Robin feeding mulberry to nestlings (photo by Beth Bergman, thebethlenz blog, 2008)

As noted elsewhere (see “Special Topics:  Lancaster’s Mulberry Trees”), White Mulberry today lives everywhere in Lancaster County even though the county’s involvement in the silk industry is long gone.   The reason:  After flowering, Morus alba bears an abundance of delicious fruit that robins and other songbirds relish.  Mulberry seeds then pass through the birds’ digestive system and are poop-planted everywhere that robins perch (songbirds also poop while flying!).  Unfortunately, this chain of events (flowers-berries-birds-new trees) can affect the quality of the air we breathe each spring.

Mulberry is a dioecious species — male and female flowers are on separate trees, unlike monoecious trees where male and female flowers reside on the same tree.   (Actually, Morus is a species that is occasionally “gender fluid”— a given tree may first be male, then female, then change back again.).  Those trees that are male produce prodigious amounts of light-weight pollen which is dispersed by wind…pollen that can be inhaled deeply into a person’s lungs to trigger asthma and, especially in children, other respiratory problems as well.  In the wide open spaces of rural Lancaster County, this may or may not be much of a problem.  But White Mulberry sprouts in backyards, hedgerows, and weedlots all over Lancaster City where its pollen adds to other urban air challenges (e.g., exhaust fumes from high concentrations of cars and furnaces).   In fact, because this pollen can cause allergies and trigger asthma attacks, the planting of White Mulberry trees has actually been banned in numerous American cities (Tucson, Albuquerque, El Paso and Las Vegas, to name a few).

To carry these thoughts a step farther:   Hardly anyone intentionally plants White Mulberry these days; instead it is a self-sustaining population in the city.   And since it is more likely to sprout and then be permitted to grow where there is less yard-care and where abandoned lots are more common, more White Mulberries probably live in the lower-income portions of the city than in the higher.  This means that more mulberry pollen pollutes the springtime air of poorer neighborhoods, which in turn suggests that children in those neighborhoods are more likely to develop respiratory problems than children in the city’s wealthier neighborhoods.    

Environmental justice is a concept that emerged in the United States in the early 1980s and focuses on the fair distribution of environmental benefits and burdens across people of different races and economic status.  If the above speculations about the urban distribution of White Mulberry are correct, then Lancas- ter provides an example of environmental injustice because poor people are more heavily burdened by pollen pollution than people of greater wealth.  

Another species that probably fits this pattern is Ailanthus (the Tree of Heaven), which also thrives in harsh urban environments and sprouts in untended spaces.   Moreover, some individuals have been shown to experience allergy and hypersensitivity to Ailanthus pollen specifically.  Here again, children and other residents in the poorer parts of town can be expected to be affected more.  (It should be noted that Ailanthus is insect-pollenated rather than wind-pollenated, and may produce less pollen overall than mulberry trees.)

A third species that may play a significant role in Lancaster’s tree pollen status is the Red Maple (Acer rubrum).  This tree is superabundant in the city; indeed, a study of Lancaster’s trees a few years ago found that fully 20% of the city’s trees in parks and on streets are Red Maple.  A. rubrum also happens to be dioecious, with males who produce wind-borne pollen (actually Red Maple is “polygamodioecious” — some individuals are male, some female, and some monoecious).   Because of Red Maples, then, Lan- caster probably has more pollen-laden male trees than other cities.  

To sum up pollen pollution in Lancaster with regards to the above three species:  (1) Red Maple and White Mulberry, because of their superabundance in the city, present special challenges to Lancaster that per- haps other cities don’t face; and (2) White Mulberry and Ailanthus, because of their greater distribution in the city’s poorer neighborhoods, are probable sources of environmental injustice so far as tree pollen is concerned.

None of this is to say that these three species of tree should be demonized; each is wonderful in its own way.  The fruit of mulberry trees contains Vitamins C, K, B, A and E as well as iron, potassium and mag- nesium; and mulberries are delicious in muffins, pies, jam, wine, salads and ice cream.  Ailanthus was originally brought to this country in part because, when planted in the right place and cared for in the right way, it’s a spectacularly gorgeous and exotic-looking tree.  And Red Maple grows just as fast Silver and Boxelder Maple and yet in some ways is superior to those species.  Red Maple thus provides quick urban shade by a quality tree that displays brilliant colors in the Fall.

Robin feeding mulberry to nestlings (photo by Beth Bergman, thebethlenz blog, 2008)


Featured Topics

Tree of Heaven’s Better Angels

Tree of Heaven (Ailanthus altissima) is a much maligned, even demonized plant these days. It...

Read More

Measuring Trees

A Word About Measuring Tree Size American Forests, the conservation organization that administers the National Big Tree...

Read More

Mulberry Trees and Lancaster’s Silk Industry

Links With The Past:  Mulberry Trees And Lancaster’s Silk Industry It’s obvious that aged trees...

Read More