Sunlight-Sensitive Park Resources and Shadow Impacts
Sunlight-sensitive park resources are those resources which are dependent on sunlight to maintain the overall usability and/or health of a park space, whether it be for human activity or horticultural needs.
As the city develops, the extent and duration of shadows cast increases. Direct sunlight exposure becomes all the more important as a resource for people and nature, particularly in the city’s central greenspaces, the Boston Common and Public Garden, which are used by millions of people each year as places to relax, gather with the community, walk to work, and recreate.
In considering the impact of shadows on these parks, it is necessary to assess how they affect the growth cycle and sustainability of the parks’ natural features, as well as the comfort and enjoyment of their users.
The issue of human use and comfort is particularly important during the cold winter months when there is less available sunlight, especially during morning and afternoon commuting hours, when thousands of people pass through these parks daily. Human-related sunlight-sensitive resources during the warm months include use of the wading pool at the Frog Pond in the Common and the lagoon and Swan Boats in the Garden.
Horticulture and Shadow Impacts
Trees and turf need 4 – 6 hours of direct sunlight.
Less sunlight = less photosynthesis = less energy for trees and turf to grow
Full day and yearlong analysis of cumulative shadows show that the Common and Garden are under significant shadow pressure
The Tremont and Boylston edges of the parks in particular experience significant shadow pressure
When trees and turf are in the shadows of buildings, soil surface temperatures may not reach normal levels.
A lag time in warmth, and a shaded condition that can keep soil wetter, favor disease development. This is a contributing factor in the root rot some trees have suffered in the Garden, and the decline and removal of trees in the Tremont/Boylston corner of the Common
Shade impacts the success of seed growth, with colder soil temperatures slowing and shortening their growing period.
Grass is less tolerant of shade than trees. It is easy to grow grass, and easy to kill grass.
These days our parks are quiet, often covered with a blanket of snow, the trees dormant and flowers having long disappeared…is the staff on vacation? Not at all! The winter months are full of activity. Friends Project Manager Bob Mulcahy, Collections Care Manager Sarah Hutt, and consulting arborist and soil scientist Normand Helie are hard at work planning for the year to come, and overseeing winter tree work.
Sarah and Bob evaluate every piece of sculpture throughout the year, planning out the annual cycle of care and cleaning. Together they prepare the budget, receive proposals from conservators, ready contracts for the proposed work, and notify Boston Parks and Recreation Department, the Boston Arts Commission and Boston Landmarks Commission about upcoming annual maintenance. Once the contracts are signed and paperwork filed, Bob and Sarah will work with the conservator contractors to schedule when the Women’s Memorial, among many, will get the TLC they need.
The ginkgo tree is the oldest living tree species in the world. It is a botanical “living fossil,” dating to the time of the dinosaurs -250 million years ago. It was the very first seed-producing tree, which means it was the first tree that could live and reproduce on land. Ginkgo biloba is a “single species,” which means it has no living relatives, and fossil ginkgo leaves have been found on every continent, but the tree primarily survived in the protected valleys of China. In the eighteenth century, European visitors discovered the ginkgo on the grounds of Buddhist temples in China, Japan, and Korea.
Ginkgos are well represented in two of our parks, with thirteen gingko trees on Boston Common, and eight in the Public Garden, both male and female trees. The first Ginkgo biloba in the parks was transplanted to Boston Common near the Joy Street steps in 1835. Already 40 feet tall at the time, that tree was moved to the Common from the nearby estate of Mrs. Gardiner Greene, daughter of John Singleton Copley.
The male tree produces small, cone-like structures that hold pollen. When female trees are about 25 years old, they produce small fruits with a fleshy, apricot-colored skin around the nut inside. The pulp is quite pungent when mashed and has a strong odor that many find unpleasant.
Long valued for aesthetic, religious, and medicinal reasons, it is said that Confucius studied under a ginkgo tree, giving it special meaning to Buddhists. The nuts are thought to have healing properties and can be eaten as a special delicacy. Extracts of the leaves are also used in medicines to enhance memory or stimulate blood flow.
Due to its resilience, the ginkgo has become a popular ornamental tree in the last fifty years. The tree tolerates pollution, extreme temperatures, road salt, and the confinement of being planted along a street. It is also both pest- and disease-resistant. It is easy to see why the tree is a symbol for longevity in Asia. There is an individual Chinese ginkgo said to be 3,500 years old.
The ginkgo is easy to identify due to its unique, fan-shaped leaves, with veins that fork outward from a central point. Its common name is “maidenhair tree” because the leaves resemble the leaves of a maidenhair fern. Carl Linnaeus, the renowned Swedish naturalist, gave the tree its Latin name, “Ginkgo Biloba,” which refers to the lobed shape of its leaf. In autumn, the leaves turn a beautiful golden hue before falling to the ground, typically all at once, resulting in a wonderful carpet of yellow surrounding the tree
Look for the ginkgos as you walk in the Public Garden and the Boston Common.
The Friends has been funding the expert care of trees since 1970 as part of our mission to preserve and enhance the Boston Common, Public Garden, and Commonwealth Avenue Mall. This work would not be possible without the financial support of our Members. We are always delighted when Members express a genuine interest in learning more about the details of the work we do. We noticed an increase in questions about tree care coming in over the past few months, and in particular inquiries about our work related to the elm bark beetle and Dutch elm disease, which seem to have been sparked by our Members Reception presentation (Digging In: Beyond the Roots of Urban Tree Care). We asked our contractor Christine Helie to explain some of the work she does for us. She is an entomologist and field scientist who works with her husband Normand at The Growing Tree. Chris is directly involved in developing an Integrated Pest Management program to preserve the mature and young elm trees in our parks. Here is what she had to say:
Among the trees in the Public Garden, on the Boston Common and on Commonwealth Avenue, is a unique collection of elm trees. This valuable assortment of European, American and Asian elms are susceptible to Dutch elm disease (DED). This disease is caused by a fungus that compromises the conductive tissue of the tree and eventually kills it. The primary vector of the fungus is the European elm bark beetle. Through its breeding and feeding behavior, this bark beetle transfers DED from diseased trees to healthy trees.
In 2012, with the support of Greg Mosman, Tree Warden of the Boston Parks & Recreation Department and on behalf of the Friends and its tree care program, a monitoring and management system for this insect was designed as part of a new elm tree preservation program for the mature and young elm trees in the three parks. The manner in which insects are monitored and managed can vary depending on the habitat in which they exist. For our purposes, a three sided box of plywood, painted green was built to house an 18”x25” sticky trap with a pheromone lure attached in the center.
Pheromones are chemicals produced by an organism that elicit a response from another organism. They are used by insects or animals to communicate with individuals of the same species. Depending on the type of activity, different pheromones will be used to relay a message.
For example, ants use a trail pheromone to mark a path leading to food that other ants in the colony can find and follow. However, when encountering a dangerous situation they use an alarm pheromone to warn their nest mates. The pheromones used in our beetle traps signal to both male and female elm bark beetles that this is a great spot for breeding and laying eggs.
There are over 24 traps in use throughout the parks. From the beginning, our goal was to
make them easy to access but discreet. Rather than placing unsightly posts throughout the parks, we decided to install our traps on trees at least 150 feet away from any elm trees.
Because the bark beetle is attracted to elm trees weakened by stress, one of the components in the pheromone mimics volatiles released by a stressed elm tree. As a result, the trees that we chose to place our traps on became substitute elms, luring the elm bark beetles away from the elm trees.
Pheromones are effective at very low concentrations and insect specific. This fact becomes evident when you compare some of the trees with traps to an actual elm tree. The vase shaped elm tree with upright branching is quite different from the pyramidal shaped Norway spruce with drooping branchlets.
The elm has a broad leaf with a serrated edge, whereas the spruce has needle-like foliage. The bark of an American elm tree has deep crevices that form diamond-shaped furrows, while the bark of a Norway spruce tree has thick round scales.
Regardless of these features though, the Norway spruce in the Public Garden has consistently captured high numbers of the European elm bark beetle on its trap.
Below are images showing the physical differences between Elm trees (top photos) and Norway Spruce trees(bottom).
Bark beetles appear to use different methods when locating a proper host tree. By crawling on the bark, they can sense the texture and determine whether the tree is susceptible to attack. Dispersing beetles are also guided by odors from weakened trees. From what we have observed in our program, it seems apparent that when the beetles land on a potential host, one of our stand-in elms, the odors detected override the physical clues they pick up from the tree. As a result the beetles continue to search for the source of the pheromones until they are caught on the trap or die trying to find the “elm.”
These traps have also allowed us to monitor the location, concentration, and pattern of movements of this disease host, helping to indicate the optimal times to treat, prune and, in some cases, remove a diseased tree.
Since their implementation, the elm bark beetle traps have become important tools in our fight against Dutch elm disease. The twenty four traps in use throughout the parks and surrounding areas are installed on thirteen different tree species. While these trees may be Oaks, Locusts, Maples, Lindens, or even a Norway spruce, they actually serve as substitute elms and are important allies in the preservation of our real elm tree population.
Landolt, Peter J. “Sex Attractant and Aggregation Pheromones of Male Phytophagous Insects.” American Entomologist Spring 1997 12-22. Print.
The white dusting appearing on the grassy areas of Boston Common over the next few days is not made up of snowflakes that we traditionally expect to see this time of year. Warmer temperatures have made it possible for us to fit in one more needed treatment to help trees, turf and soil on the Common. The treatment will support better root growth and development, provide plant nutrients, increase disease resistance, and correct several conditions that are causing additional stress on the plant life in this heavily used urban park.
For November our Tree of the Month is the Oak, one of only a few local trees to be the last to lose its leaves. Well into the winter season, Bostonians will be able to look up at shivering branches almost uncannily cloaked with tenacious brown oak leaves.
Oaks are an astoundingly diverse group of trees – there are 600 species all over the world, and 90 species in North America. These mostly deciduous trees hybridize easily and are sometimes difficult to identify. The wood of oaks, as well as their acorns, is high in a type of polyphenol called tannin—the source of its strength, resistance to rot and insects, and the flavor oak barrels can impart to their contents (well known to lovers of “buttery” chardonnay).
Here on the Boston Common and in the Public Garden, Pin Oaks, Red Oaks, and White Oaks are among the largest trees. The Commonwealth Avenue Mall has over 30 specimens and four species of oaks, the Garden over 20 specimens and seven species, and the Common over 50 specimens and five species.
The tree fruit—mostly acorns—carpeting the forest floor this time of year is collectively called “mast”, from the same Old English word (“maest”) that gave us the word “meat.” Bumper acorn crop years are called “mast years” and are directly associated with huge fluctuations in wildlife populations, which can then have ripple effects on populations of other creatures like ticks that feed on oak-dependent wildlife such as mice and deer.
Early humans all over the globe ate acorns, after processing them in various ways (soaking, drying, etc). Until relatively recently in human history, acorns were a significant source of calories for humanity – by some estimates up to 10 percent.
When Europeans came to North America, the old growth oak forests here were an attractive natural resource, as oak was an important building material that had been mostly exhausted in Europe. Oak was used in shipbuilding, quarter sawn for oak furniture, and prized for barreling wine and spirits. The old-growth White Oak planks of the USS Constitution withstood so many English shells that the sailors nicknamed it “Old Ironsides.”
Oak forests still thrive in Southern New England, and are characterized by dry, sandy soils, other fire adapted plants such as blueberry bushes and various pine species, and hickory species. On Cape Cod, Martha’s Vineyard, and Nantucket, the familiar low twisty maritime forests are also dominated by fire adapted oaks and their associated understory species and pollinators, some of which are rare and endangered species of moths and butterflies.
In some parts of New England, oaks face a new kind of threat – overgrazing by deer. Without many predators, deer populations have exploded in New England, and excessive browse of oak seedlings and saplings by deer is exerting pressure on successional change in the forested landscape.
As you walk through our three parks, look for urban wildlife feeding on acorns—squirrels are ubiquitous, of course, but you might also see crows, jays, ducks, geese, raccoons, opossums, and perhaps even foxes. Then take a moment to look up at the familiar lobed leaves that not so long ago shaded us on those hot summer days. We have so many reasons to feel grateful to the oak!
Claire Corcoran is an ecologist and member of the Friends of the Public Garden Board of Directors. She is a self proclaimed “tree hugger” and dedicated advocate for greenspace in Boston and beyond. Claire lives in the South End of Boston with her husband and three children.
Thanks to all who joined us for the Friends of the Public Garden 2015 Members Reception at the Four Seasons Hotel! It was a night to celebrate trees and our Friends who support them.
Roughly 200 turned out to learn how they are helping 1,700 trees in the Boston Common, Public Garden, and Commonwealth Avenue Mall. The presentation, Digging In: Beyond the Roots of Urban Tree Care, was led by Lyn Paget, Swan Boats owner and Friends Council co-chair. Panelists Margaret Porkorny, longtime greenspace advocate and Friends Board Member; and Friends Project Manager Bob Mulcahy explained the trials and tribulations these trees face while living in the heart of Boston, and how the Friends efforts continue to help them persevere in a stressful city environment.
Guests were invited to mingle over drinks and hors d’oeuvres following the presentation. Our special thanks to the Motor Mart Garage, our lead sponsor for this event.