Walking in the Wissahickon last year during the windy winter months, I was enjoying the solitude when I heard a steady rustling that had been going onfor a while. Focusing on the sound, I could see American beech (Fagus grandifolia) leaves vibrating in the wind. These faded, thin, ghostly leaves created wonderful sounds in the otherwise quiet winter landscape.
I wondered why beech and other tree species, like the Arboretum’s large Bender oak (Quercus x benderi)sometime hold on so tenaciously to their leaves all through the winter months, many to drop only when new growth forcefully pushes them off in the spring.
It turns out that these leaves that don’t want to leave are called marcescent [mahr-ses–uhnt].Marcescence is when plant parts, in this case leaves, remain attached to the plant whenwithered. If you walk through the woods in our area in mid-winter, you will find marcescent leaveson a handful of trees, especially young or juvenilespecies such as oaks (Quercus), American beech, eastern hophornbeam(Ostryavirginiana), musclewood (Carpinuscaroliniana), and perhaps maple (Acer). Each species’ leaves are different, and produce a different sound in the breeze. So if you listen closely, you can identify the tree species in the winter without looking, by sound alone.
But what functional or evolutionary adaptation caused this clasping characteristic?There have been many reasons proposed since the word appeared in the early 18th century.
Protection:Some speculate that marcescent leaves protect new buds and deter browsing animals such as deer or the Pleistocene magafauna that existed millions of years ago.The leaves may also absorb and radiate heat from the winter sun, which may provide frost protection or allow limited photosynthesis (energy production) in the chlorophyll under the twig’s thin bark.
Nutrient Boost:Another hypothesis is that releasing leaves in the early spring facilitatesthe release of nutrients to help the tree during the early growing season, whereas those nutrients may have been washed away or leached too far into the soil if the leaves fell and decomposed during the normal leaf drop.
Moisture Management:Others suggest that marcescent leaves can trap snow and reduce the velocity of dry winter winds, leading to more moisture at the tree’s base in the spring.
Regardless of the reasons that marcescent leaves exist, they are a wonderful feature of our winter landscape. Next time you are out in the winter snow near a young American beech, close your eyes and listen. It is remarkable what you will hear.
I leave you with the 2nd stanza of the poem Reluctance by Robert Frost:
The leaves are all dead on the ground,
Save those that the oak is keeping
To ravel them one by one
And let them go scraping and creeping
Out over the crusted snow,
When others are sleeping.