February 19, 2024


The skin provides another feature that identifies us, like the color of the eyes, hair or height. Everything adds up to contribute another characteristic that individualizes us and that makes every human being a beautiful animal. In our eyes, of course. In trees you can also find an endless number of attributes that facilitate their identification: the shape of the crown, the height, the peculiar branching of each species, the size and type of leaf, as well as its edge, the colors… So much information as you can imagine, because, although at first glance many trees look like the same plant, in reality they are very different, if all these are handled microdata rigorously.

The bark is a part of the tree that is like a fingerprint, capable of giving the key to a specific species. It is true that, sometimes, it is more difficult to identify it only through this, but now that the trees have leaves, a small intensive course can be done on any walk in a park or garden.

Bark of a cork oak in the garden of Cap Roig, in Girona.
Bark of a cork oak in the garden of Cap Roig, in Girona.Edward Beard

Bark characteristics are used to determine deciduous species in the winter, when most trees are leafless, but now is the right time to learn about the hardened tissue that protects these beneficial friends. Because that is one of the functions of the bark: to act as a first line of defense for the tree against everyday dangers, such as herbivores, microorganisms such as fungi and bacteria, dehydration or sunstroke. Or even from fire, as the well-known bark of the cork oak does (Quercus suber), thickened to such an extent that it gives this species an unmistakable appearance within the Iberian flora. The Romans already designated the cork oak as go up, which means cork, appealing to the main product obtained from it. This lightweight and incredibly insulating material protects the tree from drying out and from fire. When its branches burn, the cork oak will regrow without problem thanks to this prodigious defense, rebuilding with its buds everything destroyed by fire. For this reason, cork has come to be used in the aerospace industry, as part of the protective equipment against the extremely high temperatures that ships have to endure when entering the atmospheres of our planet or others in the solar system.

Something very curious is that not all the barks will serve to stop the fire, but in some species they will spread the fire, behaving like tinder. This is the case of many eucalyptus (eucalyptus spp.), whose outer bark continually breaks into shreds, as if it were dressed in rags. Both that bark and its leaves are full of highly flammable substances that enhance the fire, thus renewing the forests that make up. Those tattered bits of bark even fly up in flames, quickly spreading the fire to distant places. Eucalyptus trees will sprout again after the apparent disaster, as the cork oak does, because it maintains dormant buds under its bark. In addition, the ashes resulting from the fire will serve as fertilizer to encourage new growth. From what you can see, all this pyrophilous vegetationfriend of fire, needs this mechanism to grow healthy.

Bark of a white poplar with etched wounds on its trunk and the eye-shaped mark of an old cut to a branch.
Bark of a white poplar with etched wounds on its trunk and the eye-shaped mark of an old cut to a branch.

Other times, the barks are covered with a defense intended exclusively for animals that can harm the tree, as in the case of the terrible spikes of the three-spine acacia (Gleditsia triacanthos). This North American tree is found in almost any park, where it also mixes with a thornless ornamental variety, making it easy for gardeners and arborists to maintain. Unfortunately, the skin of the trees cannot do anything to defend itself from other animals that go on two legs, and that crack and engrave letters and hearts on its bark that tear it apart. It would be better for those lovers to have empathy and resort to a tattoo on their own skin, if they need to lacerate something alive.

The fierce spines of a three-spine acacia.
The fierce spines of a three-spine acacia.Edward Beard

The bark also becomes a place of gas exchange between the plant and the atmosphere, through the lenticels, which are superficial structures that, to the naked eye, appear as distinctive marks. Sometimes they can be seen as dots or more or less thickened lines, giving a peculiar touch to many barks, as in the case of the diamond-shaped lenticels of white poplars (populus alba). On other occasions, the lenticels are found at the bottom of the cracks in the bark, caused by the accumulation of dead tissue cells. These grooves also serve the tree to channel and pour rainwater to its base, as if they were many funnels.

In the same way, small animals such as insects will hibernate and hide throughout the year in these cracks. Thus, they serve as a pantry for birds, for example, when species such as treecreepers, nuthatches or chickadees roam the trunks in search of them. Undoubtedly, the tree is itself a complex ecosystem that supports many living beings.

Wet bark of a plane tree showing its colors in a street in Madrid.
Wet bark of a plane tree showing its colors in a street in Madrid.Edward Beard

Finally, we must not forget the beautiful range of colors of each of the species. At one extreme we would have the rainbow eucalyptus (Eucalyptus deglupta), whose bark lives up to the popular name of the plant, although it is difficult to find it cultivated in Spain. On the contrary, we have another very ubiquitous species such as the shade plane (Platanus spp.), with a very characteristic and striking coloration, especially when wet, when it looks like a sample of camouflage tones. Regarding these colors, we might be surprised to find a completely white bark, and not be from a birch (Betula spp.). It would be an ancestral cultivation technique, whitewashing, consisting of painting the trunks of fruit trees with lime to prevent egg laying and insects in general, although it seems to be ineffective. Better to appreciate the natural color of the bark, so as not to stain ourselves and to be able to hug the tree, to feel its skin against our skin.

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