Marcescence

Marcescent leaves on an American beech tree (Fagus grandifolia) growing at Cedar Swamp Wild Management Area in Torrington, CT

When we observe organisms that are, broadly speaking, similar to humans, the purposes of their characteristics and behaviors are often quite intuitive for us to figure out. When watching a deer browse on a young sapling, it quickly becomes quite clear that she is using her teeth to chew her food because we do the same. When observing a fish moving through the water, we interpret the motion of his fins, and its association with the forward momentum of his body, in a loose analogy to our own arms and legs. There has been some fascinating scientific research detailing the physics of exactly how teeth and limbs do their jobs so well, but their general purpose is pretty easy for anyone to figure out.

This is not always the case. Sometimes we struggle to interpret the behavior of other animals because our own cultural baggage prevents us from seeing the truth. For hundreds of years, western society’s prejudice against homosexuality resulted in scientists assuming that any two animals engaging in mating behavior must be of the opposite sex; in reality, same sex couples in the animal kingdom are quite common. In other cases, the behaviors or characteristics involved are so different from our own that we really need to think outside the box in order decipher them.

One such biological puzzle that we have yet to solve is that of marcescence. Marcescence refers to the tendency of certain kinds of deciduous trees to hold onto their dead leaves from the previous growing season during the winter, rather that shed them as many other species do. Here in New England, the two main kinds of marcescent trees are the American beech (Fagus grandifolia) and members of the oak family (Quercus spp.), but other trees that exhibit the same phenomenon can be found in temperate habitats throughout the world. Compared to senescence — the scientific name for trees losing their leaves in the autumn — marcescence is very understudied and nobody is quite sure why it occurs in some trees and not in others. We humans don’t have leaves (obviously) and so our own bodily experiences aren’t much help when it comes to generating hypotheses to test. Instead, we must take what we already know about plant ecology and try to think about what kind of advantage, if any, holding onto one's leaves would have in the environments where these trees live. 

In general, proposed explanations for marcescence can be divided into two categories, each attending to a different problem that a marcescent plant might face. Explanations in the first group suggest that marcescence has something to do with maximizing access to nutrients. As we talked about in October’s post on invasive earthworms, fallen leaves are a crucial part of the decomposition cycle in deciduous forest ecosystems, with nutrients from those leaves being recycled into the soil as they are broken down. Hypotheses explaining marcescence as an adaptation for obtaining nutrients suggest that it may allow individual trees to increase the likelihood that they will be able to re-collect the nutrients contained in their own dead leaves. In cases where marcescent trees drop some of their leaves in the fall, hold onto others over the winter, and drop them in the spring, a more gradual decomposition process may result in more efficient nutrient extraction. For species that do not follow this schedule, there is still some evidence that marcescent leaves undergo chemical changes that make them easier for soil organisms to decompose. Because marcescent leaves tend to be concentrated in the lower branches of a tree, it is thought that they may be more likely to land close by when they fall. This would mean that any nutrients extracted from a theoretically more efficient decomposition process would go right back to the tree that shed the leaves in the first place. 

Marcescent leaves on a sapling oak (Quercus sp.) growing at Sessions Woods Wildlife Management Area in Burlington, CT

Another way that marcescence could help with nutrient retention is by giving trees more time to recover materials from their leaves before they fall off. During fall senescence, trees stop producing chlorophyll — the main pigment used to capture light in photosynthesis — and allow what remains in their leaves to breakdown, exposing other pigments and causing the leaves to change color. As this happens, the tree removes and stores valuable nutrients from its leaves, a process which requires energy and thus enough remaining chlorophyll to perform photosynthesis. Once all of a tree’s chlorophyll has broken down, it is no longer able to photosynthesis and so is unable to recover and recycle any more nutrients from its leaves. Deciduous trees therefore must balance the potential rewards from keeping their leaves active for longer with the risk of losing resources should an early frost come along and kill all their leaves while they are still active. In cases were holding onto their leaves for longer allows them to recover more nutrients, marcescent trees may have an advantage; alternatively, marcescence may result from a tree “miscalculating” and having its leaves die off during an early frost. 

The second group of hypotheses explaining marcescence proposes that it has something to do with protecting vulnerable parts of a tree during the winter. As with the first nutrient-retention hypothesis described above, both of the hypotheses in this category include possible explanations for why marcescent leaves are most common in the lower branches of mature trees and in saplings. The first hypothesis suggests that marcescent leaves deter herbivorous animals, such as deer, from browsing on the tree. Compared to overwintering buds and twigs (not to mention living, green leaves), dead leaves don’t provide a large animal like a deer with very much nutritional value and so they tend to avoid them in favor of other options. By covering their buds and twigs with dead leaves, the thinking goes, marcescent trees in the sightline of browsing deer (i.e. saplings and mature trees with low branches) make it more difficult for the deer to nip off their buds or, at the very least, make them harder to spot. Marcescent leaves may also help to insulate buds from frost damage, especially in the lower branches, which tend to experience the lowest temperatures. This appears to be the case, for example, in young individuals of the New Zealand cabbage tree (Cordyline australis), though it is not clear whether or no marcescent leaves serve this function in other species as well. 

As is often the case in situations like this, none of these hypotheses are mutually exclusive — that is to say, it is entirely possible that marcescence provides advantages in both nutrient retention and bud protection and that these advantages together drive its evolution. Conversely, it is possible that marcescence has no evolutionary advantage at all. If a trait is neutral — providing no particular advantage or disadvantage — it won’t make reproduction any less likely for the organisms that have it, but it also won’t necessarily become more common with time like an advantageous trait would. Where neutral traits are common in a particular population, its usually either because a past bottleneck event happened to favor individuals with that trait or because the trait is residual of something that was once more useful. Perhaps marcescence is just the byproduct of an incomplete switch from an evergreen growth habit to a deciduous one. Or maybe this is the case in some species, but not in others — there is not rule that says a similar trait must have the same explanation across species, though this is often the case if they share a similar environment. At least for now, we do not know.

Over the past few years, there seems to have been a jump in the number of scientific studies looking into marcescence, its evolution, and its role in deciduous forest ecology. Hopefully, this will continue and we will eventually gain a better understanding of this fascinating phenomenon. If you are an aspiring scientist interested in studying plants, it's a great mystery to get involved with. If not, marcescent trees are still a fun thing to look for during a winter stroll — they are actually quite common and now that you know they exist, you will start to notice them everywhere, adding their textural and auditory beauty to the landscape, buzzing and vibrating in the wind like wings of flying insects in the dim twilight air. 

For more sources and detailed information on the hypotheses discussed here, plus a few more, see Herberling and Miuzika’s recent review paper.