Aerobiology at the continental scale

Long distance transport of pollen and spores has been documented many times in both the Eastern and Western Hemispheres. Generally these incursions are one time or occasional events; however, two systems have been widely studied that show regular seasonal transport over hundreds to thousands of kilometers. The continental transport of Puccinia graminis uredospores along the “Puccinia pathway” was first described in 1923 and continued to be studied for several decades. This fungus is the cause of stem rust of wheat, one of the most important wheat diseases worldwide. In the spring, uredospores of P. graminis have been carried northward from contaminated wheat plants in northern Mexico and southern Texas on prevailing southerly winds. Uredospores can also be carried back to the south in late summer and fall on northerly winds associated with frontal systems. Transcontinental movement of the spores from this pathogen has also been described in Europe and Australia.

Starting in the 1980s the long distance transport of Juniperus ashei (mountain cedar) pollen from populations in southern Oklahoma and Texas has been recorded. Pollen from this species is highly allergenic and affects a significant percent of allergy patients in central Texas and other areas of the southern plains. The mountain cedar system is unique because pollination occurs in December and January, a time distinct from other plants, including other species of Juniperus. Pollen from this species is registered in our air samplers in Tulsa on 30% to 40% of the days in December and January. Starting in Dec 1998 mountain cedar pollen forecasting has been undertaken to warn downwind populations of the potential threat of high pollen concentrations in the atmosphere. The downwind forecast is based on trajectories generated using the HY-SPLIT dispersion model from NOAA Air Resources. Based on air sampling in Oklahoma, our forecasts have accurately predicted most pollen incursions. Initially, it was believed that the transport was only regional carrying pollen into the Tulsa area from southern Oklahoma and Texas. However, when our trajectories were compared to data from other pollen sampling stations in the National Allergy Bureau, evidence of transport over greater distances is clear. In fact, during the six years of forecasting, Juniperus ashei pollen was registered over 20 times in London, Ontario.