A friend invited me over to help identify the trees in the backyard of his new house. Lichen coated the trunks, rendering bark a less than useful tool, so I looked skyward for other clues. Like any good naturalist, I squinted in an effort to see if the branches were alternate or opposite, trying to follow the twigs with my eyes out to a point where individual lines were visible from the mass of branches. As I leaned even further back for a better look, Jared interrupted my focus by asking, “And why do the trees on the left still have their leaves?” I widened my gaze to take in the whole stand and realized that the three trees to the left were all topped with a halo of brown leaves, while the ones on the right stood bare. Aha! Who needs the fine, botanist-approved details of opposite/alternate when a much more obvious clue looms overhead?
The leftward trees weren’t just a few confused individuals who hadn’t quite gotten the memo about autumn—they’re red oaks, a species that exhibits marcescence, meaning that the trees keep their leaves after they’re dead, only dropping them in the spring when the next generation is ready to burst forth. And for most folks, that’s quite a sufficient explanation. Some trees hang onto dead leaves, others don’t. The end. But then there are those among us whose personal motto could be “Wait, but why?”. But why would some trees hold onto dead material when all of their comrades are dropping them to the ground? Explanations for marcescence haven’t reached the level of scientific consensus but a few theories have gained traction.
The most prevalent theory here is that trees with marcescent leaves are just a few rungs behind on the evolutionary ladder toward full deciduousness. Coniferous trees, with their svelte green needles that seem to last all year (they’re actually shedding some needles constantly), were the first to evolve. Hanging onto green needles year-round increases the amount of time a tree gets to photosynthesize and reduces the nutrient loss of dropped leaves. But as the millennia passed, some trees evolved to drop all of their leaves at one time in response to changing environmental conditions such as day length and temperatures. Deciduous trees reduce water loss and frost damage by kicking all of their leaves to the curb at one time as the weather gets cold and days get short. Oaks, and their relatives beeches and hornbeams, exist in an evolutionary middle ground between these two strategies, neither fully one nor the other. Check back in a million years or so, and perhaps these trees will have embraced a completely deciduous lifestyle. Or perhaps another strategy will have evolved in response to warming temperatures and…I digress.
But science wouldn’t be science (or all that interesting, really) if there weren’t some other contending ideas. A Danish study from 2001 suggests that marcescence has a protective effect for trees. In this experiment, Claus Svendsen showed that deer preferred to nibble on twigs that had been hand-stripped of their leaves instead of twigs with those pesky marcescent leaves intact (I too prefer hand-stripped twigs when possible—bonus points if they’re artisanal). The dead leaves have a lower nutritional content than the twigs and buds, making them less appealing to deer, which can’t navigate their way past the leaves to just the twigs. But this effect only held true for beech and hornbeam, not so for oaks. Other scientists postulate that marcescence is a strategy that helps trees growing in poor soils. Trees that drop their dead leaves in the spring give themselves a boost of nutrients right when they need it the most. Or perhaps it’s because marcescent leaves tend to be found on younger trees or the lower branches of larger ones, those leaves can trap snow and keep that moisture close to the tree come spring. They could also provide a degree of protection for buds and twigs over the winter.
So which theory is right? All of them? None of them? A little bit of each? Yes. Exactly. Check back in a million years or so.