Leaves are Leaving
(Though some stick around!)
Watching colorful leaves swirl from the tree tops in a gust of autumn wind is one of the pleasures of being outdoors this time of year.
Like so many things in the natural world, tree behavior in our northern forests takes its cues from the sun. Once deciduous trees detect a reduction in the amount of daylight, they start to reduce the amount of chlorophyll they produce. When chlorophyll production stops, it is broken down and absorbed back into the tree. Until this point, other pigments present in the leaves have not been visible, overshadowed by the green of the chlorophyll. But now, as the chlorophyll disappears, the vibrant reds, yellows, browns and oranges we enjoy at this time of year are revealed.
In addition to changing color, deciduous trees are preparing to shed their leaves. Dropping their fragile leaves helps trees conserve resources and protect themselves from freezing temperatures, a lack of available water, and the potential for damaging wind and heavy snow. They do this by growing a layer of cells between the leaf stem and the tree branch, called an abscission layer. This layer stops the transport of nutrients and water to the leaf. Once the leaves have fallen off, the abscission layer also protects this sensitive area of the plant from winter cold and dryness.
Interestingly, there are a handful of trees that hold onto their dry, brown leaves throughout winter. This interesting phenomenon, known as marcescence, is not fully understood. The abscission layer on these trees does not completely form until spring, causing their leaves to linger. In our area, this is commonly observed in oak and beech trees. Marcescence almost always occurs on sexually immature parts of the tree, the parts that have not yet formed flowers. Trees may often lose this characteristic as they age. Ultimately, marcescent trees lose their leaves come springtime when swelling buds push the old leaves out of the way.
There is no consensus in the scientific community as to why some tree species keep their leaves, but there are a number of theories. One theory is that hanging on to the withered leaves protects next year’s buds from browsing deer or the drying winds of winter. Another theory is that these trees benefit from a spring drop and the timely compost it provides around its base. In either case, the leaves are cycled back to enrich the soil which is critical for next years’ growth.
Photos: At top, these beech leaves were observed in April, still in place on their stems. Below, white oak leaves reflect vibrant red and orange hues as their chlorophyll disappears.