Transsexual Trees: Can Trees Change Gender?
Trees And Botanical Sex
Over their long existence on Earth, trees have developed a strong reproductive fervor that allows them to thrive in many kinds of ecosystems. One of these adaptations is to have a variety of reproductive arrangements across species, with some featuring male and female plants and others featuring individual specimens with both types of reproductive features. Some species have been documented changing their sex altogether, particularly under stressful conditions (including old age). Here, we’ll look at why botanists and foresters think this happens and what it means for choosing a type of tree to plant in your yard or neighborhood.
The Black gum (Nyssa sylvatica) is another dioecious tree that can switch from male to female when stressed. Photo credit: Richard Webb, Bugwood.org, CC BY 3.0 US via Wikimedia Commons
What Are The Tree Sexes?
Trees, like other plants, reproduce with stamens, pistils, pollen and seeds, which we observe as flowers, fruits and nuts. Among tree species, there are three sexes: male, female and trees that have “perfect flowers,” which contain both male and female components. Tree species with both male and female individuals are known as “dioecious,” or “two-housed,” while trees that contain perfect flowers are known as “monoecious” or “single-housed.”
Most trees are pollinated by insects carrying pollen from one flower to another, or by winds that carry and disperse pollen. Once a female or monecious tree has been pollinated, it grows a nut (like an acorn or walnut), fruit (like berries or apples) or another kind of seed that contains the genetic material that grows into the tree’s next generation.
Why Would A Tree Change Sex?
There are a few tree species that are dioecious but also contain individual specimens that have switched their sex (typically from male to female) under certain conditions. Many investigations, including one done at Rutgers University, have examined why this happens. This study shows that around 75% of dead striped maples (or goosefoot maples) in a sample had produced female flowers the year before they died, while most living maple tree populations have a higher male-to-female ratio than that. The same research found that male tree branches of striped maples that were cut and then preserved in water produced female flowers.
Observations like these have led to the belief that some male trees have a latent potential to produce seeds when placed under duress, and some will make the change toward the end of their lives. This phenomenon has been observed in trees other than the striped maple, including blackgum, honeylocust and ginkgos, as well as other maples – particularly the red maple. While the striped maple is usually found in Eastern Tennessee, these other types of gender-morphing trees are widespread in yards and parks across Nashville.
Gingko trees have both male and female specimens and are one of several tree species that can change their sex over the course of a long life.
Identifying A Tree’s Sex
Trees don’t always reveal their sex right away: honeylocusts don’t reach maturity until 10 or 15 years of age, while ginkgos take 20 years – and maples up to 25 years – before they begin to produce flowers. This developmental delay might cause some issues down the line for suburbanites and city-dwellers who may prefer to plant a male tree to avoid fruits or seeds making a mess on sidewalks or front yards. Planting a ginkgo, for example, without knowing its sex may result in some bad smelling fruits produced years down the line.
In general, however, a tree’s sex doesn’t make a difference in its ability to contribute to the community canopy and provide ecosystem services like temperature moderation, stormwater control, and noise and air pollution reduction.
If you’re considering purchasing a tree through the Nashville Tree Conservation Corps’ Tree Sale this year, but you’d like to avoid a tree that drops lots of seeds or berries, be sure to mention this as one of your landscaping requirements so we can advise you on the right species and, if necessary, sex of the tree you order.
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