In early May national news media warned of a “pollen tsunami” descending on the northeast. In 2014 a “pollen vortex” threatened to follow the polar vortex that brought us such a hard winter. Headlines can be misleading; we tend to forget that this happens every year. It’s still important news, because millions of people suffer from spring allergies or even asthma. Both pollen stations in southern New York were registering counts of 1500 to 2000 grains per cubic meter of air for a few days early this May. Yes, this is very high but it’s also very normal. In years past we’ve seen over 10,000 per cubic meter in one 24-hour period around this time.
Springtime is mating season for trees in the northeast – one reason that I haven’t posted anything here for two months. Although I teach biology at Fordham University, I also do the airborne pollen counts for southern New York State. Fordham’s pollen-monitoring program comprises two counting stations, one in mid-town Manhattan, the other in Armonk, about 30 miles up in the northern suburbs. Some years ago, I found myself in charge of both operations, so the annual blizzard of airborne tree pollen engulfing the Tri-State area in April-May keeps me more than busy.
Allergies have bothered me since I was 16, but they stopped when I traveled to Africa at 19, only to return the following year when I got back to England. It turns out that pollen allergies are not nearly so common in the tropics, but I’ll come to back to that in a moment.
Pollen travels from male to female flowers, where eggs become fertilized. Seeds form and the next generation gets started. Magnolia, cherry and dogwood have pretty flowers with big petals to make themselves attractive to insects who do the work of getting pollen from flower to flower. But oak, elm, birch have adopted a more ancient evolutionary strategy by which they release vast amounts of pollen directly into the air to be carried in the wind. Grasses and many wildflowers like ragweed do the same thing; that way they don’t have to rely on insects that may not always appear at the proper time.
All seed plants make pollen. They have been doing so since long before there were dinosaurs. Wind carries the microscopic pollen grains, even though most miss their target and are wasted. But late in the age of dinosaurs the first flowering plants evolved by getting insects to carry their pollen directly. This grand evolutionary alliance between animals and plants meant that flowering plants didn’t need to spend so much energy and resources on pollen that mostly landed on the ground. In the early days of flowers, the climate was a lot warmer and more stable (there was a lot more greenhouse gas in the atmosphere too). As in the tropics today, insects were abundant and very dependable.
But by the time humans appeared in the Pleistocene Era, the earth got cooler and the climate less stable. The last 2 million years have seen a steady tidal rhythm of ice sheets going back and forth across the higher latitudes. Today, in the temperate regions, trees that flower in the spring cannot count on insects to be around when they are needed. A late frost can come unexpectedly and upset the insect life cycle, but the flowering schedule of trees is more tightly linked to the number of daylight hours. So, many species like oaks, birches and hickories have decided to give up on insects and go back to doing what pines and junipers have always done: make vast clouds of pollen and let the wind distribute it at the proper time.
In fairness to the folks who write news headlines, we did have a long winter, which can delay the beginning of tree season. This can make the flowering times of different tree species overlap more; and more trouble for people with multiple allergies. But our pollen records didn’t really show that in the end. By the peak in early May it was turning out to be a quite normal spring pollen season in southern New York, a bad time to be here if you have allergies. Yes, weather matters a lot. Rain can wash pollen out of the air for a few hours, but otherwise the order and timing of tree pollen is very predictable. The bad news is it’s been happening since the last ice age and will go on every year unless we have sub-freezing temperatures without a break all the way through into late May. But that would be a different kind of story.