Friday, February 13, 2015
If you are out in the woods this time of year you may have noticed that while most trees have shed their leaves for the season, there are a few trees that have not. These trees have leaves that turn brown, but do not fall off, and instead dangle in the breeze and stand out among the many tree branches that have lost their leaves for the season.
Trees and shrubs in the Beech family (Fagaceae), the Hazelnut family (Corylaceae), and the Witchhazel family (Hamamelidaceae) retain their leaves in the dormant season through a phenomenon called marcescence. Marcescence means “withering but not falling off” and it refers to plants that retain their leaves through the winter. Normally, plants develop an abscission layer, where the connection to the vascular tissue is cut, and this is what causes flower petals, leaves, and fruits to be released from an individual plant. But plants that exhibit marcescence do not develop an abscission layer. This allows the plant to sequester the nutrients trapped in the leaves, instead of allowing them to fall to the forest floor to decay. It is also thought that marcescence evolved to deter herbivory. This makes sense since marcescence tends to only occur on young trees. Larger trees shed their leaves since herbivores cannot reach them.
Regardless of the reason, marscesence makes it easy to identify young beech trees in the forest, as demonstrated by the photo.