It’s the first week of spring, and with the promise of longer days and warmer temperatures comes the annual plight of the allergy sufferer. Look carefully, and you’ll see the earliest flush of flowers dangling whimsically throughout the Arboretum. Perhaps you’ve wondered why some trees begin their reproductive cycles so early in spring, before hardly an insect has shown up to work, so to speak. The secret is in the windy days, as torrents of microscopic pollen rise up with each gust, and the line at the pharmacy grows ever-longer.
Anemophily is a form of pollination caused by wind. In temperate climates, where the activity of insects is limited during colder months, many trees depend on the flow of air currents to move pollen grains from male to female reproductive parts. These wind-blown grains are smaller and less sticky than those of animal-pollinated plants, measuring in at a mere 20-60 micrometers, or half the diameter of a human hair. Unburdened by the aesthetic tastes of animal pollinators, anemophilous flowers emphasize function over form, eschewing the showy anatomy and nectar production necessary to attract insects. Wind-pollinated flowers are often minute, assembled in long pendulous arrays called catkins, such as on the Szechuan filbert (right), or groups of dangling flowers in pedicels, seen on a cultivar of American elm (below, left). Nearly all gymnosperms (mainly conifers, cycads and ginkgos) also rely on wind pollination. The branch of the Olga Bay larch (below, left) sports both downturned, pollen-bearing male cones and upright, pistillate female cones. Anemophily is especially effective in temperate deciduous and boreal forests, among hardwoods and conifers whose canopies are exposed to strong wind.
Relying on wind-pollination is somewhat of a gamble; its effectiveness is determined by fluctuations in weather and air currents, which are famously unpredictable in early spring (as Monday morning reminded us). Furthermore, the lighter pollen grains of anemophilous plants are less viable over long distances. Facing these challenges, wind-pollinated trees must hedge their bets by producing enormous quantities of pollen–often billions of grains per tree. As another method of mitigating wasted energy, many species (most notably oaks, aspens, ashes, alders and birches) time their reproductive cycles to occur well before leaf-out, preventing new foliage from physically interfering with air currents. Anemophily is often complemented by other adaptations that prevent inbreeding. On the flowers of the American elm, female organs will mature faster than the male organs in a process called protogyny, which further reduces the chances of self-fertilization.
As you explore the Arboretum this week, keep your eyes open for these early signs of spring romance. While species like the American elm and the Szechuan filbert have nearly completed their reproductive cycles, many, such as Japanese white birch (right), are only starting to wake from dormancy. Within weeks, the birches, cottonwoods, walnuts and oaks will begin to flower and rain down a fantastic amount of pollen on humans and vehicles alike. And with luck, some of this prescient pollen will find its way to a neighboring pistil, to begin the miraculous process of fertilization.
If you’re interested in learning more about phenology, botany, or ecology, join the Arnold Aboretum Tree Spotters, a group of citizen scientists responsible for collecting data from 11 species throughout the Arboretum. Our next training session will take place on Thursday, April 14 from 5:00pm–7:30pm at the Hunnewell Building. Click here to register.