Mountain cedar, Juniperus ashei, is a member of the cypress family of gymnosperms, Cuppressaceae. It is an exceedingly prolific producer of pollen, and during peak pollen anthesis, counts may be greater than 8000 grains per m3 at sampling sites miles away from the trees.
Great clouds of pollen often rise off the trees, mimicking smoke from fires, and occasionally causing "white-outs."
Mountain cedar is especially prevalent in south and west Texas, where it is now the dominant tree. The numbers of such trees have increased dramatically over the past 200 years because of the impact of such factors as control of the number and extent of natural wildfires, burns which would ordinarily keep the numbers and size of these trees limited.
In south and central Texas, the trees ordinarily pollinate in mid-December through January, the only significant plant aeroallergen at that time of year. Further north, other related cedars such as Rocky Mountain cedar, Juniperus scopulorum, pollinate later, usually beginning in late February or early March, but frequently continuing into June. Along the Rocky Mountains, counts are not as dramatic, probably because of the semiarid conditions.
Other members of the cypress family are quite common in other parts of the United States and Canada, such as white cedar, Thuja occidentalis, in the northeast, red cedar, Juniperus virginiana, in the south and central states, and western red cedar, Thaja plicata, in the northwest. Japanese red cedar or sugi, Cryptomeria japonica, is a major aeroallergen in Japan, as is Japanese cypress, Chamaecyparis obtusa.
The importance of these cedars in pollinosis has been long appreciated, in distinction to the relative impotence of members of the pine family, Pinaceae. Allergenic cross-reactivity is extremely strong among Cuppressaceae. This was first noted by Black in 1929, who reported that an extract of a non-endemic cypress was as effective in treatment as the local cedar extract.
Adapted from: Annals of Allergy, Richard Weber, MD, January 2001.