Who drops and who doesn't?

It's not as simple as that in the tree world...

Years ago I was asked to participate in a green walk in the city centre. The leader of this walk was a cycling enthusiast who was also vehemently anti-car. I think I was the plant person. I guess he was the everything-else person. I recall stopping in front of a young oak tree and the man saying, “Look - this is the kind of damage cars cause. This tree is already dead from pollution.” Sadly, it was not true. And even more sadly, I said so. Perhaps not surprisingly, he never invited me back.

Do you know why some trees keep hanging on to their shriveled, brown and crispy leaves, long after every other tree has become barren? Have you ever wondered?

Columnar or pyramidal English oak (Quercus robur ‘Fastigiata’) holding onto its leaves through the winter - a beautiful colour echo with this Spanish inspired house

Turns out no one really knows. But there have been lots of guesses amongst botanists and tree scientists. Imagine! Something we don’t know for sure. Such a great lesson for us in these days of thinking we know it all…

Go figure that in the tree world, things are not simple. I grew up thinking that evergreens kept their needles all year, every year, and that deciduous trees dropped their leaves in the fall, often after making a spectacular show. But it turns out that not only is this not altogether true, but many trees are out there bucking the system. For example, evergreens do drop their needles — this is known as seasonal needle drop and affects the oldest needles, usually those closest to the trunk of the tree.

Red pine plantation forest in Ottawa, with fallen needles creating a copper coloured forest floor

I had thought needle drop was due to stress brought on by disease, insect damage or drought, which does indeed happen, often to both old and new needles, but in fact old needles drop regularly after they’ve reached the end of their lifespan, which is between two or four years. Here is what the folks from the renowned Morton Arboretum in Illinois say:

Typically, white pines will retain needles for three years, but in autumn, 2-or-3-year-old needles will change color and drop, leaving only the current season’s growth still attached. Austrian and Scots pines usually retain their needles for three years. Red pine drops its needles in the fourth year. Spruce and fir needles also turn yellow and drop, but the change is usually less noticeable because their older needles are thinned progressively, making the process more gradual than in pines.

Cedars, on the other hand, hold onto their browning awls (their version of needles) for a while before finally dropping entire branchlets. You will certainly have seen this on your cedar hedges and probably swatted them yourself to get them to fall and leave only the more attractive green growth. And yew needles take three years to finally give up the ghost, dropping in late spring or early summer - just in time for you to be alarmed and think that there might be something drastically wrong with them.

And the weirdness doesn’t stop there. Have you paid attention to the free spirit in our native forest, that is, the larch?

Larch, or tamarack, are frustrated leafy trees because not only do they turn a brilliant gold in the fall like their neighbouring aspen, sugar maple and elm, to name but a few, but they drop their needles as well.

Golden autumn needles on larch or tamarack (Larix laricina)

Larch are actually known as deciduous conifers, believe it or not. All eleven species of larch, some native to North America and others to northern and southern Eurasia, are all deciduous conifers and belong in the Pine family. These are long-lived trees; the oldest larch was found in Montana in 1997 and was identified by its rings (from taking a core sample through its trunk) at being just over 1000 years old. Other deciduous conifers include the dawn redwood (Metasequoia glyptostroboides), the bald cypress (Taxodium distichum), the pond cypress (Taxodium ascendens) and the golden larch (Pseudolarix amabilis), which is a larch wanna-be.

Why do they do this? No one knows. It may be some kind of weird environmental adaptation to their surroundings, as larch can live in some very inhospitable places and for a very long time. Perhaps holding onto their needles requires too much of the trees energy. Or perhaps all “evergreens”, once upon a time, dropped their needles for half of the year and that was normal. All of the deciduous conifer species prefer the same kinds of conditions: that is, wet, boggy, acidic soil, and all can grow from between 80 to 100’ tall. Indeed, scientists believed up until the 20th century that the dawn redwood was extinct, until it was found in China in 1941.

So, that leads me to the question: does that mean that trees that hold their leaves through the winter are called coniferous deciduous trees? Maybe not, but the phenomenon of holding onto leaves all winter is called something, that is, marcescence. This word comes from the Latin marcescere, meaning "to wither, languish, droop, decay, pine away”.

If you go into the forest, you will see the dried, shriveled coppery leaves of beech (Fagus) , oak (Quercus), hornbeam (Carpinus), hophornbeam (Ostrya) and witchhazel (Hamamelis) hanging onto their branches. These leaves finally fall in the spring when the new leaves push them off, creating a carpet underneath. Like the larch dropping its needles, these trees holding onto their leaves is a mystery.

Mature beech tree (Fagus sylvatica) holding onto its old leaves; photo courtesy Rosser1954 Roger Griffith

Some have speculated that the old leaves hide the succulent new buds and deter deer and moose grazing; others say the fallen leaves in spring provide nutrients for the tree when it breaks dormancy and requires them; and still others speculate that the leaves trap blowing snow, allowing it to provide extra moisture at the base of the tree when the spring melts come. And by falling in the spring, these leaves also create a mulch, thereby insulating the trees roots from the stress of drought and heat.

Whatever the reasons for these topsy-turvy behaviours — losing needles, keeping leaves — I’m grateful that there are things nature shows to us that we still cannot decipher. It makes life much more interesting.