When winter approaches, leaves turn yellow and then fall. Trees remain bare until it gets warm again. But this doesn’t happen with all trees. For instance, spruces, pines, firs, junipers, and thuja trees do not bother with these conditions and keep their needles regardless of the weather. Why is that? It all comes down to differences in the structure of leaves and needles.
Both deciduous and coniferous plants rely on photosynthesis. They absorb water from the ground through their roots and capture sunlight with their leaves or needles. In essence, needles are the same as leaves—just much narrower and tightly coiled.
Using the sun’s energy, plants convert water and carbon dioxide into various sugars and starches, which they then consume as food.
In winter, the days are shorter, and plants receive less sunlight than in summer. Broad, juicy leaves become vulnerable to the cold. They freeze, causing the plant to lose precious water.
It becomes unprofitable for the tree to continue photosynthesizing and to spend its stored sugars on keeping the leaves alive. So, it sheds them and goes into dormancy to conserve as many nutrients as possible until spring, when there will be more sunlight and warmth.
Coniferous trees also receive much less energy for photosynthesis in winter. However, their needles are resistant to the cold. First, needles have a smaller surface area than leaves, and second, they are covered with a special waxy coating that prevents moisture loss.
Therefore, spruces and firs don’t need to shed their needles for the winter. This allows them to be adapted to a wide range of temperatures, growing both in warm and very harsh climates.
Nevertheless, it would be incorrect to say that coniferous trees never shed their needles. In fact, they change them regularly. According to the University of Iowa, needles can last 1–3 years. In the fall, old needles turn brown and drop, and the trees constantly grow new ones.
The phrase “green in winter and summer” isn’t entirely accurate either, because spruces, pines, firs, and thuja regularly change color every year. Sometimes the changes are noticeable, and sometimes they are not. Conifers can be bright or dark green, yellow, or even brown.
