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Ah, spring. Ah, ah, ah-choo!

Posted: April 26, 2019

Spring is a time of warmer weather, beautiful flowers, green growth, but it also heralds tree pollen season.

As the world greens up in the spring, tree pollen affects many of us with “hay fever” symptoms. According to many resources, in the United States, those symptoms are more often linked to pollen allergies. Spring is a time of warmer weather, beautiful flowers, green growth, but it also heralds tree pollen season.

As with all vegetation, trees can’t relocate themselves. The ability for tree populations to respond to and thrive in changing conditions requires genetic diversity. New combinations of genes present in seeds may give some young trees advantages over others of the same species when confronted by insects, disease, or changing conditions. To get that diversity, trees look to pollination. And they need a “middle man” to make it happen.

Most trees, especially those dependent on wind for pollination, put out their flowers before their leaves. This increases the chances that wind will pick up and carry the pollen farther to a tree of its own kind. These flowers are not often showy. The reddish tint on red maple in the early spring is due to their flowers, but you have to get up close to really notice that’s what they are. 

If you remember your biology, you’ll remember that for angiosperms, which are most of our deciduous trees, there are male and female flower parts. Male flower parts are called stamens. Stamens have anthers, which contain pollen, at their tips. The female parts are called carpels. The carpel is made up of an ovary with a protruding structure called a style, and at the end of the style is a structure to receive the pollen, called a stigma. When the pollen reaches the stigma, it germinates, sending a tube through the style to fertilize the egg in the ovary. 

Flowers are often structured in a way to reduce the chances of self-pollination. Afterall, if the goal is genetic diversity, it pays to prevent that from happening. Some flowers position stamen and stigma to prevent inadvertent interaction. Some species have separate male and female flowers on the same plant. And some trees keep them separate entirely by having male and female trees, each with their flowers that release and receive the pollen. Species that use this strategy are called dioecious (meaning “two houses”). Winterberry holly is dioecious. To get those female shrubs bearing beautiful red berries, there must be a more unassuming male plant in the vicinity. The invasive tree-of-heaven is also dioecious. The female trees are recognized by their seeds – with the attention now given to tree-of-heaven because of spotted lanternfly, aerial surveys are serving to identify areas of tree-of-heaven by looking for the female trees.

Gymnosperms, our conifers, don’t have flowers; instead they use male and female cones to enact a similar reproductive strategy. One type of cone produces the pollen grain. Another forms the ovule with the female reproductive structure (similar to the carpel of an angiosperm), which stays attached to the cone. Conifers pollinate solely by wind, blowing the pollen grains to the female cones for fertilization. Once mature, the seeds release from the female cone and are dispersed by wind or animals. Interestingly, to reduce self-pollination, conifers have the male cones low in the crown and the females towards the top.

Trees that rely on wind pollination must produce copious amounts of pollen to ensure that at least some reaches desired destinations. These pollen-producers are responsible for the yellow clouds of pollen, images of which have been circulating the Internet from the southern states: the so-called pollenados, pollenapocalypse… You may notice that the plethora of pollen tends to appear when the oak and pine flowers are out and active. (The oak tassels currently are rapidly developing in central PA.) They coat our cars, houses, sidewalks, and “feed” our allergies.

Trees that rely on animals to pollinate their flowers have adaptations that accommodate relevant pollinators. These trees produce less pollen, but it’s stickier, allowing its attachment to the animal. The trees also must produce a benefit to the animal to ensure they will access the flower and move the pollen around. For some plants that is nectar; others offer up parts of their flower as food. The flowers also must be visible against the tree leaves. These animal-pollinated flowers are showier, often fragrant, and sometimes stinky. Flower color ties into preferred pollinators as well: purple flowers tend to attract butterflies, red attracts hummingbirds, white attracts bees during the day, and night blooming white flowers attract moths. Stinkier flowers attract flies. Bees and wasps prefer the more fragrant. 

For the allergy sufferers out there, hang in there. Spring is moving quickly and by middle May, we should be through the worst of the tree pollen. (But then we get to summer and the grass pollen. And in the fall, we get ragweed). Follow your best practices: keep your windows closed if you can, don’t hang your clothes out to dry on the clothesline if you can, take a shower when you come inside for the day. 

Enjoy the beauty that is spring and appreciate that work involved in creating tree seeds and maintaining the diversity of species of our incredible forest.

Contact Information

Allyson Brownlee Muth, Ed.D.
  • Interim Director, Center for Private Forests
Phone: 814-865-3208