On Maple Trees (and Treefrogs) Protecting Themselves

An Hypothesis

In the summer of 2018, my family moved one hundred miles South, from Olympia, Washington to Portland, Oregon. Both are in the Pacific Northwest, a landscape famously full of mossy forest whose canopy is made up mostly of evergreen trees: Douglas firs, Western red cedars, hemlocks, and more. In among those evergreens, however, are some deciduous trees, among the most abundant of which is the bigleaf maple. The bigleaf maple is a glorious tree, broad and sweeping, with branches that extend far from its trunk—sometimes its many trunks—providing dappled shade in the summer through a lattice of leaves. In the mild winter months, those same trees become dramatically sculptural, made of many shades of browns and greens, their trunks covered in pillowy moss and lacy licorice ferns.

When we were looking at houses in Portland, the maples were putting out abundant samaras, the winged seeds that helicopter down, allowing maples to disperse their progeny farther than if they were to just drop from the tree. And these samaras, unlike most that I had previously encountered, had a bite to them. Their tiny sharp hairs embedded in our skin and caused irritation. They fell on the ground and made walking barefoot an itchy experience. They dropped onto chairs such that when we sat down, they became embedded in the fabric of our pants, working their way in to irritate the skin underneath.

Back in Olympia, just one hundred miles north, the samaras of the bigleaf maples were kinder and gentler. The trees were of the same species—although “species” is a broad concept that can likely never be fully pinned down—but they were putting forth distinctly different samaras.

It is possible that there is a genetic distinction between the bigleaf maples in Portland, and those one hundred miles north. More likely, I think, is that the environment in which these maples live is causing them to reveal phenotypic plasticity.

Phenotypic plasticity refers to the many outcomes that are possible from the same genetic starting materials. That is: if two organisms have the same genes, but look different, how did those differences come to be?

Broadly speaking, a genotype (say, the alleles for brown eyes) produces a phenotype (the actual brown eyes). Phenotype is the observable form of an organism. But for many traits—for most traits, in all likelihood—a particular genotype encodes information that can produce a whole range of possible phenotypes. What phenotype will actually be produced is due not just to the underlying genotype, but also to interactions with the molecular, cellular, developmental, and external environments.

It turns out that humans don’t have very many genes—far less than the 100,000 that was roughly imagined as the Human Genome Project got underway, but the particular number is mired in both technical and definitional concerns. It turns out that no species on Earth has as many genes as would be required to code in a precise and static way for the resulting individual. Of course, “precise and static coding” wouldn’t be effective anyway, in any environment that changes over time, or in which the inputs or conditions are variable over space. Phenotypic plasticity thus allows individuals to respond in real time to changing environments, to avoid being canalized into set patterns by their genes.

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Here are just four examples of phenotypic plasticity—the first two of which are also in the Childhood chapter of A Hunter-Gatherer’s Guide to the 21st Century, the last two of which are not.

The skulls of dominant wild hyenas are big and robust. They have large sagittal crests on top of their skulls, and broad zygomatic arches at their cheeks. Both of these structures provide places for muscles to attach—much needed if you’re in the business of asserting your dominance with your teeth. In contrast, the skulls of hyenas born and raised in captivity have no such structures. The different environments of wild versus captive hyenas affect what form (morph) they have¹.

It is also true that human children who grow up chewing soft, highly processed foods have smaller faces as adults than children who grow up chewing hard and tough food.

In the same vein, if your first few years of human life are spent in a hot climate, you develop more sweat glands than if your early years were spent in a cool climate².

Finally, we have what is perhaps my favorite example of phenotypic plasticity: frog eggs responding to a hungry snake³. The picture-postcard-famous red-eyed tree frog, Agalychnis callidryas, a denizen of Central America’s lowland rainforests, lays its clutches of about 40 eggs⁴ on leaves overhanging ponds of fresh water. After a period of development, the eggs hatch and fall into the ponds below, where they swim about as tadpoles for some months⁵ before metamorphosing fully into frogs, ready to begin the cycle anew.

There are risks everywhere. Eggs sitting on leaves are an easy lunch for anyone who finds them, but even so, there are fewer potential predators on a subtle leaf overhanging a pond, than in the water below. Can there be a perfect ratio of time spent immobile on a leaf over a pond, with just a few predators, to time spent mobile and growing as a tadpole in a pond, with more potential predators? There cannot. Again, a simple, static rule is impossible, because conditions are never exactly the same twice.

Left undisturbed, the egg of a red-eyed tree frog will spend up to ten days on a leaf before hatching out as a tadpole into the pond below. But sometimes disturbance arrives. Often the disturbance comes in the form of a snake—specifically, the cat-eyed snake, Leptodeira septentrionalis, which seems to find frog eggs delicious. When cat-eyed snakes move in on eggs of red-eyed tree frogs that are as young as five days old—half the age at which they often hatch—many of the eggs hatch early. These eggs are young, and fragile, and more likely to be eaten by fish and shrimp in the pond below than if they had spent ten days as eggs. Some of them won’t make it. But in the face of a predatory attack by a cat-eyed snake, all of the eggs that do not hatch early become lunch.

Having the ability to respond to environmental conditions rather than being on a rigid timeline helps frogs: more survive, and fewer end up as lunch. That’s phenotypic plasticity for you.

If the differences between the bigleaf maple samaras of Portland, Oregon, and Olympia, Washington, are due to phenotypic plasticity, what environmental conditions are sufficiently different between Portland and Olympia to cause such a difference in the samaras of the maples in these two places?

One possible answer, among many, is the amount of English ivy covering the landscape.

English ivy is both introduced, and invasive, the latter being a subset of the former which implies that it is a fierce and able competitor in the land to which it has been introduced. In the Pacific Northwest, English ivy smothers landscapes. It takes out the most fragile understory plants first—the maidenhair fern and the trillium and the bleeding heart. Next it covers the woody but still smotherable shrubs—huckleberries and dogwood and salal. And finally, the ivy climbs trees to reach the light, sometimes taking out the very trees that it climbs. This latter result is not a particularly useful strategy for the ivy—when you kill your host, you also tend to kill off the benefit that it was providing you. In Portland, much of the forest is now a monochrome sea of ivy, rather than a rich and varied landscape of ocean spray and columbine, salmonberry and sword ferns.

The ivy climbs bigleaf maples very well indeed, and some of them have succumbed to it. Many trees, though, are pushing their way through, continuing to produce leaves with which to photosynthesize and seeds with which to propagate. The seeds of those maples who live amidst ivy, I posit, are better armed than are maple seeds in places without so much ivy, better able to resist the ivy, than if they were not thus armed, and that this is an adaptive response to the ivy itself.

How might “armed” maple seeds—with hairs that irritate the skin of us ape interlopers—be helping the maples in their struggle against the ivy? If you have walked in a forest that is encrusted with ivy, you will recognize the cover that it provides. Other understory plants are less abundant in an ivy-dense landscape, but it is also true that the ivy itself provides a dense network of tents and hidey holes for potential seed predators—for mice and squirrels, for instance—who might well take advantage of the ivy’s protection to predate more seeds. The ivy itself might also directly compete with maple samaras, wrapping its roots and tendrils around them, rendering them incapable of developing into new maples. In both cases, a maple seed that is more irritating to outsiders, outsiders who would eat or otherwise destroy it, while more expensive for the parent tree to create, may increase the chances, in an ivy-infested landscape, of a new maple coming into existence.

The hypothesis on the table right now, though, is simpler than any of that. The hypothesis is that, in the presence of English ivy, the samaras of bigleaf maples are more urticating, hairier, and better armed.

It’s a hypothesis. It might be wrong; it might be right. It is testable. And it can point the way to other hypotheses as well. For instance: In addition to the hypothesis of physical adaptations in the presence of ivy, perhaps the seeds of bigleaf maples in areas with a high amount of English Ivy have chemical defenses that are not present, or present in lower amounts, in maple seeds from low-ivy areas.

And if either of those are borne out by research, one might then ask the bigger question—not yet precise enough to be a hypothesis: What else is the smothering of the landscape by an invasive plant doing to the native flora and fauna?

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In the three years that we have lived in this little piece of Portland forest, this forest that was nearly drowning in English ivy when we moved in, I have gone out into it every week, and pulled some of the ivy out. It seems endless. It is beyond daunting. It even brings out the woo in me, as I imagine, having carefully cleared a red huckleberry of the ivy around it, or another sword fern, that I can hear the native plant breathing more easily, that I can feel its gratitude.

Another observation that I have made, having now spent three samara seasons in this forest, each one with less and less ivy in the immediate vicinity due to my efforts—an observation based on qualitative experience only—is that the maple samaras seem to have less bite with each passing year. They seem less intent on burrowing their sharp hairs into the soles of my feet when I walk on the deck, or into my fingers when I pick one up that has made its way inside the house. I have not measured this. I do not know for sure that it is true. The hypothesis—that prevalence of English ivy on a bigleaf maple prompts that maple to develop better protected samaras, ones that are less pleasant for humans to interact with—still awaits a test.

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1. Example from p41 of Mary Jane West-Eberhard’s excellent 2003 book, Developmental Plasticity and Evolution. New York: Oxford University Press.

2. Both the face size and sweat glands examples are from older original research that are cited on p163 of Daniel Lieberman’s (also) excellent 2014 book, The Story of the Human Body: Evolution, Health, and Disease. Published by Vintage.

3. This amazing work, which came out early in my time in grad school, began with this paper: Warkentin, K.M., 1995. Adaptive plasticity in hatching age: a response to predation risk trade-offs. Proceedings of the National Academy of Sciences, 92(8): 3507-3510.

4. This number—~40 eggs in a clutch—has been confirmed by later researchers, including Warkentin, but was originally noted in this 1963 paper, before the genus change from Phyllomedusa to Agalychnis: Pyburn, W.F., 1963. Observations on the life history of the treefrog, Phyllomedusa callidryas (Cope). Texas Journal of Science, 15(2): 155-170.

5. More, slightly later work from the same researcher: Warkentin, K.M., 1999. Effects of hatching age on development and hatchling morphology in the red-eyed tree frog, Agalychnis callidryas. Biological Journal of the Linnean Society, 68(3): 443-470.