In our yard, I’ve planted 10 oak seedlings of 5 different species. I’ve noticed that of these, 9 of the seedlings hold on to their dried leaves all winter! Driving around our area, I can pick out young oak trees in the winter by the way they hold onto their leaves.
This made me wonder – Why is it that young Oak trees don’t lose their leaves in the winter? It doesn’t seem like mature oaks do the same, at least not the whole tree.
The scientific term for a plant holding onto dead leaves is “Marcescence” – These young oaks and the related beeches, holding onto their dried, dead leaves through the winter, are said to be marcescent. There are a few theories as to why these trees hold onto their leaves, but nobody knows for sure why they do. The natural world is full of mysteries!
Here are a few of the theories, some more grounded than others:
Protecting Trees from Browse
This is a fairly plausible reason for trees to hang on to their dead leaves through the winter – Browse is the term for animals chewing twigs and leaves from a plant. Dead leaves are far less palatable than live leaves, and it has been observed that deer and other animals are much less likely to chew on twigs that still have dead leaves on them than bare twigs.
Even if this isn’t the main reason for marcescence, it certainly is a benefit to the tree to discourage browse. The very youngest seedlings in our garden will often get nipped by rabbits, but our more established saplings that are several years old and 10+ feet tall don’t get touched by anything.
Nutrient Cycling
Because oaks often grow in dry, poor soils, they need every advantage they can muster to conserve nutrients. Tree leaves are a great resource for nutrients, as the roots of the tree draw them out of the soil and deposit them in the leaves. If these oaks dropped their leaves right away in the fall, they could either blow away in the winter winds before they have a chance to decompose, or they might decompose while the tree’s roots are dormant, meaning the tree can’t take up the recycled nutrients.
I don’t quite see this theory as being as plausible as the first one – If you’ve ever grown oaks, you probably notice that the leaves are thick and waxy, taking a long time to break down. In a mature oak forest, the leaves often take more than a year to break down, so it wouldn’t make much of a difference from a nutrient cycle standpoint whether the leaves drop in November or April.
Moisture Trapping
One other theory for marcescence is that by holding onto their dead leaves, trees can trap moisture in the form of snow and ice in the winter. Since oaks typically inhabit dryer soils, it could be a benefit to young trees to trap as much moisture as possible to give them a boost in the spring.
I definitely notice that brushy areas trap a lot of the drifting snow that blows in off the fields, and young oaks do make for a good snow fence. This drifting snow often carries silt off the fields as well, which could boost nutrients in the soil under these trees.
Around here, soil under hedgerows at the edges of fields and open areas often is more moist and rich than the field soil right next to it! So, the Moisture Trapping Theory could definitely be a reason for marcescence.
Benefits for Gardens
While the reason for marcescence in oaks and beeches may not be known, I do know that this trait is does have some real benefits in the landscape! If you like a naturalistic landscape style, or plan your garden for the benefit of wildlife, the tendency for oaks to hold onto their dead leaves into the winter offers a unique look and function in the landscape.
Shelter for Wildlife
One of the biggest benefits of oaks and other trees that hold onto their leaves is that they offer shelter for wildlife. The large, waxy leaves of oaks make for good protection from sleet and snow in the winter, and I notice that our young oaks are a favorite of songbirds when winter really sets in.
Because of their longevity, oak leaves are a favorite of squirrels for building their nests, called “dreys.” In newer developments where there aren’t many hollow trees for squirrels to nest in, planting a few oaks can give them what they need to survive.
Less visible are the countless caterpillars that utilize oaks as their food source – These caterpillars, the larval stage of various moths, often overwinter in rolled up oak leaves or bark crevices. Having somewhere for these caterpillars to overwinter and pupate is extremely important for the continuation of their populations.
Aesthetic Appeal
While the look may not be for everyone – if it isn’t, I highly suggest that you adopt a view shift – there’s no question that an oak sapling full of dried leaves in the middle of winter is a striking sight. Love it or hate it, there’s no mistaking an oak tree in the middle of winter!
I find that because there isn’t much else to look at in the winter, the marcescent oaks give the eye something to look at in the landscape. The sound of the dried leaves rustling in the winter wind is unique, too, adding to the whole “alive” feeling of the naturalistic landscape.
Screening
In addition to shelter for wildlife and aesthetic function, oaks make a great winter screen for views you don’t want to see! Whether it’s an ugly warehouse or a neighbor you’d rather not see much of, planting a grove of oaks offers nearly year-round screening.
The British have been wise to this for years, using marcescent European Beech for hedges! If you’ve ever been to England in the middle of winter, you’ve probably seen a hedge or two of beech – These show that you don’t have to solely use evergreens if you want a screen!
Hopefully this article helps to shed a little light on Marcescence in Oaks and the benefits to the garden. More importantly, though, I hope this article prompts you to go out and buy an oak! You can buy oak seedlings for a few dollars each from either online nurseries or your local DNR nursery. That’s a very low cost involved to do something incredibly beneficial for the environment!
If you’re interested in learning more than you ever cared to know about oaks, Dr. Doug Tallamy has written a whole book on them – Check it out on Amazon:
The Nature of Oaks: The Rich Ecology of Our Most Essential Native Trees
Also, speaking of Dr. Doug Tallamy – if you are a gardener who loves to plant native plants and garden for wildlife, I highly suggest registering your garden on https://homegrownnationalpark.org/ – Even if you aren’t much of a gardener, I highly recommend learning more about this movement to do your part for conservation!
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