Betula sp.

Birch

By: H. Wayne Shew, Ph.D.
NAB Certified Pollen Counter
AAAC Collection Station—Birmingham, Alabama

Betula is the type genus for the family Betulaceae. This family contains six genera and between 120-150 species. Five genera are found in temperate regions of the northern hemisphere and extend into tropical mountains of South America through the Andes. There are five genera native to North America, birch, alder, hazelnut, hop hornbeam, and ironwood, and one to China, hazel hornbeam. The scientific names for these genera are: Betula, Alnus, Corylus, Ostrya, Carpinus, and Ostryopsis respectively. All members of the family are monecious shrubs or trees. Members of the family have simple, alternate leaves, that are straight-veined with petioles and stipules (basal paired appendages of a leaf). Staminate flowers are borne in long, pendulous catkins and have zero or four sepals, no petals, and 2-20 stamens. Pistillate flowers are borne in shorter, pendulous or erect catkins and have no sepals or petals. The catkins have bracts (modified leaves) that may be mistaken for sepals. Pollen produced by all of the plants in this family is wind disseminated and is known to trigger allergic rhinitis in some individuals. The fruit formed from a pistillate flower is either a samara or small nut. The plants in this family contain varying amounts of tannic acid that result in the members being astringent (bitter and causing contraction of skin cells and other body tissues). One genus, Corylus produces edible nuts—hazelnuts or filberts.

There are five species of Betula native to the eastern U. S. and all are important economically. These species are harvested for timber, used for ornamental/landscape purposes, and for the extraction of oil of Betula. Oil of Betula is obtained from birch twigs and smells and tastes like wintergreen. As an aside, oil of Betula is used in tanning Russian leather.

River Birch Bark

River Birch Bark

Yellow birch (B. alleghaniensis) has strong, hard wood with a close grain and as a result is used in the manufacture of tool handles, interior finishes, and veneers. Sweet birch or black birch (B. lenta) is a strong, hard wood that has a color that deepens upon exposure to air, giving it an appearance like mahogany. It is common in parts of New England and is used for construction of pallets, manufacture of railroad ties, lumber, and veneer. The wood of paper birch (B. papyrifera) is also hard and is sometimes used for veneer. The most commonly found birch in the U.S. is river birch (B. nigra). It is used in the manufacture of baskets and furniture, and is commonly used as fuel.

Birch trees are wind pollinated and its pollen shows up in our pollen collections in late winter to early spring—March & April typically. The pollen grains are approximately 20 X 26 (range of 18-23 x 21-30) microns in size, and are described as having an oblate shape (compressed or flatten along the poles) with a triangular appearance and convex sides in polar view. The pollen grains have three aspidate (strongly protruding) pores.

River birch pollen, polar view—triporate, aspidate

River birch pollen, polar view—triporate, aspidate

River birch pollen, a near equatorial view

River birch pollen, a near equatorial view

People who suffer from birch pollen allergy may also find themselves allergic to other genera in the family Betulaceae and should try to avoid exposure to pollen produced by any of the genera in this family. Reduce exposure to pollen by limiting outside activities in the early spring, and using AC or heat sources that filter the indoor air. Individuals having birch pollen allergy may also have oral allergy syndrome (OAS), a type of food allergy characterized by a cluster of allergic reactions in the oral cavity and throat following ingestion of certain (usually fresh) fruits, nuts, or vegetables. The best-documented cross-reactivity is between apples and birch pollen. However, being allergic to apples does not mean that you are allergic to birch pollen. Cross-reactivity should not be assumed and foods should not automatically be removed from the diet without appropriate allergy testing and medical diagnosis. The most common reactions observed are itching and a burning sensation of the lips and mouth, but other common reactions can be triggered in the eyes, nose, and skin. Some individuals may even experience swelling of the lips and tongue and a tightness in the throat. Rarely, OAS can lead to hives, wheezing, and even anaphylaxis. Some estimates are that as many as 70% of people with hay fever triggered by birch pollen may exhibit oral symptoms triggered by specific foods; see a list of foods triggering OAS at the following websites:
foodallergens.info
healthwithfood.org

In most cases, not all of the foods listed trigger OAS so it is important to determine which foods trigger oral allergies and which do not and thus which foods can be eaten as part of a healthy diet. Some foods can be eaten after being cooked without triggering OAS. The cooking alters the allergenic proteins such that the immune system no longer reacts to them. Certain foods such as nuts and celery are not affected by cooking and can cause oral allergic symptoms even after processing.

OAS can occur at any time but is most likely to be experienced during the pollen season. Individuals with OAS usually develop symptoms within minutes following ingestion of allergy-causing foods. OAS is also known as pollen-food allergy and it is estimated that up to 60% of food allergies seen in adults are the result of cross-reactions between foods and inhaled allergens.