“And to this very day, when all the other trees lose their leaves, the great oaks (and for the sake of this post, the American Beech) keep theirs in memory of their ancestor’s promise. Only in Spring’s time, when the other trees grow new leaves, do the oaks drop theirs. Winter cannot steal spring away and control the weather forever.”
As the days get shorter and colder, trees have an important “decision” to make. They must shed their leaves or hold tight through the winter. Evergreens hold tight to their needles or leaves, but in evolutionary terms, evergreens are old. Over time, trees diversified and accumulated many beneficial adaptations. One of those was the ability to lose leaves. It is these deciduous trees that dominate areas like Statesville, where there are patches of mixed evergreen and deciduous forests. Between evergreens and deciduous trees, there seem to be some that do not obey all the rules. Tree such as oaks and beeches belong somewhere in the middle. To understand why these trees hold their leaves longer than others, we must first understand why deciduous trees shed in the first place.
Shorter periods of daylight coupled with colder temperatures trigger a hormone in deciduous trees that sends a chemical message to every leaf. This message is to, essentially, “get off.” Cells then appear at the exact point where the leaf and stem meet. These cells start to rapidly divide, creating what is called an abscission layer. The division of these “abscission” cells prevents nutrients from reaching the leaves and also prevents chlorophyll from being replaced. Therefore, the leaves start to die just as other pigments (yellow, red, and orange) start to be seen and enjoyed by many people. Abscission comes from the Latin word abscissio, meaning “breaking off” (think scissors).
Leaves are the main food producers for trees through the process of photosynthesis. The leaves trap energy from the sun and use it to convert water and carbon dioxide into glucose (food) and oxygen. On short, cold days, food production decreases significantly. Deciduous trees choose to push these food producers away rather than keep them around with decreased productivity. Not that there’s anything wrong with retaining what gives you food. Evergreens do it. For example, one of the luxuries that a southern magnolia, a broad-leaved evergreen, has is it is able to produce food through photosynthesis all year long. Another advantage of retaining leaves would be that the tree would not have to allocate energy and resources into growing new ones every year. For deciduous trees, though, the bad outweighs the good.
If there happened to be several warm days in the winter, the leaves would resume photosynthesis (which requires water). If this warm spell was following by a frost, the leaves would be in trouble. The water would be trapped and would freeze. Because water expands when it freezes, the ice crystals would damage internal structures beyond repair, killing cells and tissue. In areas where snow and ice accumulates, entire branches could even be pulled off of trees, putting the tree in serious danger. So, for many trees, abscission keeps trees from becoming damaged and also helps conserve water and energy.
Even with the risks, the American Beech tree, Fagus grandifolia, decides to hold most of its leaves throughout the winter. The process of retaining dead plant organs that are normally shed is called marcescence. How do beech trees get away with this strategy and is there any advantage? There are three main theories, but these theories trigger more questions than answers.
Marcescence could be a trait found in both juvenile trees and/or lower branches of older trees. Holding on to leaves throughout the winter could benefit younger trees simply because after the taller trees shed leaves, more sunlight would reach the bottom. This would allow these understory trees to take advantage of the increased sunlight and photosynthesize longer. The American Beech tree does really well in the shadows of larger maples, oaks, and birches. Could it be that retaining leaves is what allows the beech to be so shade-tolerant? It’s hard to imagine that the beech trees are able to take advantage the increased food production process since their leaves look to be out of chlorophyll in the winter (see the yellowish, brown leaves above).
Another idea is that having these leaves “stick” around plays an important role in nutrient cycling for the tree. If the leaves shed in autumn, they would join all the other leaves on the forest floor and start the decomposition process. There’s a chance that the nutrients produced from this decomposition could leach away and the trees would have none for the spring when they are growing. Instead, the beech trees wait to drop their leaves in the spring to ensure that are some nutrients in the soil. This could be important for small understory trees that have small root systems. However, the problem with this theory is that big leaves often take months to decompose. Maybe there’s another answer.
Retaining leaves may help to deter large herbivores, such as deer, from browsing. At least one researcher thinks that the leaves protect buds and twigs from being chewed off because the leaves are less nutritious and palatable. Svendsen (2001) showed that the European beech tree, Fagus sylvatica, was browsed significantly more by weight and number of branches when the leaves were removed. Chemical analyses revealed that the protein and fiber content of beech twigs was higher of higher quality when compare to marcescent leaves. Maybe these leaves do act as an herbivore deterrent.
No one seems to know for sure why these trees chose to not give in to peer pressure and drop their leaves. It could be one of the previous theories. It could, in fact, be a combination of these theories. It could be something yet to be discovered.
Ah, the beauty of nature.
Svendsen, C. R. 2001. Effects of marcescent leaves on winter browsing by large herbivores in northern temperate deciduous forests. Alces, 37: 475-482.