Marcescence is when a plant part dies but is not shed. It’s most frequently noticed in the winter, when certain deciduous tree species don’t lose their dead leaves. Oaks and beeches have normally marcescent leaves. That’s an oak at left, and marcescent beech leaves are pictured below the jump.
Most deciduous trees lose their leaves when they are no longer photosynthesizing, the process by which a plant gets nutrients. When photosynthesis slows in the fall, the base of the leaf stem produces an enzyme that forms a separation layer, known as the abscission zone, which prevents infection and eventually causes the leaf stem to separate from its branch and fall to the ground. This process is called abscission.
Marcescent leaves don’t form an abscission layer, and only drop in heavy winds or when new leaf buds push them off in the spring. It’s not known exactly why marcescence occurs. The hormone that stimulates abscission is called ethylene. It is believed that the hormone auxin inhibits abscission by preventing the abscission layer from forming. But no one seems to know why some trees are normally marscescent and others aren’t.
Enjoy the texture that the papery, withered leaves bring to the otherwise somewhat two- dimensional winter landscape, courtesy of marcescence.
Earth Friendly Gardening 