The Birds and The Bees of Trees

If I asked you what “the Birds and the Bees” means, would you think of trees? No, you would probably think of something more, umm, inappropriate.

The “Birds and the Bees” has been a “coming of age” euphemism used for centuries to innocently describe “the act of reproduction”. But the exact origin and meaning of the statement remains unknown to this day. The saying is found in significant poetry from the 1600s, stories from the 1800s and iconically in music lyrics throughout the 20th and 21st centuries, but nowhere is there a definition or explanation. It is inferred that the “birds” refers to the motherly act of laying an egg and the “bees” refers to the act of fertilization (or pollination) with genetic material. While this may be a strong and reasonable explanation, this phrase was used well before the concept of genetics was discovered by Gregor Mendel in the late 1800s. 

While we may not know the origins of this statement, everyone seems to be well aware of its meaning. In a way, it insinuates that reproduction is a natural and necessary act of life. From the evolutionary lens of Charles Darwin, success in life is determined by one's ability to spread genetics to build future generations. With respect to “the Birds and the Bees” of Trees, this holds true. 

For a tree to “reproduce”, it must combine pollen of one tree with a seed of a different tree from the same species. But you must be thinking, how can a tree “get with” another tree if they are rooted in place? Well, this is where the birds and the bees come back into play. For pollination to occur, the pollen must be transferred from one tree to another. This occurs via wind pollination or animal-assisted pollination. 

Wind pollination is the most common way trees pollinate. Everyone has experienced wind pollination at some point whether you realize it or not. Around mid to late spring yellow clouds drift about, creating layers of yellow dust on every surface and aggravating seasonal allergies everywhere. This may seem like a nuisance at the time, but what if I told you this was essentially tree sperm? Weird imagery, right? Some trees specialize in wind pollination by creating long dangling flowers that will be easily caught in the wind, such as maple trees. Conifer trees are also infamous for wind pollination using this method almost primarily.

Animal-assisted pollination is most well known in flowering plants and trees. Everyone has seen a bumble bee collecting pollen on its fuzzy little legs while drinking a snack of nectar from a beautiful flower. Well, that is the act of pollination in the process. That bee collects the pollen from one flower, spreads the pollen to the next flower, collects more pollen, and so on and so forth. In addition to bees, other insects and birds are common pollinators for trees. However, animal-assisted pollination is rarely the sole pollen transfer method because it is much less reliable compared to wind pollination. Animal-assisted pollination depends on the population of that pollinator, which varies greatly year to year. For a successful future generation, the pollination process must be relatively consistent each time a tree plans on reproducing. For example, if a tree solely pollinates via a certain bird species, then that species of bird experiences a disease, there will be fewer birds to pollinate the following year, resulting in fewer young trees of that species. 

Trees have adapted to this variation in pollinator population by alternating “mast years”. A mast year is the year which trees of one species will collectively decide to pollinate. Rather than hoping for a few offspring to grow every year, the inconsistent mast years allows for more offspring to be fertilized during a shorter period. The concept of varying mast years also comes in handy when avoiding predators. If the mast years are inconsistent, predators will not be able to predict or rely on a consistent seeding cycle for food. A good example of this is seen in Oak Trees. Acorns (the seeds of the Oak Tree) are a favorite snack for deer and squirrels. If an Oak Tree had their mast year consistently every year, the deer and squirrels would be able to predict this influx in food and consume/store all the acorns before they can grow into trees. By alternating mast years, the Oak Tree is able to outsmart the deer and squirrels with unpredictability. 

Now, at this point you might be wondering, “how does pollen and a seed become a new tree?” That is an excellent, but complex question!

Trees have specific male and female counterparts, just like we have different male and female "parts". Some species have separate genders, i.e. male trees and female trees (ex. Soft Maples, see images above) , while others will have both male and female counterparts on one tree (ex. Pine trees, see image below). The pollen from the male counterpart is transferred to the female counterpart via the methods discussed above. Once the pollen is transferred, the true magic of science begins. 

Not just any pollen will pollinate any tree. Most trees are able to detect what species the pollen came from, and reject any “non-self” pollen to avoid incest. Yes, incest is also a genetic concern in the plant world! Some species go even further to avoid incest by developing their male and female parts at different times to ensure only compatible pollen is making it to the female counterparts. 

Once pollinated, the embryo begins to develop into a baby tree. While the concept of an embryo is the same for humans and trees, the development is very different (obviously). For deciduous trees, the embryo becomes the fruit. Ready for more weird imagery? That apple you pack in your lunch box is essentially a tree womb, protecting the fertilized seeds. Fruit can come in many shapes or sizes, such as multi-seed fruits like apples, single seed/stone fruits like peaches, or nuts like walnuts or acorns. Conifer trees are similar, but instead of flower petals and fruits, they have seed cones and pollen cones. The pollen gets caught in the scales of the seed cone, fertilizing the ovule to become an embryo within the scales. 

Once the embryo within the fruit/cone is developed, the seed must be dispersed. This is where “the birds and the bees” come back into play…or at least the birds. In many circumstances the seeds will be dropped at the base of the mother tree. But for a tree to be truly successful, the offspring will be dispersed away from the mother tree. This ensures the spread of genetics far and wide for stronger future generations. Many trees rely on birds or mammals such as squirrels to consume the seed and then excrete (or poop) out the seed at a different location. Conifers rely less on animals and more so on the wind for dispersal. As the cones mature, the scales are released from the cone. These fully developed scales are thin and papery, allowing them to be caught in the wind and wisp across the landscape to spread the genetics of future generations. 

Seeds only remain a seed as long as it is dehydrated. Once the seed comes in contact with water and soil, the seed begins to germinate or “break seed dormancy”. The first step in germination is the production of a radicle, which becomes the first root breaking ground. Eventually, more roots develop and the seed coating disintegrates, revealing the first leaves known as the cotyledon. These first leaves are extra sensitive to light, allowing photosynthesis to start right away, despite the seedling being so little. Conifers also produce cotyledons, but in varying needle shapes. As the seedling grows, the first true leaves will develop and the cotyledon leaves will be shed. The development of true leaves/needles is like a child being upgraded from diapers to pull-ups…”I’m a big kid now”!

And there you have it, the Birds and the Bees of Trees. While there are innuendos for just about everything, “the birds and the bees” of life is arguably one of the most well known and innocent of them all. New life can be seen as magic, but it is so much more than that. It is proof that science conquers all. From the adaptation of pollination and dispersal methods, to the germination of a fertile seed, nature has taken everything into consideration when it comes to growth. Only the strong survive, and those that don’t go down in history giving back to the land they came from. Whether you are looking through a genetic lens, an evolutionary lens, or environmental lens, new growth and young trees are the key to the future!