First in Flower

Springtime and sexual identity expressed in maple flowers

Female flowers of Silver Maple and Red Maple

A Note from Bryan: Because I’m out of action owing to knee surgery on Tuesday, here from the archives is a timely and updated dispatch that most of my readers have not seen, and which makes me happy because even while enduring severe post-operative pain (expected) I can nonetheless bring you vernal opportunity.

FOR MANY OF US, the early hints of spring are not green — they are red. Maple trees are exploding into displays of crimson flowers.

Flowers? Where are the petals in those images above? They’re tiny and inconspicuous, granting us an intimate view of the raw reproduction happening now among some of our most common trees across the U.S, southern Canada, and parts of Europe. And when it comes to maple sexuality, well, it’s dynamic to say the least. Some trees are entirely male. Some are female. Some are both. Some even change.

First, let’s get you oriented. The left image above shows female flowers of Silver Maple (Acer saccharinum). Emerging from the buds are wiry ruby stigmas, which receive pollen carried mostly on the winds from male Silver Maple flowers emerging on a separate tree. Above on the right, dangling on their long stalks, are flowers on a female Red Maple (Acer rubrum) similarly showing their kinked, wiry stigmas.

Male flowers (below) are typically on separate trees. Their pollen-producing anthers are borne on short stalks (filaments) emerging from the buds. On the left below are male flowers from a Silver Maple showing anthers with their yellowish pollen. On the right are flowers with spent brown anthers in a neighborhood of Washington, D.C., in February, proof that all this sex is happening now among many of us, before the leaves emerge (and amid the chaos emerging from the nation’s capital).

Male Sliver Maple flowers / © Bryan Pfeiffer

But also know that nature is nothing if not complex, especially when it comes to reproduction. As Silver Maple trees grow old, their unisexual flowers (either entirely male or female on a given tree) can sometimes shift to become bisexual. You can see that in my image below: ruby female stigmas are just starting to emerge (like in the top left image); but deeper in each flower bud you can make out bunches of male brown anthers yet to emerge and looking like chubby hot-dog buns (especially in the clusters on the right). (Click and zoom that shot for a better view.)

Bisexual Silver Maple flowers / © Bryan Pfeiffer / (Click and zoom.)

Among Maples, “It’s Complicated”

Some maple species can become sexually fluid. A Red Maple that produced only female flowers for decades might in one or more years produce some male flowers as well. And male Red Maples might in certain years bear some female flowers.

I’m not certain of the evolutionary strategy in play here. But first know that female maples (like females generally) bear a greater burden than males in the arena of reproduction — and therefore require more resources. In the case of maples, females produce those winged fruits, each called a samara (an all-around lovely word). Drought, soil depletion, or stress affecting a female maple tree might favor the production of male flowers in a given year. Similarly, in a good year (enough rainfall, for example, or reduced stress from insects) a reliably male maple might enhance overall seed output by producing some female flowers.

Whatever the case, you yourself can witness all this flowering and reproduction, which is happening now for many of us. Two bits of advice: find maples in bloom and look closely at low-hanging flowers. These final two images feature reproductively active Red Maples: ruby female flowers and orangish male flowers (owing to their yellow pollen) on separate trees (although their branches overlap). (Click for a bigger view.)

So please look up — now or in late March and early April across northern-tier states, southern Canada, and northern Europe. Spring is or will soon be emerging before the leaves, not only among maples but other trees as well. So before the green arrives, our landscape displays fifty shades of flower.

Red Maples — females (red) and males (orange) in bloom / © Bryan Pfeiffer

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Postscripts

• In botany parlance, we call unisexual plants (individuals that are either male or female) “dioecious” — a counter-intuitive term that means the male and female reproductive parts are borne of “two houses.” (In another words, it takes two separate plants to reproduce.) Bisexual plants are “monoecious” — male and female flowing parts existing together of “one house.”

• Incidentally, I’m using “fluidity” because the botany terms established for some of this stuff can be a challenge: polygamodioecious, for example, or polygamomonoecious.

• The red is reduced across Europe, where the common native maples — including Sycamore Maple (Acer pseudoplatanus), Norway Maple (Acer plantanoides), and Field Maple (Acer campestre) — tend to flower in yellows and greens after the leaves emerge. The same is true for Bigleaf Maple (Acer macrophyllum) in the far western U.S. and Striped Maple (Acer pensylvanicum) in the East. But Silver Maple (Acer saccharinum), although native to North America, has spread in cultivation to occupy a scattered distribution (with red flowers) across the U.K. and Europe.

• For more on maple sexual fluidity, I suggest Richard B. Primack’s excellent article titled “The Sex Life of the Red Maple” published in Arnoldia, the quarterly magazine of Harvard’s Arnold Arboretum. (Sorry to say that you’ll need academic or other access in order to read from the journal.)

• Thanks to my friend and colleague Kent McFarland for that top shot of female Silver Maple flowers.