I spent a large part of the day last Saturday raking leaves again. It wasn’t a great comfort, once I’d finished, looking up and seeing thousands of leaves still clinging to the two oak trees in my neighbor’s yard, (that would fall in my yard) but neither was it a surprise.
I’ve learned over the years to make peace with the late falling oak leaves. After all, spring cleanup should always include raking the lawn, and a few scattered leaves on it aren’t going to matter in the grand scheme of things. Even though it would be so much more convenient if all the trees dropped their leaves at the same time, why they don’t is part of nature’s plan.
In the fall shortening day length tells most deciduous trees that it’s time to stop growing, so the tree forms a layer of waxy, corky cells at the base of each leaf. This is called an abscission layer, and it slows and finally stops the flow of sap to the leaf. Once the sap stops flowing to the leaves they lose chlorophyll and the reds, yellows and oranges that the green chlorophyll was hiding finally become visible.
In some trees like oak (Quercus), beech (Fagus), and hornbeam (Carpinus) , this abscission layer forms much later, so even though the leaves might freeze dead and turn completely brown they still cling to the branches. Pin oaks (Quercus palustris) don’t form an abscission layer until spring, so their leaves stay on the tree all winter. This retention of dead leaves is known as marcescence.
Nobody really seems to know for certain why trees retain dead leaves but some believe that one reason might be to ward off foraging moose, deer and other animals. Animals will eat the bud bearing twigs from the lower parts of older trees and from nearly all parts of younger trees. One theory says that they don’t like the taste or texture of the dead leaves so they stay away from the trees, which means the buds stay safe and can grow on.
Another theory says that dead leaves clinging to the lower branches trap snow and ensure that the trees get plenty of water in the spring when it melts. I’ve seen nature do some pretty amazing things, but this idea seems a little far fetched to me. What, I have to ask, if the tree is on a hillside-doesn’t all the water from the melting snow just run down hill?
Even though we know how the leaves remain on certain trees, we don’t really know why, so I’ll view the “why don’t some leaves fall in the fall?” question as just another one of nature’s great mysteries. It will give me something to ponder as I rake more leaves today.