Spring in the Willamette Valley is heralded by the awakening of trees seen as bright green growth, showy flowers, and prodigious amounts of billowing tree pollen. Some days the pollen is so thick that every horizontal outdoor surface in the Portland area is coated with the powdery substance. Although it can be a nuisance to those with seasonal allergies, pollen serves a crucial role in the reproduction of most tree species.
Most trees reproduce with others members of their own species, which mixes the genes of the parents and creates genetic diversity in their offspring. Because trees are rooted in one spot, they must have a way to transfer genetic material from one tree to another. This transference of genetic material is called pollination. Pollen is the vehicle of transference. Each microscopic pollen grain contains the male genetic material of the tree. A pollen grain has a hard outer shell to protect the precious genetic material inside during transport from one tree to another.
Pollen is produced in the male pollen cones of conifers. The small brown pollen cones of Hinoki cypress can be seen above, left. The fallen spent male pollen cones of the Douglas-fir surround the female Douglas-fir seed cone in the photo at right.
In the flowering trees and shrubs, the male anthers of flowers produce the pollen. The yellow pollen-coated anthers of the camellia flower (left) and big-leaf maple (right) are obvious above.
In order for reproduction to occur, the pollen must be transferred to a female seed cone of the same species in conifers. The female seed cone of the Douglas-fir can be seen above, left. The female seed cone of the Mugo pine (right) is growing just to the right of the male pollen cones.
In order for reproduction to occur in flowering trees and shrubs, the pollen must be transferred to the female stigma of a flower of the same species. The stigma of the rhododendron flowers above, left, are the obvious longer curved structures at the center of the flowers. The sticky green stigma of the cherry flower above, right, is encircled by anthers. A more detailed explanation of pollination in flowering trees is described in our Northwest Treebune article Tree Anatomy: Flowers.
Once the pollen has reached its goal, a pollen tube forms and transports the male genetic material to the female reproductive structure. This is when fertilization occurs and the genes of both parents are blended to make the seeds of the next generation.
The powdery pollen mixture collecting on your windshield in spring is made of tiny pollen grains so small that magnification is required to see the individual grain. The pollen grains range in size, shape, and color. The shape of pollen varies from smooth and round to spiky to pitted, depending on the species. The various shapes and sizes of pollen can be used to identify species living today and in the distant past. Pollen found in layers of ancient rock formations have been used by paleobotanists to identify which plants grew during ancient times.
Method of Pollen Transfer
Because trees cannot move, they need something to carry their pollen from one plant to another. Most trees employ either the wind or animals.
Most conifer pollen hitches a ride on the wind. This makes sense because most conifers (like spruces and firs) grow in forests where many of their own species grow nearby. A gust of wind can easily carry the lightweight and sometimes buoyant conifer pollen over acres of forest containing trees of the same species. Wind pollination is not specific in its delivery, though, and requires prodigious amounts of pollen to be produced to ensure success. On a breezy spring day, clouds of yellow pollen can be seen rising like smoke above a stand of Douglas-fir. Conifer tree pollen is the earliest pollen of the spring season to be released. The white pollen of the Hinoki cypress seen above looks like smoke as it rises from the pollen cones in a Lake Oswego backyard.
Some flowering trees are wind pollinated, such as oaks, alders, and some maples, but most flowering trees and shrubs lure their pollen couriers with their flowers. The scented oils and the visually alluring colors and markings of the petals signal pollinating insects, birds, and mammals to the promise of food in the form of carbohydrate-rich nectar and protein-rich pollen. The pollinators become coated in sticky pollen grains from the anthers during feeding. When feeding on another flower of the same species, they transfer pollen grains from their body to the sticky stigma of the flower.
Pollen is the ingenious solution to the challenge of reproduction between immobile trees. Transported by the wind and animals, the pollen encases half of the genetics of the next generation of trees.