WHY THE SYCAMORE SHEDS ITS BARK
Why is the sky blue? Why is the earth round? And why do sycamore trees shed their bark?
OK, the last question might not be as existentially important as the first two, but it might be even harder to answer. Sycamore, or rather its hybridized cousin, the London planetree, is the second most common street tree in New York City (18.4 percent, according to the 1996 Street Tree Census) and a trademark tree in our City parks. Therefore, New Yorkers often ask about them. When the question about shedding bark was recently posed to us at Central Forestry and Horticulture, it sent us scrambling.
The American sycamore attains the largest girth of any tree native to Eastern North America. In colonial times, families used hollow sycamores as temporary shelter, and a single trunk cavity was known to hold up to 15 men on horseback! The current record holder in Ohio is 15 feet in diameter. The sycamore is very long lived, sometimes reaching 500 years of age. After 200 or 300 years, it becomes hollow. Sycamores grow in rich, alluvial or bottomland soil, usually along river banks and streams. Sycamores prefer well-drained soils, but tolerate periods of waterlogged, anaerobic conditions (lacking oxygen). Urban soils are notoriously compacted with poor aeration, so this natural adaptation allows sycamores to function well in the urban environments.
American sycamore and London planetree are very similar and are easily identified by their classic bark pattern. The American sycamore has one seedball hanging from a long stalk, while the London planetree has seedballs in pairs. The thin, peeling bark is unmistakable, with patches of brown, green, gray, and white that resemble army camouflage. The bark is brittle and can’t accommodate the fast growth and annual diameter accruals of the trunk and branches, so it cracks and exfoliates. According to the United States Forest Service, low winter temperatures may injure the bark and cause excess sloughing. However, New York City property owners who maintain their lawns and sidewalks know that the tree sheds bark year round and throughout its life.
But why? Digging deeper, we get more questions than answers. Is there an evolutionary reason why sycamore has thin, shedding bark? Why do some thin-barked trees shed while others don’t? Is there an environmental survival strategy in effect?
"This is not an issue that has received much attention," says Dr. Marc Abrams, Professor of Forest Ecology at Penn State University. "The only explanation I have ever heard for exfoliating bark, in general and not specific to sycamore, is the rapid growth idea. On the other hand, shagbark hickory is not particularly fast growing."
Exceptions abound. One hypothesis is that thin bark is an adaptation to the floodplain environment. Water conservation, an advantage that thick bark offers, is not necessary with the continual availability of moisture. Thin bark may conversely allow for increased transpiration of water through and out of the tree’s system, speeding growth and ultimately leading to exfoliation. Additionally, the anaerobic soil conditions may cause a need for compensatory oxygen or other gas exchange between the trunk and the atmosphere, hence a need for thin, newly exposed bark. However, green ash and pin oak are floodplain trees that don’t exfoliate, making the theory inconsistent. Silver maple is another floodplain tree that does shed, but has thick bark.
Another theory is that trees exfoliate as a protective strategy against herbivory. Frequent shedding may prevent fungi, parasites, and epiphytes (mosses and lichens) from persisting on their trunk and stems. Smooth barked trees lack the standard protection from herbivory given by thick bark, so they must have another strategy to protect themselves. Fungi and epiphytes thrive in the moist bottomlands where sycamores have evolved, lending some credibility to this theory. But how do we explain beech, yellowwood, and other trees with thin bark that don’t peel?
One last reason might be connected to the sycamore’s ability to conduct photosynthesis, as pointed out by tree biologist John A. Keslick, Jr. "This tree has a unique cortex that as the outer bark is shed then this tree can photosynthesize even with no leaves on the tree." Perhaps shedding bark is not just coincidental but necessary for the increased photosynthesis along the trunk and branches, generating an increased growing season that contributes to rapid growth and survival.
Says Dr. Abrams, "We can always blame genetic variation when we have no where else to turn." Sometimes a simple question can be deceptively complex. Amazingly, none of the tree physiologists we consulted had an answer, or even knew of scientific studies that had explored the question in any depth. The three theories we’ve advanced – floodplain adaptation, herbivory protection, and bark photosynthesis – await a Ph.D. candidate’s rigorous analysis.
Written by Douglas Still and Fiona Watt
FOR THE DAY
"The living language is like a cow-path: it is the creation of the cows
themselves, who, having created it, follow it or depart from it according to
their whims or their needs. From daily use, the path undergoes change. A cow
is under no obligation to stay."