(Illustration: CHS)

It seems that most people cannot help but plant trees whenever they have an opportunity. Over years, the settlers of Capitol Hill have cut down and replaced countless trees — native old growth firs and hemlocks turned to introduced oaks, ashes, elms, and maples. These new residents are mostly deciduous — but have you ever noticed that not every one of them drops their leaves?

They say nature abhors a vacuum. The typical understanding of this is that if there’s an opportunity to be had, some species will jump in and give it a try. Another similar way of thinking of this is that if there’s an adaptation to be had, some species might give it a go. Sometimes we might not know exactly why something is the way it is, but boy, do we like to equivocate when we aren’t quite sure. In this case, the focus of our equivocation are trees that demonstrate marcescence.

The root of this word tells a tale (as most words do if we bother to follow their trail) — marcere, is a latin word that refers to something drooping or withering. This is quite descriptive of a desiccated brown oak leaf still hanging on the tree that grew it, months after others have been shed, attached but no longer alive.

A tree demonstrating marcescence keeps some, or all of its leaves attached throughout the winter.

We aren’t talking about the evergreens that give us our State’s moniker. Instead we’re talking about oaks (Quercus), beeches (Fagus), hornbeams (Carpinus), and a few smaller trees like witch hazel (Hamamelis), which hold onto their leaves, or at least some of them throughout the winter. The catch is that these leaves are dead but not abscised.

Abscission is the term used to describe the leaf dropping process. Most deciduous trees go through a familiar process: when the right moment comes, their leaves turn various, often stunning colors, which is followed by leaf drop. Abscission happens in three steps.

One: The plant greedily takes back as much chlorophyll as it can from the leaf, revealing the other chemicals present in the leaf (our fall color). Two: The plant forms a protective layer at the abscission zone (afterall, having a bunch of open pores after dropping leaves wouldn’t be helpful to a tree concerned with pathogens and moisture retention). Three: The leaves are forced off in a variety of ways, from enzymatic dissolution to cellular explosion. And then all the leaves drop. (Other things can be abscised – like flowers and fruit – I ponder this when the Christmas cactus on my desk makes its annual mess of wilted, discarded blooms.)

A mature oak showing marcesence in lower branches. (Imagine: Brendan McGarry)

Of course, as is nature’s way, some trees don’t totally adhere to this process as strictly as we’d like (if only for the sake of simplicity in classification). A slender young oak might be entirely marcescent for several years of early life. A mighty mature oak might keep leaves on lower limbs while losing the leaves in its crown on a “normal” schedule.

My favorite thing about marcescence is that we don’t know exactly why trees do it, though there’s plenty of speculation.

Even when humans observe a phenomenon, it can be difficult to demonstrate empirical evidence (as is the way of Western science) for why something happens the way it does. And I would wager sometimes scientists stall out at “there are more important things to focus on.” Valid, even if it feels lacking in whimsy to me.

Imagine though that you are a young oak tree, tender, and still small enough that a passing deer could munch on your slender branches or leaf buds. Having unpalatable dead leaves all over could possibly be a deterrent. Or maybe pushing past those crinkly leaves would be too noisy for a herbivore that’s constantly on the menu?

Herbivores aren’t the only thing that could hurt those leaf buds. After all, a tree stored energy all year to develop buds, which are the springboard for the coming growing season and are worth protecting. Some botanists speculate that marcescent leaves help reduce cold damage to buds.

And it’s possible that these trees are just holding onto precious nutrients for the right moment. When marcescent trees finally abscise their leaves, it’s often right before it’s time to break bud and spread their leaves. Possibly this is just a coincidence, because leaves are sometimes mechanically thrust off by a swelling bud. But it is also the perfect time for some extra compost around a tree’s roots – especially if you grow in tough soils as oak and beech are known to do. (Also why they are often chosen for street trees – it’s not a place for sensitive plants.)

A witch hazel showing marcescence and blooms at the same time. PC: K M via Flickr

Whatever the reason, I recommend you go out and look for this phenomenon on the Hill yourself. Oaks, beeches, and hornbeams abound and you can find them using the Seattle street tree map. You may even find a witch hazel growing in someone’s yard by the blooms they are just starting to put forth – tell me if they still have their leaves or not. Age, conditions, location, and a number of other factors might decide whether a tree is fully or partially marcescent or even if they managed to hold onto all their leaves through those particularly windy days of the winter.

To me they seem like the last veil of winter and when those leaves finally drop, I know things are finally turning up. We might have an extra day between us and spring this year, but the groundhog says we’re on track for spring arriving on time. I’m be curious if those marcescent trees agree.

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