Pollen Season

 

Pine (Pinus sp.) pollen coating a car.
Rowan County, North Carolina. 

Every spring pollen fills the air.  On some days you can see pine trees exhaling yellow clouds of microscopic pollen grains.  Pollen coats cars, and other outdoor surfaces and it aggravates allergies of multitudes of people.  What is going on? Why is it seasonal?  How does it cause the discomfort of allergies? 

Pollen dusting the lid of a trash can.
Rowan County, North Carolina. 

Pollen grains are produced by most seed plants, and each contains a sperm nucleus, the plant's male gamete.  The pollen is transferred to another cone or flower, often on another plant, and there it may fertilize an egg cell giving rise to the next generation.  Some plants use pollinators such as insects, birds or mammals to carry pollen.  Others release their pollen into the air.  Wind pollination is inefficient so lots of pollen must be produced. This wind pollination is the source of pollen season problems.     

Male flowers of Red Maple (Acer rubra).
Rowan County, North Carolina. 

Pollen season starts in our part of the world in late winter.  Maples (Acer sp.) start to flower in late February or early March.  Red Maple (Acer rubra) makes separate male and female flowers on the same tree.  The male flowers have anthers that dangle in the breeze and shed their pollen.  Female flowers are fertilized, and in a few weeks, tiny, winged fruits emerge from the female flowers. 

A pollen producing male catkin
of River Birch (Betula nigra).
Rowan County, North Carolina.  

River Birch (Betula nigra) is another early tree that releases pollen.  Like the maples, River Birch has separate male and female flowers on the same tree.  The male, pollen releasing flowers are borne on cylindrical, hanging bunches of flowers called catkins.  

Male cones of Loblolly Pine (Pinus taeda).
Rowan County, North Carolina. 

Male cones of Loblolly Pine that have released 
their pollen and fallen from the tree.
Rowan County, North Carolina. 

Pines (Pinus sp.) probably produce more pollen than the other spring trees.  Pines are conifers and do not have flowers but release pollen from male cones.  These are smaller than female cones and each cone can make many thousands of pollen grains.  These float through the air and a small number land on the female cones that produces the pine seeds.  Pines are the main culprits in giving that yellow dusting to our cars in spring. 

Catkins on a male tree of Black Willow (Salix nigra).
Rowan County, North Carolina. 

Black Willow (Salix nigra) is a flowering tree that prefers to grow near water.  It has separate male and female trees and in the spring catkins on the male trees release pollen.  Some of this pollen pollenates flowers on female trees but much of it lands on the surface of water.  This pollen can form rafts that are pushed around by the wind.  After a few weeks cottony fruits float from the female trees to disperse the willow.    

Male catkins of Willow Oak (Quercus phellos).
Rowan County, North Carolina. 

Oaks (Quercus sp.) are magnificent trees and also produce abundant, airborne pollen.  Oaks make separate male and female flowers on the same tree.  The male flowers are in catkins and the product of wind pollination is the familiar acorn.  After the pollen has been released the catkins fall off the tree and accumulate in windrows beneath the tree and clog gutters on houses.  

Male Catkins of White Oak (Quercus alba).
Rowan County, North Carolina. 

Airborne tree pollen is a major cause of spring allergies.  We know the symptoms; runny nose, watery eyes, sneezing, shortness of breath.  Pollen that is inhaled can evokes an immune response that causes all these problems. Our immune system recognizes pollen as foreign and releases powerful chemicals like histamine and cytokines.  This response causes the side effects we experience as allergies.  Not all wind pollenated trees are to blame though. Oaks, willows, maples, birches and other trees can all cause allergic reactions, but pines usually do not.  While pines produce great clouds of pollen, but most people do not react to it.    

Masses of oaks catkins littering a street.
Rowan County, North Carolina. 

Plants are going about their reproductive business in pollen season.  Many organizations, including several weather forecasting groups track pollen levels and report on them each day.   Climate change has caused plants to flower earlier in the year and prolonged this season of discomfort.  But as summer comes along, the pollen season passes and so do the allergies. 

Oak catkins that have shed their pollen mark
the end of spring pollen season.
Rowan County, North Carolina.