Nature is changing colour this season

Fall is a magical time of year when leaves change their color and trees are the stars of Mother Nature's fashion show. But there's a scientific explanation behind this transformation.  Leaves have color because of chemicals called pigments and there are four main types of pigment in each leaf. Chlorophyll causes the green, xanthophyll makes yellow, carotenoids make orange leaves, and anthocyanins make red leaves. Through spring and summer, we don't see the other pigments in the leaves, just the green chlorophyll. Chlorophyll is important because it helps plants make energy from sunlight, a process called photosynthesis. The summer sunlight triggers the leaves to keep making more chlorophyll, but trees are very sensitive to changes in their environment.

As summer turns to fall and the days get shorter, less and less sunlight hits the tree's leaves. This is a signal for the leaf to prepare for winter and to stop making chlorophyll. Once this happens, the green color starts to fade and the reds, oranges, and yellows become visible. The energy that the leaves have made flows into the tree and gets stored. Without the chlorophyll to help them make energy, the leaves don't really have a job to do, so when the timing is right, they break off their branches and fall to the ground. The tree can then live through the winter using the energy it's saved up until the days get longer and warmer. Then it grows new leaves to capture the sunlight and the cycle starts all over again.

If the soil was able to drain well enough, the trees might show very good fall color. Where I live, trees that have been stressed by extreme heat and drought do not color up as well, especially the trees that turn red in autumn. As days shorten and temperatures cool down, the green pigments in leaves are no longer produced. The chlorophylls break down, which allows the oranges and yellows (the carotenoids) to show. Red pigments (the anthocyanins), though, develop when sugars break down and as phosphates withdraw from the leaves and are deposited in the stems and trunk. Unlike the carotenoids, anthocyanins have not been in the leaves all summer. The brighter the sunlight at that time, with cool temperatures, the brighter the red colors will be.

So, it’s a combination of day length, temperature, soil moisture, and sunlight that determine how vivid the colors will be: mild sunny days, crisp chilly nights but not below freezing, longer nights, and moist soil…Enjoy the magic and science of fall while it lasts!  When chilly temperatures come around in autumn, often the first leaves to turn color are those near the tips of the branches. Sometimes the first leaves to turn color are the oldest leaves. If the effect is evenly distributed through the tree, then that’s normal.

But when a certain branch begins to turn color before the rest of the tree, there’s most likely an issue with the roots supplying water and nutrients to that branch. The root or the section of the trunk under the branch could have been partially girdled by other roots, or by something buried in the ground…. a boulder, other trees’ more vigorous roots. Or a foot thick layer of discarded shingles, which happened to the sourwood I planted in the Maryland garden. After a few years, it started turning red very early (and sourwoods start early to begin with), and new spring growth was sparse. I dug it up and discovered the shingles. The tree did just fine after removing all the shingles, bringing in soil, and replanting.

Soil with poor tilth, or a dry slope exposed to searing sun will often result in that side of the tree getting its fall colors earlier. If lightning struck the bark, then branches above the damage will color up early or even die off. So, sections of the tree will show their stress by turning color earlier than the rest of the tree. It could be as simple as an old hammock cord wrapped around a branch, or leafy debris collecting and rotting in the crotch of the tree. When the bark, the cambium layer (where trunk growth originates), and phloem or xylem tubes are damaged anywhere along the length of the trunk or branch, the result is early color.

Soil disturbance, caused by trucks parking under one side of the tree (compacting the soil), limestone gravel added as a parking surface (oaks like acidic soil and will turn pale with alkaline limestone), and new road construction too close to the tree are more reasons for this to happen. I often see forest edges suffering when the soil surface was scraped too close to the tree trunks, and replaced with grass. Those trees, whose roots extend well beyond the dripline, have lost a large percentage of their roots, and are severely stressed. They might recover, but parts of the trees often die, after their leaves turn color early.

Two separate trees have different colors because they are either from 2 different species or environmental conditions vary from one tree’s location to the other tree’s location. Fall color generally is consistent within the species, although different varieties of the same species can have different fall colors. Even among trees belonging to the same species, they can take on fall color a couple of weeks apart because soil moisture, drainage, and fertility, and sunlight can be quite different from one spot to the other.

Today I woke up to the sound of gnawing and the pitter patter of little feet outside my window. When I looked outside, I saw the gorgeous hues of Oak trees, and squirrels running across them, busy gathering acorns. And then, as the fog continues to lift, the sun begins to peek out. And that, without doubt is the most magical part of my day. Every single morning as I drive to work, I have a massive rush of endorphins when I see this. I cannot help but call my wife and tell her how lucky we are to live in a place where this is not a vacation but a daily commute to work. She smiles silently on the other end of the phone at how excited I am about these little things.

Also lack of rainfall. For most winter-deciduous plants, the ability to perform photosynthesis is the primary cause of leaf drop. Once the amount of sunlight received by the leaves is insufficient to provide the plants with enough energy, the leaves are cut off from the plant and fall to the ground. Cold temperatures stress the plant, and contribute to the leaf drop-off as well. Drought-deciduous plants grow mostly in the tropics and subtropics, and drop their leaves when they no longer receive enough moisture during the start of the dry season.

However, this mechanism also exists in some temperate plants. Once the ground water freezes, and precipitation comes in the form of snow, the plants no longer have access to moisture and drop their leaves. Interestingly, light is not so much a factor in growing new leaves. After all, there is no photosynthesis going on to tell the plant that it’s time to grow new leaves. Instead, plants rely exclusively on warmer temperatures and rainfall for that. Enjoy the colour changes of the nature every season. Cheers!