Wintertime trees
The trees have turned to gray and skeletal, perhaps they’re even covered in a fine layer of snow, so why are some of them still clinging to last summer’s leaves? What good could it do them to hang on to those dry brown clumps?
This strange winter-long leaf retention is called marcescence and it is most common in American beech, witch hazel, hornbeam, hophornbeam, and many species of oak. When autumn rolls around deciduous trees excrete enzymes that create an abscission layer between the branch and the leaf, causing the leaf to fall off. Remaining leafless throughout winter allows trees to reduce water loss and put energy toward preforming next spring’s leaves in over-wintering buds. Sometimes, leaves on low lying trees or leaves on the lower branches of large trees don’t go through the abscission process as quickly as other leaves because they are exposed to less sunlight. Then, when the cold comes, the enzymes that prompt leaf dropping are inhibited and the leaves are stuck on for the winter.
Marcescence could also be a strategic defense mechanism against winter herbivory. Crunchy brown leaves could deter hungry herbivores and keep them from eating the buds that will eventually form next spring’s leaves. The trees might also be saving their leaves as a snack for themselves. When leaves fall in autumn, they sit on the forest floor all winter, slowly decomposing under the snow. As they decay, their nutrients are released in the soil, but some of those nutrients are liable to wash away when the snow melts. By hanging onto some leaves and dropping them when warm weather comes around, trees can ensure that there will be at least some dependable nutrient source in their soil.
No matter the reason a tree keeps its leave over the winter, when spring comes around, spring bud expansion in the spring time will pop them right off. I think it’s safe to say that no one knows exactly why marcescence occurs but there are plenty of plausible, interesting ideas.
Source
Finley, Jim. “Winter Leaves That Hang On.” Center for Private Forests (Penn State University). PennState, n.d. Web. 25 Nov. 2016. <http://ecosystems.psu.edu/research/centers/private-forests/news/2012/winter-leaves-that-hang-on>.
