February 25, 1951: 67 years ago

Why do some deciduous species still have leaves in the winter?  This specimen of American beech (Fagus grandifolia) was collected by Bayard Long in Philadelphia 67 years ago.  Surprised to see leaves in February?  Signaled by shortening days, most deciduous trees in our region drop their leaves in the fall when a layer of cells form at the base of the leaf, forming an “abscission zone.”  This abscission zone is formed by weakening cell walls, thereby separating the leaf from the stem.  However, this doesn’t happen in all deciduous plants.  The retention of these leaves is called “marcescence.”
 
In our area, some species keep their dead leaves through the winter, and it can be quite beautiful on a snowy day.  In particular, beech and oak species retain their dead, brown leaves through the winter, especially on lower branches and in small plants. Why?  Well, I’m not convinced we really know.  There are a few reasons that have been suggested.   
 
Some of these ideas include:
1) serves no adaptive function, but just a byproduct of evolution,
2) deters animals (like deer) from eating stems in the winter,
3) facilitates better nutrient absorption and/or ensures leaf litter mulch layer in the spring when the leaves fall to ground (leaves decays faster once on the ground),
4) protect the leaf buds for next year from drying out or frost damage in the winter. 
 
I’m sure there are other hypotheses too.  Species differ in their leaf out times, leaf drop times, leaf lifespans.  The who, what, when, where, and why of evergreen and deciduous leafing strategies is fascinating.  And much remains unknown.
 

The persisting, dead leaves that remain on American beech can make for quite a scene in the winter.