Have you ever noticed leaves hanging on to certain trees throughout the winter? This phenomenon is known as marcescence, and why it happens is still widely unknown, though scientists have theorized a variety of reasons. Typically, most deciduous trees drop their leaves during the fall season, however those who hang onto their leaves are marcescent.
Swamp white oak retaining leaves in February.
As the autumn season starts and hours of sunlight decreases, chlorophyll in leaves begins breaking down, revealing a variety of colors in different tree species. Shades of red, yellow, orange, and brown shine through before the leaves fall from the branches. This occurs when shorter days and cooler temperatures prompt the trees to prepare for the winter ahead by conserving nutrients through the dormant period. It is at this point that most North American deciduous trees create what is known as an abscission layer between the leaf stem and the branch, causing a separation, allowing the leaves to drop to the forest floor without harming the tree. The fallen leaves soon provide nutrients in the soil as they decompose. However, marcescent trees have a delayed reaction forming the abscission layer, causing many leaves to remain on the branches through the winter until they are finally released in the spring.
This phenomenon is most common in young species of the beech family (Fagaceae) including beech (Fagus) and oak (Quercus). It also occurs in the birch family (Betulaceae), including American hophornbeam aka ‘iron wood’ (Ostrya virginiana), and American hornbeam aka ‘muscle wood’ (Carpinus caroliniana).
Juvenile beech trees in winter.
Young red oak tree in winter.
However, marcescence is not limited to juveniles and these species only. It has also been seen less commonly in witch hazel (Hamamelis virginiana), sugar maple (Acer saccharum), and more.
So why does this happen? Well, that is the tricky part. Scientists believe there are many reasons as to why some leaves hang on throughout the winter. However, like many natural occurrences, there haven’t been any singular reasons affirmed. Scientists believe that marcescence can occur from genetics or environmental stressors, such as periods of drought caused by climate change. Periods of drought or excessive precipitation can have an impact on the available ground water needed to sustain normal plant biology. Some believe that there is a symbiotic relationship that occurs between winter birds and marcescent trees. The belief is that the leaves act as a windbreak to protect birds from cold winter winds.
Since many marcescent trees are juvenile, there is also reason to believe that holding on to the leaves is a protection measure taken to deter deer from browsing on the buds of next year’s growth. Another hypothesis points to a delayed nutrient source. The leaves that have dropped already provided nutrients to the soil through the fall and winter season; those leaves that hold on and drop closer to spring can now provide nutrients through the spring growing season and act as a natural mulch, maintaining soil moisture in dry sites through the active season.
Although we haven’t completely nailed down the reason for marcescence, it provides a unique addition to a snowy forest landscape. So next time you’re out in the woods, take a look around and see if you spot any marcescent trees.
