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Pollen season arrives, blame the trees

Submitted by Ned Rozell
Phone: 907-474-7468

05/05/08

Photo caption below.
The bark of a birch tree that is now releasing millions of grains of pollen.
UAF photo by Ned Rozell
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With your next breath of spring air, you'll pull dozens of invaders through your nose. These intruders may make your nose drip and your eyes red and watery.

The airborne invaders are grains of tree pollen, specks so small that it would take eight of them to cover the period at the end of this sentence. The air is rich with pollen because spring is the mating season for trees.

The first step in a tree's reproductive dance is to release sperm, safely held in the center of a pollen grain. Trees release an incredible amount of pollen to improve the odds of finding a female flower. One birch catkin (the cluster of tiny flowers that looks like a caterpillar) can release millions of pollen grains.

Birch is the worst of the Alaska pollen types for allergy sufferers, said the late Jim Anderson, former biosciences librarian for the University of Alaska Fairbanks. In addition to being a collector of neckties, teddy bears, typewriters, and other things, Anderson had a passion for pollen. He studied it for years with an air-sniffing instrument mounted on top of a building. Looking at years of data, he found a few trends in the way trees and other plants release pollen.

Anderson found that birch trees begin releasing pollen about two days before leaves emerge from buds, and the concentration of birch pollen is greatest about three days after the leaves come out. He measured more than 4,500 grains of birch pollen per cubic meter on the highest pollen days.

Allergy sufferers are hit hardest by birch pollen because it contains irritating proteins. Each grain of pollen consists of a center containing the male genetic material, surrounded by a protective wall called the exine (which was the word on AndersonĀ¹s vanity license plate). When pollen comes in contact with moisture, such as that on the nose's mucous membranes or the lining of the eyelid, protein molecules from the exine leach into a person's tissues. An allergic person's immune system produces antibodies against the protein molecules. Antibodies then trigger the release of histamines and other potent substances, leading to the cold-like symptoms familiar to allergy sufferers.

Anderson discovered that watching the weather could help him predict the day trees release pollen. By taking the average of the daily high and low temperatures and subtracting 32 degrees Fahrenheit, he came up with a number called degree-days. By looking at past years' pollen outbreaks and comparing them with temperature records, he found that, in the Fairbanks area, poplar and aspen release pollen first. Next come birch, alder, spruce and grasses, all at specific numbers of degree-days.

Though the dates of pollen release from year to year vary with the warmth of spring, Anderson said he was able to keep a running tally of degree-days. He knew, for example, that when the number approaches 232, birch trees are ready to release pollen. He also found that birch trees tend to release pollen in greater amounts every other year. He didnĀ¹t know the reason for the biennial cycle, but thought it had to do with the biological makeup of birches.

Luckily for those with allergies, pollen season doesn't last very long. According to pollen calendars Anderson has made for Fairbanks and Anchorage, birch trees typically shed large amounts of pollen May 10th through the 20th.

This column is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. Ned Rozell is a science writer at the institute. This column was first printed in 1999. Rozell is currently out of state.