Shade Trees on the Playground


Over the last decade or so, educators, parents and doctors have begun strenuously emphasizing the need for children to get outside and play. Often, the driving force is obesity prevention, but children also benefit psychologically from spending time outdoors.

The Problem with Good Intentions

There is a small problem with this goal. Increased time spent outdoors (particularly in California) means an increase in sun exposure, and increased sun exposure carries significant health risks. According to the Centers for Disease Control and Prevention, as many as 90 percent of all melanomas may be caused from exposure to ultraviolet light. (Centers for Disease Control and Prevention, 2002) Accordingly, it is important to provide children with access to outdoor playgrounds, but those playgrounds should be shaded.

Trees: The Obvious Answer

Trees are the easy answer, and given the additional benefits they provide – such as cooling the area and reducing the amount of particulates in the air – it is hard to envision anything else being as well suited for the task. However, shading a playground is not as simple as buying a few seedlings, sticking them in the ground and waiting for the shade – you must carefully select and place the trees to maximize the amount of shade provided, and minimize the potential for failure and increased labor.

Principles of Placement

The sun’s rays approach California from the South (check out this neat solar calculator to find out exactly where the sun is relative to your location). Yes, the Sun traces a generally east-to-west path across the sky, but the path lies offset to the south. In the summer, the arc occurs higher in the sky, while in the winter, the arc remains closer to the southern horizon. Use this information to your advantage. By placing a group of trees along the southwest side of the playground, the trees will block the sun’s afternoon rays. While trees on the northeast side will certainly provide numerous tangible and intangible benefits, they will not provide very much shade for the playground.

Local Lumber

Because they usually require little supplemental water and do not harm the habitat, native species are the preferred choice for shade-tree plantings. If native trees are not available or desired, select non-native trees and plants that have a low potential for spreading. Check out the Cal Poly Tree Selection Guide for more information about specific species.

Water Use

Avoid sabotaging your efforts by planting species that have significant water requirements. Instead, choose species like redbuds, wax myrtles or any of the native California oak species, which have evolved to tolerate periodic droughts. Be aware that some oaks are “drought deciduous,” meaning that they drop their leaves in response to droughts. Avoid these trees, as they will often lose their leaves when you need shade the most.

Changing of the Seasons

One of the benefits of using trees to provide shade is that with careful selection, you can design a shade concept that changes with the seasons. For example, you can use deciduous trees in places that would benefit from winter sunshine, yet use evergreen species in places in which shade is always desirable. Additionally, because of the educational value alone, it is always a good idea to incorporate a few trees that display excellent fall color changes.

Wildlife Considerations

It is important to consider the effect the new trees will have on the local critters. While some animals may be welcome additions to the playground, others tend to spoil the party. For example, oak trees are likely to attract squirrels, jays and woodpeckers, while blackberries, cherries and black locusts may be too popular with bees. While bees are an important part of the terrestrial ecosystem, and children should learn about their value, playgrounds are not the ideal place to teach them these lessons.



Centers for Disease Control and Prevention. (2002). Shade Planning for America’s Schools.


The Limits to Tree Height: Why Redwoods Don’t Grow 500 Feet Tall

To a large extent, the ultimate height of a tree is determined by its genes. Most of the remaining influence comes from the quality of the site in which the tree is growing. In other words, an oak tree may be genetically predisposed to reach about 60 feet in height, but the amount of sunlight and rain it receives determines if it will be a 40-foot-tall runt or an 80-foot-tall behemoth.

However, even trees with great genes and ample resources are still constrained by physical factors, such as gravity and the surface tension of water. It appears that these physical constraints create a cap on the ultimate growth of trees. Accordingly, even California’s tallest redwoods (Sequoia sempervirens), British Columbia’s tallest Douglas firs (Pseudotsuga menziesii) or Australia’s tallest eucalyptus trees (Eucalyptus regnans) are unable to grow much taller than they already do.

How Trees Drink

First, a review of some basic tree physiology:

  • Water enters a tree via the roots, and then travels throughout the trunk, branches and twigs, via an assortment of vascular tissues, until if finally reaches the leaves.
  • Water escapes the leaves (a process called transpiration) via small holes in the leaf surface, called stomata. The individual stoma open and close to alter the rate of this process, which varies over the course of the day.
  • The leaves use a very small percentage of the water reaching them for basic cellular processes and photosynthesis, but most escapes into the atmosphere.

Passive Pumping

The movement of water through a tree is not an active process. Trees do not “suck” water from the ground, nor do they “pump” water up their trunks. Instead, trees rely on the surface tension of water and something called capillary action to draw water up through the tree. The process works because of both the attractive forces between individual water molecules and the attractive forces that occur between water molecules and other molecules.

Surface tension is produced by the cohesion of individual water molecules – this is why water forms droplets instead of spreading out. Capillary action is exhibited when liquid is placed in a narrow tube. The water molecules are attracted to the molecules in the tube (adhesion); but normally, gravity pulls harder on the water than the tube does. However, if the tube is narrow enough, the adhesive forces can overcome gravity.

Sponges illustrate this principle well: If you place a sponge (which is full of tiny capillaries) half-way into a glass of water, it will draw some of the water up into the sponge via the same mechanisms that trees use.

The water transpiring from the leaves creates a void, which cohesion and adhesion work to fill. This creates tension that further helps to draw the water up the tree. The rate at which water transpires from the tree influences the tension placed on the water column. The faster the water exits the tree the greater the tension on the water column.

Tree Height Limits

As you can imagine, it takes far more tension to raise water to the top of a 400-foot tall tree than it does a 40-foot-tall tree. Ultimately, some trees reach heights where the tension necessary to draw up the water becomes too great. When this happens, the column of water breaks down and bubbles may form in the capillaries – these bubbles break the surface tension of the water and leave voids in the system. The voids cause the capillaries to stop functioning properly, and become useless. This leads to a reduction in vigor, and prevents the tree from growing any taller.

In 2004, George Koch and three colleagues examined this mechanism in order to determine the theoretical maximum height of trees. By studying some of the tallest trees in the world (including California’s own “Hyperion,” which is the world’s tallest documented tree), the team concluded that the tallest possible trees may be able to reach about 425 feet, but not much more. Were trees to grow taller than this, the tension would simply be too great. (George W. Koch, 2004)

Problems with Climate Change

The method by which trees drink not only limits their ultimate height; it also limits their ability to survive climate change.

The tension on the water column is dependent on many factors aside from the height of the tree. These factors include air temperature, solar radiation, groundwater availability and wind speed. Generally speaking, when the air gets warmer, drier or windier, the water evaporates from the leaves more quickly. This increases the tension on the water column, and as we have seen, excessive tensions can cause permanent harm.

A 2012 study by Brendan Choat and 23 other researchers showed that the threshold at which these sorts of problems occur is remarkably close to the tensions trees normally produce. This held true for a wide range of tree species, across several different habitats. This means that if global temperatures rise relatively little, trees are likely to suffer greatly, as many will begin to transpire at rates that will cause them irreparable harm.  (Brendan Choat, 2012)


Brendan Choat, e. a. (2012). Global convergence in the vulnerability of forests to drought. Nature.

George W. Koch, S. C. (2004). The limits to tree height. Nature.



A Conservation Success Story: Protecting a Rare Albino Chimeric Redwood

Thanks to the efforts of local citizens, historians and conservationists, one of the rarest trees in the world has been granted a reprieve. Now, instead of being cut down to make room for a railway expansion project in Cotati, California, crews will move the tree about 450 feet from its current location, where it will hopefully live for years to come.

The project, which has an estimated price tag of $150,000 according to NBC Bay Area, was deemed appropriate as the “Cotati redwood,” is one of only a handful of albino chimeric redwoods in the world. (Fernandez, 2014) Clad in a patchwork of green and white leaves, the unusual tree intrigues onlookers and tantalizes scientists, who are eager to explore the tree’s hidden secrets.

Why Is This Tree So Special?

The first albino redwoods were likely discovered in 1866, but in the nearly 150 years since then, scientists have only documented about 230 similar trees in the State of California, according to “National Geographic.” (Jaret, 2014)

Most of these albino redwoods are extremely small and frail; unlike their towering relatives that soar 300 feet or more into the air, most albino redwoods are decidedly shrub-like. Yet, the Cotati tree is unlikely to be confused with a shrub. The tallest such mutant ever discovered, the unique redwood stands over 50 feet tall and has a crown that is 30 feet in diameter.

Most albino redwoods are parasites that derive sustenance from the roots of other redwoods. But strangely, this special tree stands alone, isolated from other redwoods. Combined with the tree’s tendency to produce both male and female cones – the only albino chimera documented to do so — terms such as “one of a kind,” are potentially appropriate for this special tree.

treesProblematic Pigment

The term “albino” is a colloquial term that is usually used to describe amelanistic organisms. Plants do not produce melanin, but most produce chlorophyll. Chlorophyll is the pigment that enables plants to conduct photosynthesis and gives them their green color. Therefore, “albino” plants appear white or yellowish, as they fail to produce chlorophyll.

Without chlorophyll, plants cannot convert sunlight into energy; accordingly, most die early in life and are only rarely observed. This is why most albino redwoods are parasitic: They cannot produce their own food. Instead, they survive by stealing nutrients from the roots of other, healthy redwoods. The ability to derive sustenance from the roots of other plants is rare, and explains why redwoods are capable of producing albinos that survive, while most other albino plants die as soon as the energy reserves from their seeds run out.

However, the Cotati redwood does not derive resources from a host tree.

A Tale of Two Trees

The key to the relative success of the redwood in question – and the handful of others like it — lies in its genes. The Cotati tree is a chimera, meaning that it has the DNA of two different trees located in its meristems (areas of rapid cell division in a tree). In essence, this single tree is comprised of two different individuals.

This unique compliment of genes means that some of the tree’s leaves get ordinary genetic instructions. These leaves are green in color and produce food for the tree. The mutant genes supply the code for other leaves, so they fail to produce chlorophyll, and are white in color. The combination of white and green leaves gives the tree its patchy look, and likely provides the explanation for how it survives: The numerous green leaves produce enough energy to support the entire tree, including the white leaves.

Scientists are still in the dark about many details of the phenomenon, which further illustrates the importance of protecting and studying this tree. Fortunately for the world, a local landowner named Pete Tapian planted the unique specimen about 70 years ago, where it still stands today, at least for the time being.

Visiting the Rare Redwoods

The exact location of most albino redwoods is closely guarded to protect their wellbeing. While the Cotati tree is probably farther away from the Los Angeles area than most people would care to travel (approximately 420 miles), there are quite a few normal redwood trees in the greater Los Angeles area. Check out this map to see a few of the closest documented specimens.


Fernandez, L. (2014, July 28). Tallest Albino Redwood Chimera Tree in Wine Country to be Saved, Moved at Cost of $150,000. Retrieved from

Jaret, P. (2014, March 19). Rare “Albino” Redwood May Hold Clues to the Super-Trees’ Longevity. Retrieved from

Lapidos, J. (2009, January 6). How Many Albinos Are in Tanzania? Retrieved from


Trees Don’t Just Save Money, They Save Lives

Most people realize that trees provide a number of environmental and economic benefits, but few realize that trees also provide health benefits for those living near them. However, a recent national study, conducted by the United States Forestry Service and the Davey Institute, is likely to change that. Given that Los Angeles is bathed in the country’s most polluted air, this study is especially important for those of us living and working in Southern California. (BARBOZA, 2014)

The Unique StudyTree with form of human lung

Unlike many other studies, which have concentrated on quantifying the ability of trees to reduce energy costs, sequester carbon or improve water quality, this study sought to quantify the physiological benefits that humans enjoy, thanks to the air-cleaning abilities of trees. As explained in the study, trees not only help to reduce the costs associated with health care, but they actually reduce the rate of illness in the community and actually save lives.

The Results

According to the study, trees reduce national healthcare expenditures by nearly 7 billion dollars each year. More impressively, the study found that trees eliminate nearly 670,000 cases of acute respiratory symptoms. Nevertheless, the most impressive statistic produced by the study showed that trees were responsible for preventing approximately 850 human deaths each year.

How They Help

Trees provide these health benefits (among other ways) by helping to remove pollution from the air. Trees remove particulates in the air, such as soot, easily enough – the pollutants simply stick to the bark, leaves and branches of the trees. Eventually, the particulates fall to the ground or they are washed away with the rain. Trees remove toxic gases, such as carbon monoxide and sulfur dioxide, by drawing the pollutants into small pore-like openings called stomata. Once inside the trees, different species safely quarantine or discard the substances in a variety of ways. Additionally, trees reduce the local air temperature, which reduces the amount of pollutants entering the atmosphere by reducing the rate of the pollution-causing chemical reactions take place.

Geographic Variation

While 34 percent of the continental United States is covered with trees, the amount of coverage varies from one area to the next (as of 2006, 21 percent of Los Angeles was covered by tree canopy). (Million Trees LA, 2006) While some urban environments, particularly those in the southeast and along the Atlantic seaboard, have robust tree populations, most urban areas have fewer trees than rural areas do. Ironically, these urban areas are the ones most in need of the services of trees. While the bulk of the air pollution removed from the air by trees takes place in the relatively unpolluted air of rural areas, the greatest positive health effects (in terms of both reduced illnesses and reduced costs) occurred in urban areas.

You can read the entire study here (PDF).




BARBOZA, T. (2014). L.A., Central Valley have worst air quality, American Lung Assn. says. Retrieved from

David J. Nowak, ,. S. (2014). Tree and forest effects on air quality and human health in the United States. Environmental Pollution.

Million Trees LA. (2006). Tree Canopy Analysis. Retrieved from



The Psychological Benefits of Trees and Natural Areas: Beyond Supposition

Some of the most important benefits that trees provide to humans are psychological or emotional in nature. While countless authors, arborists and philosophers have mused that trees help to clear the mind and invigorate the soul, it is important to realize that these claims are not merely conjecture. Scientists have collected plenty of evidence that supports the notion that trees and natural areas improve one’s mood and provide people with an increased sense of well-being.

boy and girl sittingSimple but Profound

In 1984, Doctor Roger S. Ulrich published a study, which sought to determine if trees (and natural landscapes in general) provided tangible benefits for people recovering from surgery.

Between 1972 and 1981, Ulrich analyzed the recovery details on patients who had recently had their gallbladders removed. Ulrich grouped the patients in pairs, based on criteria such as age, gender and preexisting health conditions. One member of each pair stayed in a recovery room that provided a view of a natural-looking patch of deciduous trees, while the other member recovered in a room that only provided a view of a brick wall.

Ulrich’s results were eye opening, and forever changed the nature of hospital architecture and landscaping. In the study, Ulrich compared the length of the patient’s stay, medications administered, complications and the nurse’s notes regarding patient care. Ulrich found that those patients whose windows faced the trees recovered significantly faster, exhibited fewer complications and required fewer pain-relieving medications, than their brick-wall-facing counterparts did. (RS Ulrich, 1984)

Standing on the Shoulders

In the years following publication of his study, numerous researchers have confirmed and expanded upon Ulrich’s work. One such example was published in a 2001 issue of “Environment and Behavior.” In the study, researcher Rachel Kaplan found that residents of suburban apartments whose windows overlooked natural areas experienced a greater sense of well-being and satisfaction with the neighborhood, than those whose windows overlooked man-made structures and scenes. (KAPLAN, 2001)

In 2007, Richard A. Fuller and his colleagues investigated the relationship between biodiversity and the psychological benefits that higher biodiversity may provide. The results of the study found that the participants (laypersons) were able to discern broad patterns in plant biodiversity. Perhaps unsurprisingly, the higher the biodiversity of the green space, the more benefit the participants derived. (Richard A Fuller, 2007)

Climbing Up the Right Tree

However, none of this should imply that trees and natural areas are only helpful in a passive context – interacting with trees can also produce significant psychological benefits. Take for example, a 2006 study, conducted in Japan and published in the journal “Urban Forestry & Urban Greening.” During the course of the study, researchers administered psychological tests to people who climbed concrete towers and trees. The researchers found that the climbers scored better on a battery of psychological tests after climbing trees than they did after climbing the cement towers, despite the fact that the towers were in the same location and were of equal height. (John Gathrighta, 2006)

Just the Beginning

While it is quite clear that trees and natural spaces provide concrete psychological benefits to the people living near them, scientists still have much to learn about the interrelationship between humans and trees. Nevertheless, this much is clear: Spending time around trees is not only fun, but it is also good for your psyche. If you live in the Los Angeles area, check out this list of local nature centers, which can help you get out of the city and spend some time healing in the company of trees.


John Gathrighta, Y. Y. (2006). Comparison of the physiological and psychological benefits of tree and tower climbing. Urban Forestry & Urban Greening.

KAPLAN, R. (2001). THE NATURE OF THE VIEW FROM HOME. Environment and Behavior .

Richard A Fuller, K. N.-W. (2007). Psychological benefits of greenspace increase with biodiversity. Royal Society Biology Letters.

RS Ulrich, e. a. (1984). View Through a Window May Influence Recovery from Surgery. Science.


Healthy Forests for Healthy Water

small runoff waterThough seemingly unrelated, the health of a forest directly affects the health of the surrounding waterways. While the trees depend on the local water supply for survival, they perform valuable filtering services on this water, which helps to keep the local supply clean. One of the most valuable ways trees help to protect the local water bodies is by reducing the amount of runoff water flowing across the landscape.

Reining in Runoff

While not a type of pollution in the strictest sense, excess runoff water is a serious problem for local watersheds. Excess runoff water can overwhelm creeks and streams, causing widespread flooding. Additionally, it accelerates the rate of erosion, rapidly undermines stream banks, undercuts roads and can cause significant destruction to the local infrastructure.

The sediments collected in the runoff water eventually make their way into streams, making the water cloudy, which can harm many aquatic organisms. In urban areas, runoff water collects numerous pollutants in the form of oils, fuels and debris, which ultimately flow into the rivers, ponds and swamps of the region.

Mechanisms of Mitigation

Trees – and more importantly forests – reduce runoff water and the amount of sediment reaching local streams and creeks in a variety of ways.

  • They reduce the amount of groundwater in the area through the process of transpiration. This allows the soil to absorb more water during the rain, thus reducing the amount of runoff water reaching streams
  • Trees collect water on their surfaces, and release it slowly to the ground, which helps to reduce the amount of runoff water in an area.
  • The growth pattern of tree roots and the activities of the microorganism near their roots encourage increased rates of water infiltration into the soil, which further reduces runoff water.
  • The trees’ roots help retain soil and prevent erosion, which protects yards, roadways and – most importantly – stream banks from collapsing.
  • The canopies of trees protect the soil from the pounding of raindrops, which reduces the amount of sediment that reaches local waterways.

Complicated Considerations

One common way that trees are used to reduce the amount of runoff water flowing across parking lots, driveways and other small paved areas is by planting a small group of trees near the low point of the surface. This can help you reduce the runoff from your property and increase the property’s value at the same time. The trick for those living in the Los Angeles area is to select species that are native to the state – and thus equipped to deal with periodic droughts – yet still able to cope with the periodically saturated soil that they will be exposed to. California laurels (Umbellularia californica), western hemlocks (Tsuga heterophylla) and California buckeye (Aesculus californica) are excellent choices, but if you prefer non-native species, consider black gums (Nyssa sylvatica) or bald cypresses (Taxodium distichum).

Healthy Water through History

Humans have understood that trees affect water quality for hundreds of years. According to, residents of Rio de Janeiro began advocating for the protection of the area’s coastal rainforest to prevent declining water quality in 1658. Later, after a series of droughts reduced the available water in the mid-1800s, reforestation efforts were begun in the region of Tijuca, along the Atlantic Coast. (Messenger, 2010) Today, the Tijuca National Forest is one of the largest urban forests in the world, covering approximately 12 square miles, and helping to provide clean drinking water for Rio’s nearly 12 million residents.


Messenger, S. (2010, August 22). World’s Largest Urban Forest Was Planted by Hand. Retrieved from

Spreading Emerald Ash Borer

The Emerald Ash Borer (EAB) has become a major problem in the United States due to its quick spread and the deadly impact it has on trees around the country. First seen in the Michigan area in 2002, it is believed that it was introduced into the United States in firewood transported from Asia. Since this initial introduction, they have been responsible for over 50 million tree deaths, most often ash trees. reported that the beetle is now found in Connecticut, Illinois, Indiana, Iowa, Kansas, Kentucky, Maryland, Massachusetts, Michigan, Minnesota, Missouri, New Hampshire, New York, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia, and Wisconsin. The spread of this parasite is most often caused when infested firewood is moved through human activity to uninfested areas – the beetles usually don’t fly more than a half-mile. To try to slow the spread of this parasite, some counties in Iowa have been quarantined, which means that a permit is needed to move firewood out of the county. The Emerald Ash Borer was first sighted in Iowa in 2010 and has quickly spread across the state. Iowa has approximately 52 million ash trees located in rural areas and about three million ash trees in urban settings. After the beetles infest a tree, it takes two to four years for the tree to die. Because of the length of time it takes to show signs of infestation, it is unclear how much damage has been done before quarantines have been put in place. The beetles leave D-shaped entry holes in any tree they inhabit, which makes this a telltale sign of infestation. Although quarantines have been put in place, officials haven’t yet worked out all the logistics of the quarantine. “We hope in future weeks we will have the quarantine worked out, we will have worked with industry to fully inform everyone and hopefully draw up a good quarantine plan,” says Iowa entomologist Robin Prusiner.

Coral Tree Falls on San Vicente Boulevard

A large coral tree fell onto San Vicente Boulevard in Brentwood. The Chair of the Brentwood Community Council Board, Nancy Freedman, said that the tree is No. 15 in median No.3 at the corner of San Vicente Boulevard and Gorham Avenue. Coral trees are extremely drought tolerant and in an email Freedman noted that she had observed a pool of water from the sprinkler system next to the fallen tree, which may have contributed to the tree’s fall. Coral trees are prone to failure from wet conditions and over-watering. This is not the first tree to fall on this median and there are continuing efforts to maintain these trees.


Oak Trees

Donna Giustizia in Ontario, Canada, is asking that the oak trees near her daughter’s school be cut down due to the risk of a nut allergy reaction from the acorns off the trees. Because she felt strongly about the possible hazards, she appeared before the Vaughan, Ontario, City Council to ask the council to remove the trees. Giustizia heads the school’s allergy committee, and although the school says it is nut-free, she feels that the school isn’t doing enough to protect its students.

The Vaughan City Council has said that they will prepare a report about the nut problem that will be read at their next meeting. Giustizia has said that she is not requesting that the entire town become nut free, but wants the school to be free of all nuts, including acorns.

When asked about the possibility that acorns are a hazard to children with nut allergies, Dr. Clifford Bassett, medical director of allergy and asthma care at New York University, said he has never encountered a case of an allergic reaction by playing with acorns found on the ground. “There’s no relationship between acorns and peanuts,” said Bassett.


Drought Causes Tree Death and Limbs to Fall

Tyler State Park in East Texas has seen an increase in trees dying, due to drought in the region. Although more trees are dying, the state park says that it has been difficult keeping up with the necessary tree maintenance and removal because the state legislature took away some of their funding this year. An estimated 250 dying trees have been cut down in the park from about April through June to keep campers safe.

One tree that was not cut in time dropped a large limb onto an RV in the park. Neal Williams, his wife, and two grandsons were asleep in the RV at the time the branch fell. Although they were uninjured, the RV suffered minor damage. Williams and park officials were surprised that the limb fell in calm weather from what appeared to be a healthy tree.

Tyler State Park Superintendent, Bill Smart, says, “Since most of the trees die in winter, we don’t know until spring when the trees were supposed to come back and out, and they weren’t coming back out.” Smart warns that although they are doing the best they can, they need people to be aware of the condition of the trees and of the possibility of limbs falling.