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The Science of Tree Shade

by James H. Wandersee and Renee M. Clary

Singapore has urban forests with trees that provide refreshing shade for its citizens. Beginning in the 1960s, the country’s modern founding father, Lee Kuan Yew, had the foresight to promote tree-planting as a means to transform Singapore into a tropical garden city-state.  Anyone who comes here today is impressed by the millions of mature trees and the large number of green spaces that make Singapore a very special place to live and work.

If Singapore can be said to have a “signature tree,” a tree that is emblematic of the esteem it bestows upon its trees, most people would likely agree that it is the grand old Tembusu tree growing at Lawn E of the Singapore Botanic Gardens. Not only has this venerable tree’s image been displayed on the nation’s postage stamps, but also on its $5 note.  We think every citizen ought to make an effort to actually see this tree, to appreciate its shady grandeur, and to contemplate its symbolic meaning to the country.
  tembusu-tree-5-dollar-note

Examples of the honor given to its famous Tembusu tree on Singapore’s currency and postage

In this Eco Walk the Talk article, specifically, we wish to draw our readers’ attention to some key scientific aspects of tree shade that highlight its oft-overlooked importance to urban living. While the current state of this science is still limited and not yet definitive, we hope what we are able to present here will help you understand and appreciate the variety of benefits that city-dwellers receive from shade trees.

Environmentalist Jenny Indian observes that, “We seriously underestimate the impact of [tree] shade – it cools, calms and softens the light, makes hostile spaces useable, and allows us to embrace the outdoors.”  She suggests that we should “think about the absolute beauty of filtered light – light filtered through leaves or tracery is wonderful.  Areas of uniform lighting create dull, uninteresting spaces with direct light casting strong shadows, resulting in harsh images with strong contrasts.”

An under-appreciated human influence factor of shade trees is thе shadow pattern cast upon thе ground, sidewalks, οr thе walls οf уουr residence bу different tree foliage types.  We encourage уου to begin to consciously note these subtle, yet mood-altering differences between trees. The Rain Tree (Samanea saman) аnd the Senegal Mahogany (Khaya senegalensis)  tree аrе instructive examples οf common Singapore trees thаt cast іntеrеѕtіnɡ shadows. Aѕ уου grow in your awareness of the shade trees in your local environment, we think уου wіll discover thаt уου actually derive deep aesthetic satisfaction frοm seeing thеse patterns.

Leaf patterns cast on the walkway by sunlight

Leaf patterns cast on the walkway by sunlight

Some benefits of tree shade are less obvious than others. For example, you might be unaware that tree shade increases the life of tarmac road surfaces—which can get as hot as 130 degrees Fahrenheit in full sun. Research has shown that more tree shade means extended pavement life and lower road reconstruction costs. Just 20% tree shade falling upon on a tarmac road surface was shown to reduce pavement surface deterioration by 11%, which constituted a 30% resurfacing cost-savings over 30 years!

A mature tree can offer a canopy of 100,000 chlorophyll-laden leaves, each partially absorbing and filtering the light we experience beneath the tree when we occupy its shade.  Children often prefer to remain inside and inactive, rather than play outdoors when the mid-day sunlight is hot and glaring–making it uncomfortable for them to see things and raising their body temperature.  Researchers have found that tree-shaded environments increase the number of hours children typically spend playing outdoors, as well as the number of hours adults spend outdoors enjoying nature.

Sunlight is filtered by a shade tree leaf

Sunlight is filtered by a shade tree leaf

Trees offer shade, shelter, habitat, beauty, and food–while requiring little more than sunlight, soil, water, and a place to grow. At present, 3.5 billion of the world’s people live in urban areas, and that number is growing rapidly. Cities with inadequate green spaces can seem to be overcrowded and oppressive places for humans to live.

Cities also form urban heat islands, while shade trees help to provide valuable indirect cooling that serves to partially mitigate these islands’ environmental impact. Unchecked, the heat island effect  can add up to 8% to a city’s energy consumption. It can also have a negative local effect upon a city’s weather.

Diagram of the urban high island concept

The kind of shade that trees provide differs from the shade of a human-made structure, such as a building. Not only do trees shield people and buildings against the intense direct rays of the sun, they also cool the surrounding air through the biological process of evapotranspiration. Both are important to people’s quality of life. About 45% of a tree’s shading effect comes from its trunk and branches that block solar radiation—the rest comes from its leaves.

Cooling derived from evapotranspiration can equal the cooling produced by a tree’s sun-blocking capacity. A mature tree with a 30-foot crown transpires approximately 40 gallons of water per day into the surrounding atmosphere. This leaf-based evapotranspiration is a heat-absorbing process which cools the surrounding air.

According to research from the US Department of Agriculture, “The net cooling effect of a young, healthy tree is equivalent to 10 room-size air conditioners operating 20 hours a day” and “trees properly placed around buildings can reduce air conditioning needs by 30%.

What kind of shade tree is planted, how far from a building it is planted, and the side of the building where it is planted all affect its cooling effect. Trees growing  on the west and east sides of a building, respectively, have been found to reduce the amount of electricity used for air conditioning the building the most.

Aerial photographic surveys of urban tree cover can be helpful in determining where the planting of trees can yield the greatest energy savings.  It is important that a tree canopy’s shadow intersect the targeted building’s surface—which illustrates how important the distance a tree is planted away from a building is to its cooling effects and the resultant energy savings.

From an energy conservation perspective, the ideal shade tree is said to be one that is 25- to 50-feet high, has comparatively dense foliage with a high canopy, and exhibits a spreading growth habit.

Cities are often notorious for their poor air quality. Shade trees can help to filter the air and mitigate smog. One research study showed that the air immediately under the canopy of a large urban shade tree was less polluted with noxious gases and particulates, and thus significantly more healthful than the air 100 feet away from the tree.

Trees have also been shown to be capable of stripping from 9%  to 13% of total suspended particulates from the air that passes over their boughs. Exhaust from internal combustion engines and smokestacks in big cities can be a major contributor to human allergies and respiratory illnesses. Particles of ash in the air along with the fumes from burning fuels can also cause serious harm to the human throat and lungs.  Thankfully, shade trees are natural air filters.

Way back in 1844, the New York City Board of Health recognized shade trees as “improvers of city air” and it highly recommended their planting.  Today’s research has shown that the trees are a city’s lungs metaphor is far more apt than simply figurative. One research study found that children who live on tree-lined trees suffer from asthma less often than children who live in treeless neighborhoods.

Trees are the lungs of the Earth

The year 2011 was declared the International Year of Forests by the United Nations. Today, the only forests that more than half of the world’s population experience are urban forests. The National University of Singapore has predicted that: “By 2050, Asia is likely to have over 60% of its population living in cities. Asia will also have the largest number of megacities (over 10 million people) this century. We contend that trees are the most important green infrastructure that a big city needs.”

Once shade trees are planted in a city, they need to be maintained and replaced over time. For example, the city of Paris, France, has ~95,000 street shade trees.  Each has one has an affixed radio frequency ID tag (RFID tag)  with Geographic Information System  (GIS) data so city foresters can keep track of each tree—as to its planting, watering, fertilizing, transplanting, pruning, phytosanitary condition, over-all health, felling management, stump and root removal, and replanting status.

RFID- tagged tree data collection

 

Shade trees are, indeed, a vital investment a city makes for its people and the quality of the environment in which they live. Singapore knows it is much wiser to be a little green dot than a little red dot!

 

 

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 About our Guest Writers:

DR. JAMES H. WANDERSEE — botanist, FLS, FAAAS–is the W.H. LeBlanc Alumni Association Professor of Biology Education in the College of Education at Louisiana State University and Chair of the Teaching Section of the Botanical Society of America. His website is EarthScholars.com

DR RENEE M. CLARY—geologist, FGS–is the Director of the Dunn-Seiler Geology Museum and Assistant Professor of Geoscience Education in the Department of Geosciences at Mississippi State University. Her website is EarthScholars.com

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By the  authors James H. Wandersee & Renee M. Clary on EWTT:

1. How can I convince my friend to use the stairs?

2. Have You Thrown Something Away Today?

3. Plant Blindness: What research says

4. The Bridge Between Ecological Knowledge and Green Living

5. Teach Me About Soil

6. A Green Lesson from Mumbai about Food Packaging 

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Posted by on Mar 1 2012. Filed under Biodiversity, Green Cities/Communities. You can follow any responses to this entry through the RSS 2.0. You can leave a response or trackback to this entry

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