Dr. Robert Bardon, a professor of forestry and environmental resources at N.C. State University, gives some scientific facts behind all that yellow dust. Questions and answers have been edited.
Q. Why are pine trees so prolific in spreading their seed? How much pollen is produced by a single pine tree?
A. Pines, such as loblolly pine, are prolific at producing pollen. They produce approximately 2.5 to 5 pounds of pollen in a two- to four-week period. The reason for the production of so much pollen is to ensure the reproduction of the species. From an evolutionary perspective, large quantities of pollen dispersed by wind will ensure that the species can be reproduced over large areas.
Q. How far can a grain of pollen travel?
A. Studies show that loblolly pine pollen, the dominant pine species in North Carolina, can be found as far as 25 miles off our coast and over 200 feet into the atmosphere. The majority of pine pollen is deposited within 300 feet of the tree from which it was released.
Q. Does pollen serve any benefit outside of the proliferation of more pine trees?
A. There is little to no literature on the use of loblolly pine pollen for purposes other than the reproduction of the species. The advantage of the large amount of pine produced is the huge amount of genetic diversity that the species maintains by producing large quantities of pollen that is dispersed over large distances. The fact that we have a large amount of loblolly pine growing on private lands in North Carolina is good for our economy, our environment and societal well-being. These trees support our forest industry, the No. 1 manufacturing industry in the state, providing economic return to landowners through timber production and through recreation. They also help us by producing oxygen, cleaning our air, helping to reduce stormwater issues and providing habitat for wildlife.
Q. What typically determines how long pollen season lasts?
A. The length of the pollen season depends on biological factors, but is influenced by temperature. Peak season for pollen production in loblolly pine can be predicted based on the number of days since Feb. 1 that the temperature is above 55 degrees Fahrenheit. To calculate the peak, one can add up the number of degrees the daily high temperature is above 55. Once a total of 636 degrees is reached, pollen production is at its peak. Pollen begins shedding, though, before peak and may occur when the total is about 300 degrees. For most of the Piedmont and coastal plain of the state, the peak will likely occur this year in mid-April. Though the peak period shifts, the length of the season is about the same each year. Precipitation influences the amount of pollen in the air each year, as periods of wet weather will wash the pollen out of the air and reduce its impact on us.