Green Scene

Birds help spread mulberry trees all over Riverdale


Mystery solved! Last fall I was walking on Henry Hudson Parkway West just south of West 254th Street and really noticed some trees that have been growing there for years. 

The leaves were down. Therefore the trunks, leaning toward the sunlight, were my focus. The bark was light-toned and slightly corrugated. But what really caught my eye was the way the corrugations resembled a burning candle with the wax dripping downward, creating a unique, interlaced pattern.

I was quite taken with the sight and had no idea what tree I was looking at. As usual, I turned to the library service at the New York Botanical Garden, which helps members of the public with identifications. Without any leaves present, they were unwilling to venture an identification based on the bark alone, but they did tell me that the horizontal bulges which were causing the “melting effect” were burls (or galls), which can be caused by a virus, fungus or a bacterium.

Later, I was walking the same route and saw those unsightly purple blotches on the sidewalk that we see each year in June. Looking up, I could see mulberry (Moraceae morus) fruits in various stages of ripening, all over the tree branches. Now that I am paying attention, I should have no trouble identifying them in the future by their bark and leaves. 

I also realized that these trees can be seen everywhere in Riverdale. I’m guessing that the birds are dropping seeds through their poop.

The birds had not left many low-lying ripened berries, but I found a few to munch on and continued walking. 

Burls generally are rounded bulges that can be seen on many tree species throughout the neighborhood, and they can be quite large. They include maples, cherry, ash walnut and birch. 

Today, we use burl for wood inlay since these wood fibers develop with more complexity than standard wood grain of uninfected trees. It is true that, depending on the angle ordinary wooden trunks are sawn, there is some variety in the presentation of the grain. But burls are considerably more dramatic. 

Visualize a tree’s normal grain pattern as parallel strands of yarn. A burl would be thought of as a ball of yarn. It’s as though the tree’s cells went haywire and decided to tie themselves into a knot, which also makes the material very dense and resistant to splitting.

The fibers that form wood are called “xylem,” from the Greek xylon meaning wood. They act to transport water and nutrients from the roots upward to the rest of the plant. 

The second type of vascular tissue is called “phloem,” which transports the sugars produced during photosynthesis to the rest of the tree for fueling its nutritional needs with storage of the excess in different plant parts. These tissues are produced by a growing layer under the tree bark — the vascular cambium. 

New phloem forms from the part of the vascular cambium oriented toward the outside of the trunk, and new xylem on the cambium side oriented toward the center of the tree. The name “phloem” also derives from the Greek phloios meaning “bark.” 

Burls are caused by infection of the vascular cambium. As an aside, maple syrup is produced from sap flow inside the xylem that contains water and stored sugars from the previous summer.

Burls were also used by the Native Americans and the colonialists to make bowls, trays and utensils. Today, larger specimens also have been turned into uniquely shaped sculptures.

The burls that we have discussed so far infect the trunks of trees or the stems of various plants. However, there is another form of these growths known as “witches brooms.” This happens when the infection takes place in the apical bud, or growing tip, of a branch. Effectively, you are creating a small tree within a tree. 

Brooms can be spotted on black walnut, lilac fir trees and serviceberries among others. Although mistletoe looks very similar to witch’s brooms, mistletoe is instead a parasitic plant that grows on a supporting tree branch.

The fungus Moniliophthora perniciiosa infects the cocoa tree (Theobroma cacao) causing witch’s broom disease. It is highly destructive and hard to control. 

It was first noticed in Bahia, Brazil in 1989, an area then recognized as the third largest exporter of cacao beans, producing 380,000 metric tons. By the late 1990s, the yield dropped to 90,000 metric tons, and Bahia turned into a bean importer.

In this case it only took about six months to get an answer to my idle question. Patience is a vital virtue for nature lovers.

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