biology zoology blog benno meyer tree bark

Trees have a bark

You can see it but can’t hear it

I love going for a walk in the forest and the other day I was saying to my Korean friend that perhaps I should write a blog about the trees’ bark. She cast a surprised look at me and uttered “I’ve never heard a tree bark!”. Well, she had studied French for 5 years and her English is still not perfect, which is why she must have thought of “aboyer”, the barking of a dog. Anyway, although the bark of a tree cannot be heard, it is an interesting and in some ways puzzling part of the anatomy of all woody plants. Consulting the botanical literature, you will learn that botanists distinguish an inner bark, which transports assimilation products from the leaves or needles downward, and an outer bark, whose outermost structure is what we see when we look at the stem. You will come across some scientific terminology like ‘the vascular cambium’ as the tissue located between the inner and the outer bark responsible for giving rise to the bark. You will read about the ‘phloem’ as the tissue that is responsible for the transport of the sugars that result from the photosynthesis in the green leaves or needles and you will learn that what the non-botanist calls the bark is to a botanist the  ‘rhytidome’, a  waterproof structure of cork and dead cells that consist of cellulose and lignin and is confined to the outer bark.

With regard to the function of this outer bark, its protective function is always highlighted: protection against mechanical forces and fire, against potential pathogens and herbivores, against vines and other “climbers” and to prevent colonization by epiphytes, lichens and algae. However, why there should be this enormous variety of bark types and structures is largely left unexplained, especially in view of the fact that trees had more than 300 million years to ‘perfect’ the appearance of their bark to meet all the functions mentioned above.

Walking in the forest and seeing tree species with smooth or exceedingly rough, flaky or almost scaly barks and encountering in which the outer bark is covered in sharp protrusions and spines or resembles loose and smooth sheets that come off easily or consists of strips that either peel off horizontally or vertically, you cannot but wonder whether the explanations for the tree barks’ functions aren’t perhaps incomplete. Sure, barks with spines make it hard for tree-climbing mammals to ascend them, but not for insects. On the other hand, trees with rough and scally barks like many pine trees make it easy for arboreal mammals, e.g. squirrels, to climb them, but cause small and crawling insects a terrible problem.  Smooth barks and trees in which the bark can come off in large sheets leaving an almost polished stem behind like in many eucalypt trees, must be a nightmare especially for mammalian climbers.

I therefore believe that the bark of at least some tree species is designed not to prevent, but to facilitate the ascend of fruit-eating animal species, as the seeds are usually indigestible and the fruit-eater helps to spread the tree species by releasing the seeds with its faeces. Not all insects are disliked by trees and ants are in fact welcome as they prey upon a tree’s unwanted pest insects, deposited on them as eggs by flying species, primarily moths and butterflies. Moss and lichens growing on the bark and not harming the tree at all could assist beneficial insects like ants in ascending the tree. The idea that the bark of certain trees has evolved features to assist animals to climb them has not received much attention, but I think it should be worth a closer look.   

© Dr V.B. Meyer-Rochow and, 2020.
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