Outside along the driveway is a massive white oak with leaves that turn golden and yellow in the fall. The maples around it turn bright shades of orange and red and drop leaves quickly in comparison. Most of the maple leaves are gone by the end of fall. This white oak does drop some leaves in October and November, but I see many hanging on well into winter. Often it’s the lone stand of goldish-brown color on a bleak January day. Its dry, brown leaves continue to rustle in the cold wind, seemingly clinging to the tree branches for dear life. At some point a heavy winter storm will finally force the remnant to fall – but there again, some manage to hold tight until spring.
Why do some leaves hang onto the tree this long? This phenomenon is called “marcescence”. It occurs in sexually immature parts of the tree, which is defined as those who aren’t developing flowers yet. You’ll see this behavior more often in young trees, and as they age the proportion of marcescent leaves reduces. A lot of oaks display this characteristic, so do beech and hornbeam, even when the tree itself is aged.
A minority of trees follow this pattern – and it’s worth keeping an eye out for them when you’re on a brisk, winter walk. They create a neat soundscape to the yard and some softness to an otherwise rigid, icy day. Over the winter those leaves will progressively thin out, decay and become tattered as the elements of the weather, wind and freeze/thaw break them down.
How does this happen? All woody plants create an abscission zone where the petiole (leaf stalk) meets the leaf. These cells create the barrier that cut off the flow of nutrients to and from the leaf in the fall. The leaves change color and eventually drop once the abscission process is complete and the cells fully create the separation.But in marcescent plants (or their specific branches), the abscission zone is not activated until the spring.
Why would this be? Nobody knows for sure, but here are some of the theories:
- To provide more mulch in spring. Waiting to drop leaves in the spring creates an organic leaf litter blanket that provides nutrients to the tree earlier in the year.
- To reduce winter desiccation. This is a common injury that happens when the amount of water lost by the foliage exceeds the amount provided by the roots. It’s kind of like being freeze-dried.
- To deter herbivores’ grazing. Deer and moose would prefer to eat a twig with nutritious, tasty buds, but not with dried up, crunchy leaves on them.
it may be one of the theories above, all of the above, or none of the above! Nobody knows for sure at this point, which makes it a wonderful mystery of nature.
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