Leaf Science Sep25

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Leaf Science

Each year, after the summer season is over, the leaves morph into stained-glass colours as a visual gift of sorts before winter sets in with its dull grey tones. But how does that recurring profusion of reds, oranges, golds, and browns happen each year to signal the start of autumn?  Why do leaves change colour at all?

We’ve got the leaf science scoop.

Leaves are green in the first place because of chlorophyll, a pigment that captures solar rays and utilizes the sun’s energy to create a sort of ‘food’ for plants – simple sugars of water and carbon dioxide. In the warm summer growing season, chlorophyll dominates the makeup of the leaf’s chemistry, masking out any of the other pigment colours that may be present, and causing that summer-green brightness.

Once colder temperatures and fewer daylight hours begin to arrive, chlorophyll – which is sensitive to the cold – slows its production and dwindles, leaving room for the other colour pigments called carotenoids. Add in secondary pigments called anthocyanins, both start shining through the diminished green tones of the average leaf, and you’ll have your range of autumn foliage.

It may not be a very romantic explanation when compared to the views that have inspired many an artist, poet, and musician (see: Van Gogh, John Keats, Simon and Garfunkel, Yo La Tengo, The White Stripes) – but this complex interaction of pigments and environmental influences is still a fascinating seasonal recurrence.

Now, here’s a little mystery to temper with all that technical info above:

Scientists are still not completely certain about the function of the colour change and the subsequent dropping of the leaves — they’ve sorted the how, but not necessarily the why. The most widely-accepted theory is that deciduous plants are thought to shed their foliage in fall because it would be too difficult to maintain the regular green leaves (chlorophyll) and leaf density during the cold temperatures and low-light period of winter.

One reason for the colour change may be that the anthocyanins, in particular, protect the leaves from the lower temperatures, a kind of ‘tree sunscreen,’ if you will, that helps the plants reabsorb nutrients, such as nitrogen, with more efficiency.

Another hypothesis is that the leaves on some trees change colour to help attract birds to their berries in the colder months, in a symbiotic relationship. the birds get a meal and the tree or shrub gets, well, we’ll politely say fertilised at a time when the soil is less helpful to the tree than it might typically be in the warmer parts of the year.

And, yet, another idea is that the colours themselves are smartly devised as warning signals towards insects that might use the trees as winter hosts (and thereby damage them). Aphids, for instance, are known to have ‘red receptors,’ meaning they tend to avoid trees with red leaves. So a maple tree turns its leaves red, and voila – it’s taking action to try and keep itself pest-free for the winter months, keeping it healthy until spring. Smart.

Although autumn colours arrive wherever deciduous trees are found, there are a few regions of the world that are actually well-known for their spectacular fall foliage, namely, the eastern U.S. and Canada; Scandinavia and northern Europe; and finally Russia, and eastern Asia – where leaf-peeping is called koyo or kanpukai, which translates to ‘red leaves hunting.’

Some of the best colour tours in the U.S. can be found in New England, due to the thick forests and distinct colour changes that take place from mid-September onward. It’s difficult to predict just how long each year’s autumn colours will last, but you’re generally able to take decent photos through the end of October; and if you know your trees, you’ll even be able to pick your favorite colour pallettes for your pictures.

Remember the cholorophyll, carotenoids and anthocyanins we mentioned earlier? Well, chlorophyll, of course, is responsible for leaves’ basic green colours. But carotenoids contribute the yellow, orange and brown tones, bringing those colours to such things as bananas, carrots, and corn. Anthocyanins bring in the purples and reds (much as they do for red apples and Concord grapes.)

Those colours are distributed among particular species of trees – so, if you like your leaves in tones of brown, ranging from tan to dusty cocoa, look for oaks, elms or beech trees. If you prefer golden yellows and bronzes, then aspens, poplars, hickory trees, birches, and especially black maples will oblige. Like orange leaves? You’ll find them on sugar maples. Reds can be seen on other varieties of both maples and oaks, plus sourwoods and black tupelos. Dark purple leaves can be found on dogwood trees.

If you’re running late with your photo-taking, seek out some oak trees, which tend to hold on to their coloured leaves longer than their associates. And if you’re really behind schedule, there are always spruces, cedars, firs and pines – which keep their green needles throughout all but the harshest (as in Arctic) winters due to the anti-freeze-like substances inside their foliage cells and the heavy wax coating on their needles.

The one unfortunate thing that we all need to make note of is our glorious fall displays are being affected by global warming. According to the journal Global Change Biology, forests in North America and Europe have been staying greener longer as CO2 levels are rising. So you might find that seeking out those perfect autumn photos might take a little longer into the fall season each year.

That’s not necessarily bad news, at least not right at first. The trees will initially be more productive with the extra photosynthesis they’ll acquire throughout a longer and warmer growing season (tree growth already appears to be starting earlier in the spring). But it’s too early to know how older forests or new growth will be impacted over time. Other environmental problems, such as pollution, poor soil conditions, fungi and clear-cutting can show up in trees too – so that bright, red tree that you spy before the autumn season even officially starts, say in late August, might be a result of the tree being under stress and calling up the anthocyanins to help protect itself. Yet another reason to get out in the great autumn weather and get those photos while you can.  – Kristi Kates