HEBERLING LAB

Chestnuts (used to be) on Chestnut Ridge
And across the entire state of Pennsylvania.
 
American chestnut (Castanea dentata) was once a very common tree, native from Maine to Mississippi. In the heart of the Appalachians, the historical range covered the entire state of Pennsylvania. I say “historical” and “was once” because it is no longer.  You may occasionally stumble upon an American chestnut tree, especially small trees and saplings persisting as sprouts from the large trees that graced our landscape a century ago. Older trees, with mature fruits, are quite rare. 
 
In fact, some estimates suggest American chestnut accounted for one in four trees in some forests!  So, what happened?  In the early 1900s, a disease caused by a pathogenic fungus (Cryphonectria parasitica) was accidentally introduced with imported Asian trees. It was first recorded in New York City in 1904.  In a matter of decades, American chestnut was nearly decimated by this disease known as Chestnut blight.
 
The Carnegie Museum herbarium captures this change in our forests. 
 
This specimen of American chestnut was collected by influential Carnegie Museum curator Otto Jennings on October 21, 1922 on a field trip of the Botanical Society of Western Pennsylvania to Chestnut Ridge, near Derry Township, Pennsylvania.  Chestnut Ridge is a ridge of the Allegheny Mountains, presumably named for its (once) many American chestnuts. 
 
This specimen is from the fruit collection of the herbarium.  These specimens are different than the “standard” pressed flat specimens on paper.  Instead, they are stored to maintain their three-dimensional structure.
 
Note the note made by Jennings on the label on this specimen:
“Trees from ¼ to all killed by blight.”
 
The case of the American chestnut is an interesting one.  It served important cultural and ecological roles; some even calling a “keystone” species.  There is no doubt that the functional extinction of American chestnut ricocheted through the ecosystem, causing long-term biological changes. Many of these changes we may not know.  Yet, at the same time, despite the species importance, our forests continue.  Presumably other species have filled the functional and physical space of American chestnut. 
 
Disease and pest outbreaks in Pennsylvania’s forests continue.  Many of our critical tree species are likely to decline in coming years and decades.  Some iconic species have already declined or at risk.  These include our ash species (mortality caused by introduced Emerald Ash Borer), American beech (Beech leaf disease, Beech bark disease caused by an introduced scale insect), and eastern hemlock (mortality caused by introduced sap sucking bug, the hemlock woolly adelgid)...to name only a few threats.
 
What will Penn’s woods look like in another 100 years? 
Our collections document the past and present to inform our decisions for the future.

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.