50 Shades of Autumn

by Leanne Massie

Photo Credit: Bert Kaufmann

Photo Credit: Bert Kaufmann

Ever wonder why some trees turn stunning shades of red and yellow this time of year while others stay a bright green year round? It’s all about three important things plants need to survive: warmth, light and water.

Evergreens, which are mainly conifers, tend to live in regions that get very cold in the winter and have quite short summers as in the Boreal forests that ring the Arctic Circle. They experience shortages of all three necessary ingredients in the Arctic winter, no sun for months means temperatures of as low as 80 degrees Celsius below zero and any available water being locked away, unreachable to plants, in the form of ice.

Deciduous trees, on the other hand, lead a much more comfy life. They still have chilly winters but temperatures tend to hover around or dip slightly below the zero mark. They also get lovely, long, warm summers with plenty of time to grow and barrels of rainwater to spare. This allows them to store up enough energy during the growing season so they can afford to drop their leaves when it gets cold and have enough energy to regrow the following spring.

But why bother dropping their leaves and re-growing them? The answer is in the third element in our list: water. Leaves have very large surface areas so represent a lot of potential area to lose water from. When it’s winter and all the ground water is frozen, losing any water through the leaves can be very damaging to the tree that need to hang to on as much liquid as possible. By dropping their leaves and thus reducing their surface area, deciduous trees are able to avoid a large amount of water loss. Of course they don’t just fall off, the trees recover as much nutrients as possible from their leaves before dropping them; it is this relocation of the green, nutrient rich chlorophyll in particular that is seen since this exposes the vivid reds and yellows of the anthocyanin and carotenoid pigments that are always present in the leaves but usually masked by the much more abundant green chlorophyll.

Going back to evergreens though, they still have the problem of water loss since they don’t drop their leaves in the winter. They have developed another solution to reduce water loss which is a typical characteristic of conifers; they have reduced their leaves to needles. This again reduces their surface area to minimize water loss while not incurring the huge cost of dropping then re-growing their leaves. Dropping them entirely is not feasibly because the low temperatures slows the actions of the microbial community to practically nothing with the result that nutrient cycling is very slow so the soil is poor quality, thus the trees don’t have the resources to regrow their needles every year.

What you might not know is that cold places are not the only place where evergreens are found. Non-conifer evergreens exist around the equator due to a lack of seasonal change. With good growing conditions all year round there is no need to drop their leaves or reduce them to needles so we find broad-leaf evergreen trees in equatorial regions. Another side effect of the lack of seasons is that the trees don’t lay down the characteristic annual rings in their trunks but have a more or less consistent grain throughout.

So there you have it! Trees that change color do so not to delight us and give us material to make leaf piles with, but because it makes evolutionary sense for them to do so. It allows them to conserve nutrients while avoiding the worst of the winter drought caused by plummeting temperatures and wait for the sun to come back. That certainly doesn’t mean we can’t enjoy the brilliant colors this produces as we always do!

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2 thoughts on “50 Shades of Autumn

  1. Interesting, thankyou, will think of trees taking back green from their leaves, never questioned why the beautiful colours appear before!

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