Evergreen trees stand out like a beacon of color and hope in northern winters, with their white snowy landscape, and cloudy gray skies. Ever wonder why some plants stay green the whole year round, whilst others – deciduous trees – lose their leaves every autumn. Read on as we explore how evergreen plants retain their color all year long.
Photosynthesis and the green color of plants
First though, and before answering the question of why and how evergreen plants stay green all year long, we have to answer the question of why plants are green at all. As you probably know, plants use energy from the sun to take carbon dioxide, from the air, and water, taken in by the plant’s roots from the soil in the ground, and convert it into glucose and other simple sugars – these constitute the plant’s food and enable it to grow and flourish. The chemical that enables this entire process to occur – or acts as a catalyst – is chlorophyll, contained in the leaves of a plant, and it is this chemical that gives leaves their green color. Chlorophyll is actually just a green pigment, but it is what allows the plant to absorb sunlight at all. In fact, it absorbs all wavelengths of visible light, except for those which correspond to what we see as the color ‘green’ – which it reflects. Hence the green color of plants.
As winter approaches, the days shorten and there is not nearly as much sunlight to activate the process of photosynthesis. In addition, most trees have quite delicate leaves that are prone to damage from frosts, rain, wind, and snow. Hence these plants – known as deciduous trees – decide to shed their leaves during the colder months and remain dormant until the warmer weather arrives. (with no leaves, they cannot receive much light from the sun, and thus cannot really undergo very much photosynthesis). That way they conserve the more limited supplies of energy available to them during the colder weather for essential survival functions and don’t waste their resources growing and repairing attractive but essentially unnecessary plumage.
By contrast with deciduous trees, evergreen plants retain most of their leaves throughout the winter months. Actually, they do lose their leaves during the winter, but they also are growing new ones at the same time! So the overall effect is as if evergreen plants retain the exact same foliage throughout the winter.
You see, as long as they still receive some water, and some sunlight, evergreen trees will continue to undergo photosynthesis, and experience new growth (both to their stem, and their leaves). Due to the colder weather, however, the chemical reactions will occur a lot more slowly though.
Just like deciduous trees, evergreen trees will experience damage to their leaves during the colder months of the year, whether from frost, wind, rain, snow and the like – and even from insects. However, unlike deciduous trees with their very delicate leaves, the damage will not be nearly as severe as that which deciduous trees would undergo in the hypothetical even that they retained their leaves during the winter. After all, evergreen trees have special leaves that are resistant to the cold and bad weather of winter and don’t lose moisture as easily. Some evergreen trees, like fir and pine trees, have long and thin needles for their leaves. Others, such as the holly tree, bear much broader leaves, that have waxy, tough surfaces. Whatever the individual differences, though, all evergreen trees have touch, resilient leaves that are much more resistant to weather and insect damage than their deciduous cousins.
As a result of these twin features of evergreen trees, the number of leaves they lose due to weather and insect damage, as well as moisture loss, will not be greatly in excess of the new ones generated by these plants via the process of photosynthesis. Thus it appears that evergreen trees retain their foliage throughout the year – even though, in reality, they are constantly and at the same time growing and losing new leaves. The process of replacing old, dying leaves occurs gradually, so that these trees will always still have enough leaves to function and to undergo photosynthesis, whatever the time of year.
The evergreen cycle
Evergreens thus put new leaves on in groups, and they tend to lose them in groups also. Think of it like your hair! All of your hairs have the same lifespan and undergo the same cycle of growing and then falling out, but they don’t all do it at the same time. Evergreen trees usually keep their leaves for two to three years before they fall out. You can tell the age of the leaves by their color and where they are situated on the plant. The younger leaves will have a lighter green colored hue and will be located towards the end of the stem. Meanwhile, those leaves that are close to falling out will be of a reddish hue and will be found closer to the stem of the tree.
Why do the leaves and needles of evergreen trees change color before they fall out though? Actually, those red, yellow and orange colors were there in the tree all along, they were just covered up – and drowned out – by the large amounts of chlorophyll contained in the plant – remember that chlorophyll is what gives the plants their green color. As the chlorophyll dies along the with the plants’ leaves, those different colored pigments come to the fore – and hence the green color of the plant is replaced with reds, oranges, and yellows.
Another interesting thing to bear in mind about evergreen trees is that even though they stay green throughout the winter, during the colder months they are, like deciduous trees, basically in a state of hibernation. They use their leaves to a limited extent during the winter months, but overall they are a state of much-reduced activity. That has to do largely with the fact that the roots of the tree – which draw water and other nutrients from the ground – are locked up in frozen soil.