Welcome back to Birdbrained Science! Last month, I hinted that not all leaves fall — and no, I’m not referring to evergreen trees, when I say that. I’m talking about another nature phenomenon that starts with an m: marcescence! I discovered marcescence last February when I noticed a tree still had its leaves, despite the layer of ice coating its branches. Marcescence is pretty much exactly that — when deciduous trees (or trees that normally shed their leaves in autumn) end up holding onto their leaves past shedding time.
Autumn in New England is largely characterized by leaves changing color and falling, both of which are part of a process scientifically known as leaf senescence. Leaf senescence is the last stage of leaf development. In leaf senescence, leaves die, allowing trees to relocate nutrients from leaves towards other parts of the tree. The cooler temperatures and shorter days usually trigger the process, which involves chlorophyll (the pigment that makes leaves green) breaking down. When chlorophyll breaks down, the foliar (leaf) nutrients are reabsorbed into the rest of the tree. Chlorophyll is also responsible for the green pigment in plants, which is why the breakdown causes leaves “turn” orange and yellow in the fall — in reality, the orange and yellow pigments were always there, just covered up by green. The death of the chlorophyll allows for the orange and yellow to finally be on display. During senescence, auxins (or hormones that control plant growth and development) decrease, which allows for a layer of weakened cells to form at the base of each petiole (leaf stalk). This layer, known as an abscission layer, allows for the leaf to separate and fall from the tree.
But in marcescence, leaves simply fail to form abscission layers. The earliest records of marcescence date back to 1749, but despite the 276 years of its documented existence, it is still fairly understudied. We’re not completely clueless; we know that marcescence seems more common in smaller trees, or in the lower branches of larger trees. We know that many marcescent leaves can stay on all the way through April. And again, we know that in marcescent leaves, the autumnal abscission layer never occurs and thus the leaves stay on.
However, we’re not sure why.
There’s no shortage of hypotheses. Since many species observed exhibiting marcescence are in the plant order Fagales (including beeches, oaks and chestnuts) — meaning that they’re related to each other — one hypothesis suggests that marcescence might be a leftover trait from evergreen ancestors. According to this, marcescence might not have a special reason to exist; rather, it’s just the result of some species failing to fully evolve away from their ancestral traits.
Another hypothesis suggests that marcescence discourages predators from munching on winter buds and twigs. The idea is that the dead leaves, which are low in nutritional value, will not appeal to potential winter herbivores and cause them to search for sustenance elsewhere; thus, marcescence would be acting as a form of protection against deer and insects. This hypothesis has some evidence — more marcescent leaves grow within the deer browse line (the area within which deer can reach and eat vegetation) and there is some evidence that removing dead leaves from grass in the winter results in higher grazing — but more study is needed before anything can be concluded.
A third hypothesis proposes that marcescent leaves might prevent frost damage or desiccation to overwintering buds. Marcescence is more common among lower branches, which potentially experience lower temperatures. Additionally, marcescent leaves seem to buffer the microclimate ) of some species. Still, we need more studies to confirm this, especially longer-term studies.
Our fourth hypothesis is, at first glance, seemingly more altruistic: maybe the reason trees hold onto their leaves is actually to provide a habitat for winter wildlife to roost. But would trees really hand out animal habitat just out of the kindness of their hearts? Probably not — not even squirrels deserve free housing in today’s market. Instead, by hosting animals, the animals’ excrement might increase soil nutrients around the trees, setting them up for more resources.
One hypothesis is that marcescence makes trees more flammable. Having dead, dry leaves will usually make something more likely to catch on fire. You might think that sounds weird — shouldn’t trees want to avoid catching fire? — but some tree species actually depend on fire to help spread seeds. However, many species that exhibit marcescence are not fire-loving trees. Again, this hypothesis isn’t well-tested.
Believe it or not, there are still more hypotheses as to what the reason behind marcescence could be. For the sake of time and word limits, I won’t go into as much depth about these last few, but I’ll give a quick summary. One is that the retention of leaves throughout the winter allows trees to photosynthesize for longer and later into the season. Another is that marcescence acts to feed the soil beneath the tree for more nutrients throughout the spring, allowing for more gradual nutrient cycling. Lastly, marcescence might allow trees to gain more nutrients from the leaves, since senescence takes longer (although one study didn’t find any difference in nutrients between marcescent and non-marcescent leaves).
Regardless of whatever the true reason is, now you know — not all leaves fall in fall, after all.
Most of today’s content came from a 2023 study conducted by J. Mason Heberling and Rose-Marie Muzika. You can take a look for yourself here.
I’ll leave you with one final question from today’s topic: How do you know when it’s time to let go?