concluded.

Other work has found reduced levels of road rage among test participants who watched videos of a drive along roads lined with and without nature. In a 2003 study, some participants watched the car cruise along a normal asphalt highway, others saw a little bit of greenery in a garden highway, and a third group saw dense vegetation in a scenic parkway. Afterward they all tried their hand at an anagram that, unbeknownst to them, was impossible (e.g. DATGI—eyes back on the road, please!); those who’d observed the scenic drive showed less impatience, working on the puzzle for 90 more seconds before giving up.

Participants who watched a drive along a scenic parkway showed more patience when tackling an unsolvable anagram than did those with other views. (Environment and Behavior, 2003)

Post-operative recovery

If you’re in a hospital, the last thing you’re really worried about is the view. But having a window that looks onto trees has been shown to have a measurable difference in patient outcomes. In a classic study published in Science in 1984, Roger Ulrich found that gallbladder surgery patients whose hospital rooms overlooked nature had “shorter postoperative hospital stays, received fewer negative evaluative comments in nurses' notes, and took fewer potent analgesics” than those whose windows faced a brick wall.

And these days, if the medical trend noted by James Hamblin catches on, you just might be prescribed a walk in the park on your way out.

For whole article:  http://www.citylab.com/weather/2015/10/the-pretty-much-totally-complete-health-case-for-urban-nature/411331/

 

 

 

The (Pretty Much Totally) Complete Health Case for Urban Nature (Part 1 of 2)

An annotated, chart-filled look at the scientific evidence.

• Eric Jaffe
• @e_jaffe
• Oct 20, 2015

m01229 / Flickr

 

I’m not a doctor, but I do sit near one in The Atlantic’s New York office. So you can trust me to know that MD-in-residence James Hamblin is on to something when he writes in the magazine’s October issue about the rising appreciation among physicians for the health benefits of parks and green space. Hamblin writes of “a small but growing group of health-care professionals who are essentially medicalizing nature”:

At his office in Washington, D.C., Robert Zarr, a pediatrician, writes prescriptions for parks. He pulls out a prescription pad and scribbles instructions—which park his obese or diabetic or anxious or depressed patient should visit, on which days, and for how long—just as though he were prescribing medication.

Seems the medical community has finally caught up with insights made by the urban landscape community 150 years ago. In 1865, Frederick Law Olmsted of Central Park design fame called it “a scientific fact” that natural “is favorable to the health and vigor of men.” (And women!) Olmsted jumped the gun on the whole “fact” thing, but time and a whole bunch of modern behavioral research on the nature-health link has proved him wise.

Exactly what makes parks and trees so healthy for people remains a matter of ongoing discussion. One credible theory, pioneered by Michigan psychologist Stephen Kaplan, holds that nature restores and refreshes our brains, much like sleep, because it doesn’t require direct attention. Harvard biologist E.O. Wilson has attributed the effects to “biophilia”—essentially, that humans are more comfortable in nature because that’s where they evolved.

Here’s Wilson chatting with The Washington Post a few weeks back:

“Instinctively, without understanding what’s happening, they know that in certain wild environments, they have come home,” Wilson said.

Connecting with nature is especially important for the world’s “increasingly urbanized population,” says Wilson. To that end, CityLab has compiled a nearly complete health case for more city green space. The lit review’s purpose is to show that urban nature is (as Olmsted might say) favorable to public health and psychological well-being, and also why it’s so critical for people who live in the high-stress city to occasionally (as Wilson might say) go home again.

Depression

There’s some pretty clear evidence that walking through nature puts people in a better mood than does walking through a city setting. That’s not a huge surprise given the stressful confines of crowded sidewalks. But the findings are especially significant considering the link between urbanization and mental illness, including depression.

A 2009 study found that a 15-minute stroll through the woods led participants to have more positive emotions—and to reflect on a life problem more constructively—than their counterparts who walked in an urban setting. A 2012 study even found nature-related mood gains in major depressive cases. Research published earlier this year found that Londoners living near street trees were prescribed fewer antidepressants.

New work from Stanford’s Gregory Bratman, published this year in top journal PNAS, suggests that nature’s impact on harmful rumination might hold the key to its anti-depressive power. Participants who took a 90-minute nature walk reported having less rumination and showed decreased neural activity in the subgenual prefrontal cortex—a part of the brain linked with sadness and self-reflection. The findings “suggest that feasible investments in access to natural environments could yield important benefits for the ‘mental capital’ of cities and nations.”

A 90-minute nature walk, but not a city walk, led to a reported decline in rumination. (PNAS, 2015)

Happiness and well-being

On the flipside of the emotional spectrum, other work has linked proximity to urban parks with higher well-being. U.K.-based researchers surveyed about 10,000 Brits on how satisfied they were with their lives, as well as whether they had general signs of mental distress. In the journal Psychological Science, the researchers reported that having more green space nearby led to a clear spike in life satisfaction—“equivalent to 28% of the effect of being married rather than unmarried and 21% of being employed rather than unemployed.” They conclude:

Our analyses suggest that individuals are happier when living in urban areas with greater amounts of green space.

General health and mortality

Generally speaking, people who live near urban green space do an admirable job not dying. Past research has found clear associations between city nature and reduced morality for many different causes of death. A new meta-analysis reviewed a number of previous studies and found “strong evidence” linking the quantity of residential green space with all-cause mortality and “moderate evidence” linking it with perceived general health.

Another 2015 paper, this one published in Scientific Reports, put the health benefits in starker terms. After studying general health and tree density in Toronto (while controlling for other demographic factors), the researchers found that having 10 more trees on a city block improved perceived health on par with being seven years younger or $10,000 a year richer. Money may not grow on trees, but the keys to a healthier life just might.

Stress

Environmental research legend Roger Ulrich and collaborators captured the stress-relieving qualities of nature in a clever study from 1991. They gathered 120 test participants into the lab, stressed them out with clips from a work accident film called “It Didn’t Have to Happen,” then showed them videos of various environments. Some participants saw a video of a city pedestrian shopping mall, others watched urban traffic, and others looked at nature.

On four different physiological stress measures, including muscle tension, participants in the nature group recovered more quickly and completely than did those shown the urban environments. Ulrich et al conclude in the Journal of Environmental Psychology:

The findings strongly suggest that environments of importance to well-being and stress are not confined to settings having extreme or unusual properties, such as loud noise or extreme temperatures, but also include very common environments that most urbanites in developed nations encounter daily.

A nature video led to less muscle tension, among other stress traits, relative to sights of city environments. (Journal of Environmental Psychology, 1991)

Subsequent research reached similar conclusions outside the lab. A 2003 study, conducted in nine Swedish cities, found that people who visited urban green spaces more often reported less stress-related illnesses.

To be continued…

For whole article:  http://www.citylab.com/weather/2015/10/the-pretty-much-totally-complete-health-case-for-urban-nature/411331/

 

 

 

Trees in Paved Surfaces: Structural Stability vs Rootable Soil

Posted by Shane Carpani on Oct 15, 2015

http://blog.greenblue.com/

 

 

Soil structure is fundamental for the planting of trees, as well as the stability of roads, buildings, and other infrastructure. It refers to two things:

1. the arrangement of soil particles including silt, sand, and clay that aggregate together
2. the void pore spaces between these aggregate particles

In this article, we discuss why appropriate soil structure is critical to healthy tree establishment, and what landscape architects and other specifiers can do to ensure newly planted urban trees receive the conditions they need to thrive, so they can provide the many benefits that mature trees offer. 

Why We Need Soil Structure

Not every soil is conducive to the growth of trees and other plants, urban soils least of all. Tree growth and fertility are strongly influenced by soil structure, as it defines whether the tree will have rootable soil available and affects the movement of air, water, and other nutrients required for trees to flourish. 

Effectively the 'architecture of the soil', soil is usually the most critical element for the success or failure of urban trees. A well structured soil functions like a reservoir, enabling the tree to accept, store, and transmit water and nutrients, provide room in which roots can propagate, and allow the space required for life and the necessary biochemical exchanges for growth. 

Trees & Compacted Soil

Too often, trees are planted in cramped planting pits and poor subsoil, resulting in retarded growth, with roots tending to colonize the area immediately underneath paved surfaces, leading to structural pavement damage. 

Root colonization immediately underneath the paving stone roadway causing pavement damage 

There are different types of roots associated with tree establishment. Anchor roots function as a structural element to the tree and hold the tree in place in the soil. 

Fibrous roots (or feeder roots) are roots that intake nutrients, and typically grow in the top 6" of the soil. They can be directed lower to avoid damage to paved surfaces, if appropriate irrigation and aeration are ensuring the required nutrients are reaching lower levels. This "feeder zone" can extend between two and seven times the diameter of the canopy drip line. 

The feeder zone of any tree must be protected from compaction in order to ensure root establishment. 

"Resistance to root penetration due to soil compaction will affect
root establishment and the ultimate health of the tree"

Tree roots are opportunistic and seek out favorable growing conditions. Moisture and air trapped between impermeable pavements and compacted soil attracts nutrient-starved roots to grow into those areas. The resulting colonization of those roots causes subsequent pavement heave and infrastructure damage. 

These trees died prematurely due to unavailable rooting volume caused by soil compaction 

So how can urban trees be adequately provided for in their city settings, without compromising or damaging the structural integrity of paved surfaces? The answer is structural soil cells. 

How Soil Cells Work

As we discussed in last week's article, urban soil rarely provides the favorable environment trees need to grow and flourish. Hard compaction, lack of aeration, poor drainage, low nutrient levels, and the existence of pollutants in soil stunt root growth and make it nearly impossible for urban trees to thrive. Landscape architects, arborists, engineers, and other specifiers have the availability of strong soil support systems that, while conducive to root growth, also offer adequate support for roads and sidewalks. 

The concept behind soil cells was first developed by GreenBlue in 1992, when our UK branch installed the first tree pit using soil cells to provide uncompacted soil volume for root growth underneath a paved surround. 

Suitable for parking lots and sidewalks, soil cells prevent the topsoil in tree pits from becoming compacted by the pressure of surrounding hardscapes, amongst other things. They allow trees in urban settings to have large, healthy root systems, thriving in quality uncompacted soil.  

These modular units are assembled into a skeletal framework (or matrix) with over 90% void space to provide large volumes of soil within the tree pit for the healthy growth of roots - all while also supporting pavement loads. 

Designed to highly advanced engineering specifications in order to support heavy vertical and lateral loads, StrataCells bring tree root systems closer to the pavement surface. Engineers have calculated that with only 12" (300mm) of granular pavement depth, a StrataCell matrix can support maximum traffic loads, while providing over 94% of void space for root growth. 

With both vertical and lateral forces considered in the engineering make-up of tree pits, soil cell modules lock together, forming a monolithic framework with excellent modular strength. Highly secure connectors allow for modules to click together fast and simply, while it's enormous growth zone allows for plenteous root establishment. 

Structural Integrity of Soil Cells

GreenBlue soil cells are crush tested during manufacturing as part of our rigorous quality control standards. Whilst FEA (Finite Element Analysis) computer load testing was also conducted during the initial design stages to project the loading capacity laterally and vertically, physical laboratory tests were then used to clarify the cells actual loading capabilities. 

This physical load testing is part of an ongoing development and research program, and is the only true measure of structural integrity. GreenBlue soil cells are made using 100% recycled polypropylene, and have the highest structural integrity of any large soil cell. 

Fatigue Testing

As manufacturer of the world’s strongest soil cell, GreenBlue has subjected our soil cells to extraordinary laboratory tests, including fatigue testing. In one test, a university applied a load of 8.6 tonnes to a StrataCell® tower 10,000 times. The tower was then crushed to measure whether the ultimate load had been diminished by the cyclic loading. The high strength modules had lost no strength, verifying the design of this remarkable system. 

In closing, our streets do not have to be a battle between trees and pavement. They can coexist together, if specifiers and professionals consider the site conditions and provide an appropriate soil structure to suit both tree planting and infrastructure construction. 

Please consider sharing this article, and download our free eBook on the Soil Requirements of Urban Trees. 

Written by Shane Carpani

Shane Carpani is the Creative Director and Manager of Content Strategy at GreenBlue Infrastructure Solutions. Connect with Shane on LinkedIn or follow @ThinkGreenBlue on Twitter.

http://blog.greenblue.com/trees-in-paved-surfaces-structural-stability-vs-rootable-soil