Regardless of seasonsome plants always have a beautiful canopy of green leaves: they are the plant evergreen. Even if it seems like they never lose them, in reality, evergreens also lose their leaves, but they do it gradually and not every year, so as to optimize available resources. In fact, all evergreens, from conifers to tropical plants passing through the olive trees also present in Italy, have in common the fact of growing in soil poor in nutrients and water and for this reason they have developed leaves that do not have to be rebuilt every year. In this way, among other things, evergreen plants can carry out photosynthesis even in the harshest months.
The leaves of evergreens are thicker than those of deciduous plants (which change their foliage every year) and covered with a waxy cuticle: in this way they reduce the loss of water and nutrients, both in summer and winter, and are protected both from predators and from the winter cold. The waxy layer also counteracts the degradation of the chlorophyllpreventing the leaves from changing color and allowing them to remain Always green.
Even evergreens lose their leaves!
They are calls evergreen because apparently they never lose their leaves, unlike deciduous or deciduous plants which every year give us the colorful spectacle of autumn foliage.
We say “apparently”, because in reality even evergreen plants they replace their leaves regularlyonly that they have a longer life (from 1 to even 2 or 3 years). Even the leaves of evergreens undergo the process of senescence, that is, they age: at the end of their life, they dry out, become darker and fall. The trick is that they don’t all age at the same time as the leaves of deciduous plants, and what’s more are replaced by new leaves. This is one of the reasons why we always see them green, and why it is rare to see an olive tree or a pine with all brown leaves or no leaves (in those cases, we are often in very harsh climates or pathological factors of the plant come into play ).
The problem of “losing leaves or not” arises especially in regions with very cold climates or wintersas happens in our areas, while in tropical areas the temperatures remain favorable for the maintenance of the leaf foliage throughout the year.
Leaves resistant to cold, heat and predators
Evergreens have therefore developed leaves suitable for surviving hostile environments. First, they are thicker and harder than those of deciduous plants, because they contain more structural molecules such as lignin And cellulose. In cold climates, think for example of the conifers (pines and firs) that dot our mountains, specialization has gone further and the leaves have become small needlesalso functional for distributing the weight of the snow.
They are also lined with one waxy layer which insulates the internal tissues of the leaf, preventing the freezing of water and sap present in the leaves. The coating also allows you to reduce water loss and water vapor, a fundamental characteristic both during winter and when the climate becomes dry and arid. Thanks to this, the olive tree and many plants of the Mediterranean scrub can survive not only the winter, but also the hot and dry summers typical of the region.
These two characteristics, combined with the presence of essential oils and repellent substances, ensure that evergreen leaves also have a greater defense against insects and herbivorous animals: eating a woody, hard and unpleasant-tasting leaf (or needle) must not be a pleasant experience! And this also applies to tropical evergreen plants: in general therefore the leaves of evergreens are less palatable.
Why don’t they change color?
The green color of the leaves is given by a particular pigment: la chlorophyllwhich allows plants to carry out photosynthesis. Chlorophyll is sensitive to the amount of light and the external temperaturewhich is why normally when the days get shorter and temperatures drop, many plants stop producing it and their leaves change color before producing it.
In addition to regularly replacing their leaves, evergreens maintain a thick green foliage also thanks to their structure: the waxy layer thermally insulates the structures inside the leaf and reduces the degradation of chlorophyll, so that the leaves do not change color. In warm and temperate climates, however, such as the Mediterranean area and tropical regions, favorable temperatures do not induce the breakdown of chlorophyll molecules, therefore making it more useful to build leaves with a longer life.
Evergreens grow in nutrient-poor soil
They are found evergreen in many different climates and environments, from conifersto those of the Mediterranean scrub up to tropical plants: those with leaves needle-like (like i pines) are typical of the climates coldwhile in the temperate and tropical regions, we find those a broad leaf.
What they all have in common is the fact of growing up soils with poor nutrients and/or water. They are plants that grow on rocky, clayey (for example the olive tree), arid, bumpy (for example lavender and St. John’s wort) or acid soils. In these cases, losing the leaves in autumn and rebuilding them in spring would require an enormous waste of resources and nutrients.
Even the seasonality affects the availability of nutrients: in winter, organic residues on the ground decompose more slowly due to the cold, while the rainy season in tropical forests depletes the soil. In both cases, the plants they do not have the opportunity to absorb nutrients from the soil.
Evergreen or deciduous? There is no better choice than the other
Usually, evergreens are also said to have a energy balance better than deciduous ones precisely because they keep their foliage for more years. In reality, this theory is losing ground, because it has been seen that, if it is true that deciduous plants spend energy on new leaves every year, evergreens must create structures that make them more resistant. Net of these two different strategies, the longer duration of the leaves of evergreens represents a very slight energy advantage.
One of the advantages that evergreen plants definitely have is that, unlike deciduous ones, they can continue to grow do photosynthesis for longertaking advantage of the beautiful winter days, taking advantage of the first warm spring days and the last autumn days! The most “lucky” are the evergreens of tropical climates which, thanks to the favorable climate, can photosynthesize all year round!
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