This is the autumn in the northern hemisphere, which is the so-called season for appreciating leaves. It is time for people to go out to appreciate the annual orange, red and yellow leaves.
But among these bright and colorful patches, some trees always remain green. Why don’t evergreen conifer trees change color and fall off their leaves every autumn?
Like many other challenges, trees can solve winter problems in many ways.
As the temperature begins to drop, temperate broadleaf trees—such as maple and oak —retrieve green chlorophyll from the leaves. This is a pigment that absorbs sunlight and powers photosynthesis . Trees store hard-earned minerals, mainly nitrogen, that put chlorophyll into the wood for reuse in future growing seasons. Yellow, orange and red will be visible briefly before the fallen leaves begin to winter.
Evergreen coniferous tree - cone tree - keeps leaves all year round and has different strategies to withstand the stress of winter.
Leaves Return on Investment
Staying evergreen does not mean that photosynthesis continues throughout the winter. The cold temperature affects the metabolism of conifer , just as they affect other organisms. In fact, in the cold winter, evergreen conifers do not have more photosynthesis than their leafless neighbors.
deciduous tree must recover investment in its leaf crown during a growing season. In contrast, evergreen conifers contribute to the tree's balance sheet by hanging on the conifer leaves, giving these conifers multiple growing seasons. This is the real benefit of staying green.
Evergreen plants have longer leaves, meaning they can survive in environments that do not work for fallen cousins. In high latitudes and elevations, shorter and colder growing seasons limit photosynthetic activity. Drought can further disrupt photosynthesis. Under these harsh conditions, a year may not be enough to allow a leaf to generate enough energy to repay the growth cost of the tree.
This may explain why evergreen conifers occupy the tops of the mountain in Alaska , Canada and Nordic high latitudes and northern forests. Deciduous broadleaf trees largely withdraw from such habitats—a situation that means they cannot balance investment in leaves and the returns on photosynthesis in a season.
Evergreen needles vary greatly in life span and reflect the degree of growing season stress. Some of the common temperate trees in southern New England, such as white pine , retain needles only during the two growing seasons. Any white pine needle only overwinters once, and to the minimum extent possible, meets the definition of evergreens.
Some conifers, such as larch , do not even do this, but instead shed the entire conifer crown every fall to form a bright golden display, which may be the highlight of the gorgeous autumn leaves they find.
In contrast, bristle pine trees living in high altitude areas in the arid southwest have been firmly fixed on the needles for nearly 50 years. Given the pressures of the growing season, it may take about that long to achieve the photosynthetic returns of its construction investment.
hard bristle pine needles can last for decades in their harsh habitat.
Adapting to winter pressure
Winter is very stressful for trees.
temperatures below zero will bring the risk of evergreen needle cells freezing, which will be fatal. To prevent freezing, evergreen conifers accumulate high concentrations of dissolved substances called cryoprotectants, which can lower the freezing point of water in cells, protect the key cell structure , while not interfering with metabolism.
Cold, snow and freezing, coupled with the requirement of longevity, leads to evergreen conifers devote their energy to the resilience of the coniferous leaves. The toughness of coniferous needles is different; for example, white pine needles with relatively short lifespan are more delicate. fiber material , which makes coniferous leaves more durable, further deepens the investment in coniferous trees and extends the time required to achieve the return on conifer construction costs.
Heavy snow can cause branches to break, which is a common risk of evergreen. Usually drooping fine conifer trees catch less snow than deciduous trees.In fact, when deciduous trees lose their branches due to snowstorms, it is usually during a blizzard on the edge of the snow season (fall or spring) rather than in a storm in the middle of winter, when the canopy has no leaves. If you have ever wondered why deciduous trees miss some good growth days by taking so long to grow leaves in spring, remember that trees don’t want to risk the damage that spring storms can cause.
Evergreen branches are meant to allow snow to slide off the branches so that they do not break under heavy pressure.
The branch structure of the conifer tree is also suitable for snow. The branches of conifer trees usually sweep outward and downward from the trunk: imagine Christmas fir. Not only that, the branches of conifers are usually more flexible than those on deciduous trees. The snow piles up the branches of the coniferous tree until the branches form a shed angle.
No matter which type, in mid-latitude, the snow will fly in winter, the growing season is usually mild and pleasant, and trees need strategies to get through it. Some people create another leaf every spring. Evergreens equip their needles and branches with the characteristics necessary for wintering, thus living to another spring—and for some, there are many springs.
