Walking through Rock Creek Park on a late afternoon in mid-February you’ll notice many beech trees (and a few oaks) still clinging to dry and faded, tissue-paper-thin leaves. While other deciduous trees in the woods push their leaves to the ground every autumn in an annual act of paternal rejection, certain members of the Fagaceae family cling to their brood through the long cold winter.
Make no mistake, the leaves littering the forest floor did not, upon losing the greenness of youth and taking on the spectacular colors of maturity, leap to ground, voluntarily returning to soil. They were pushed. If you don’t believe me listen to Robert Krulwich, reporting for NPR on “Why Leaves Really Fall Off Trees”.
What made me think of this piece was a walk in the woods last week where I came across a familiar beech tree. I pass this tree nearly everyday as I drive through the park, run the trails or ride my bike down Park Road toward Pierce Mill. The hanging branch in this picture split from the large beech you see to the left during a storm almost two years ago. As the tree shed its leaves that first autumn, I noticed that the leaves–wilting, but maintaining their clorophyll green color–clung to the detached branch long past the time the their dried-up, golden-brown siblings had been pushed to the ground. They stayed through that first winter, and by the time the canopy was again green, the leaves had turned brown and crisp. They still, however, clung to the branch hanging upside down high above the path.
Now, two Februaries after the wind tore this big branch from the tree, the leaves have turned black. Though dry and molded, they continue to hold onto the dead branch–seemingly waiting for the wind (or decomposition) that will finally bring the whole big branch to the ground, leaves intact. That likely won’t happen, however. The solid thick beechwood branch, though unable to push these last leaves off, will still hang in the tree for some time after the wind, rain, and sun have stripped the last leaf from the branch.
In the NPR piece posted above, Peter Raven, president of the Missouri Botanical Garden, explains why, when shutting down food production for the winter, trees shed their leaves. Of course, we know it is in part to protect against high winds and heavy snows clinging to the leaves–a situation that as we saw in the Northeast this winter can cause limbs to crack and snap. But Raven also notes that green leaves, left attached to a tree over the long cold winter could feasibly, on a warm sunny day, begin photosynthesizing. As the food producing leaves draw fluid through the outside layers of the tree just below the bark, a quick drop in temperature might just freeze all that water, killing the entire tree in the process.
Pushing the very leaves that have nourished it all spring, summer and into the fall to the ground becomes a matter of survival for most deciduous trees. So why do some beeches and oaks hold on to their leaves? They’re no longer capable of producing food, and they would best be returned to the soil where they can decompose, providing nourishment to the tree in a new way. Scientists aren’t really sure. All they know is that the abscission cells that like scissors cut the leaf from the tree don’t seem to work all that well in trees of the Fagaceae family. Maybe, though, it’s because the ghostly pale leaves and the soft dry rustling caused by a cold winter breeze in a beech forest such as Rock Creek Park are just too beautiful for nature to pass up.