We’re in the bitter cold of January, but the Pin Oak outside Wolf Ridge’s Science Center is still holding onto its leaves as though it were fall. You’ve probably seen oak trees near you that also hold onto their leaves through the winter. What’s the story?
Did you know that deciduous tree leaves do not really “fall” off? They are pushed off by the tree. How does this happen? A decrease in sunlight during the onset of winter causes most deciduous trees to begin reabsorbing nutrients from their leaves, especially chlorophyll. This reabsorption triggers specialized separation cells where the leaf petiole meets the branch to expand and release their hold, allowing the leaf to be pushed off. The process is called abscission, coming from the Latin word for “cut.”
Oak trees also begin reabsorbing nutrients in the fall, turning colors along with their deciduous colleagues, but do not complete the abscission process until late in the winter. Instead, at the base of each oak leaf is a single cell where the stem attaches to the branch. This cell is currently holding the leaf tightly on the Science Center’s pin oak – even if you gently tug on the leaf, it will not come off the branch. All oaks (Quercus sp.) are characterized as evolutionarily “old,” which may be why they seem slow to pick up on the fact that they should be losing their leaves in winter.
Pin Oaks aren’t native to northern Minnesota. This one was planted a number of years ago to see how it would do in the region. Some scientists believe oaks will become more common in northern Minnesota as the climate changes. You can see a number of native Red Oaks as you look out over the forest from the Astronomy Deck.
Are there trees near you still holding onto their leaves, waiting to undergo abscission? Look for oaks or other deciduous trees in your community, and see what’s happening with their leaves!
– Serena Hixson and Sonja Smerud