6.2.1 What Are the Common Types of Allergenic Trees in North America?

The symptoms of spring allergy are often attributed to the pollination of trees. Trees have the greatest diversity among plants and are the least cross-reactive. Trees tend to cross-react strongly at the genus level, but typically not at the broader family level.

Gymnosperm trees produce copious pollen, but tend to be milder in allergenicity. For example, conifers produce buoyant pollen capable of traveling hundreds of miles and coating cars, windows, etc., yet it is less allergenic. A notable exception is the cypress family (Cupressaceae), including mountain cedar, which can have very potent pollens. The mountain cedar, Juniperus ashei (alternatively Juniperus sabinoides or Juniperus mexicana) is the most allergenic tree in Central Texas (▶Fig. 6.1). It is an evergreen tree with grey-brown shredding bark growing to a maximum height of approximately 30 feet and pollinating from November through March. Cryptomeria japonica (sugi) is another potent gymnosperm being the most prevalent tree pollen source in Japan. The pine family (Pinaceae) produces impotent pollen, but can be an important allergen if located sufficiently close to human habitats. In addition to pines, Pinaceae includes spruce, hemlock, and fir trees. Though common, ginkgos and yew trees are not potent allergen producers. While gymnosperm families share little cross-reactivity in general, cypress family members do share major cross-reactivity.

Angiosperm trees are quite diverse and prolific. Because of their diversity, cross-reactivity commonly occurs at a genus level. In violation of this genus-only cross-reactivity rule though, some notable familial cross-reactors include: (1) birch, alder, and hazelnut; (2) oak and beech; (3) various maples with one another; and (4) olive, ash, lilac, and privet are also capable of cross-reacting. Australian pines are an angiosperm tree, not a gymnosperm pine. Poplars and cotton woods produce seeds with cottony tufts and release them during summer season. These are often falsely blamed for allergy symptoms during summer, while tree pollen production occurs early in the year. Despite the fact that trees are geographic-specific, in general, oak (▶Fig. 6.2) is considered the most important allergenic tree in North America, while birch carries the same distinction in Europe. Due to the limited cross-reactivity among this group, the members should be tested for and treated individually specific for the practice location.