When Leaves Don’t Leave: Marcescence

beech leaves
last year’s beech leaves and bud for this year’s leaves

Quick. I have to talk about old leaves before new leaves happen. Saw a tulip poplar headed toward budburst this morning, so I need to hurry.

Have you noticed how some deciduous trees drop every leaf in fall, but others hang on to leaves all winter? Oaks, for example? I’d wondered about oaks for years, and was never satisfied with the “they used to be evergreen” explanation, despite seeing always-green Live Oaks just one state over. I mean, didn’t all trees evolve from conifers way back before the Cretaceous debut of flowering plants? So, are oaks somehow “less deciduous” than maples or hackberries, which consistently drop every leaf directly onto my lawn each fall?

Marcescence. That’s the word that describes the phenomenon of winter leaves that linger. It doesn’t explain it, but at least it names it. From the Latin marcere / wither. The leaf’s food supply is cut off, but the leaf isn’t. Apparently, the word is used with fungi too, to describe types that dry utterly but revive with a bit of moisture. (I wonder if it would apply to glade moss which, during drought, springs green and spongy at first wetting?)

After I started Naturalist training, I noticed beech trees kept winter leaves, too: pale, peachy-tan leaves warm against silver trunks. Wherever the gray hills of Warner woods are splashed with that peculiar color—peach/brown/yellow/gold—that’s where young beeches march to the ridge. They almost glow.

They also make music. On a quiet walk, the only leaves that aren’t under my feet are beech rattling overhead at the slightest wind.

And then, two years ago, research with the word marcescent led me to one more local species to join oak and beech: hop hornbeam (Ostrya virginana). I hadn’t noticed these because they weren’t on my usual beat. Until this weekend. During the Couchville Cedar Glade bioblitz we ambled streamside among young, fluttering trees I assumed were baby beeches. But when I stopped to admire the buds, expecting a long, pointy paintbrush, I found short and pudgy: hop hornbeam. What a thrill. The textbook marcescent trio is now personal. I’ve seen all three Species Most Likely to Linger, and in all their withered glory: oak, beech, hop hornbeam.*

None of this still explains why, though, does it? Or explains advantages leaf retention may offer?
I gathered a few opinions:

The Beech family occupies a “middle ground between evergreen and deciduous.”
Marcescence is an evolutionary “delay.”
Marcescence is a “juvenile trait,” for younger trees and lower branches.
Lower branches cloaked in dead leaves can protect buds from browsing deer.
Lower branches cloaked in dead leaves can provide insulation during snowfall.
Marcescence can reduce moisture loss: Oak and Beech tend to grown on dry, sometimes infertile sites.
Leaves that fall in spring act as timed fertilizer for the soil, just when awakening trees are better able to use it.

All these sound plausible, but I know one benefit for sure: winter leaves help identify trees.

You don’t have to remember marcescence or how to spell it, but the idea comes in handy as another clue to what’s around.

 

*Oak, beech and hop hornbeam aren’t the only species. See this article for more info: Winter Leaves That Hang On.

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