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Tree bark: a world to discover

Volume 6 Number 2 - October 2009

Professor Claudio Longo

French: L’écorce des arbres: un monde à découvrir

Spanish: La corteza de un árbol: un mundo a descubrir



An innovative project at Brera Botanic Garden, University of Milan, Italy, invites young and old alike, whatever the season, to follow a trail into the old garden to discover the wonderful world of bark.  Bark is the tree’s skin.  It can be smooth, chapped, cracked, furrowed, wrinkled, and so on, but it has its own weave, colour and peculiar design of plates, chips, freckles and furrows. Each kind of tree has its special skin to protect it, which also identifies and distinguishes it. In or on the bark many creatures live or travel, such as fungi and lichens, snails, spiders, bugs and ants – a microcosm to discover.  Visitors to Brera are offered a stereo-microscope to help them make a really detailed investigation. Guides assist visitors in examining the bark and encourage them to see strange worlds and lunar landscapes and to imagine fantastic stories.  This interface between art and science is a revelation of trees and their morphology.

Our Botanic Garden of Brera in Milan is located in the very centre of town, close to the world-famous art gallery of the same name. It could be described as an old historic garden, with a touch of wilderness.  The word ‘old’ may seem redundant if one is also describing the garden as historic. I don’t think it is. A historic garden may be perfectly refurbished and restored so as to make it look new, whereas at Brera you really feel the history. Most probably this feeling has something to do with wilderness.
It is a very relative wilderness. You could call it a sort of compromise between man and nature. Nature is allowed to be present in some degree. This compromise was not planned in advance, but it has been accepted. Insufficient manpower? Maybe, but in my view it has more to do with the peculiarities of the garden itself. The main cause is probably the trees that dominate the tiny landscape of our botanic garden. Trees mean shade and shedding. In this shade not many plant species will thrive and the trees shed leaves, twiglets, flakes of bark, flowers, fruits, seeds – all the time and all over the garden.

Is this touch of wilderness welcomed by visitors? Not all of them. Too many people look only at flowers and are seemingly unable to lift their gaze to the trees. Grander visitors are often scandalized and disgusted, but many ordinary people do like the garden. Also most foreign visitors like it, even those from Britain where, in my view, you can find the most beautiful gardens in the world.
Let’s go back to the trees. Trees always have something interesting and beautiful to show. As everyone knows, their aspect greatly changes with the seasons (at least in broad-leaved species) but one element is a constant, intriguing and beautiful at the same time, and this is the bark. Every tree has its own distinctive bark. But please, don’t look them up in books! You would forget the differences after a couple of minutes. Far better to look at them in real life, best of all in a botanic garden where you can find so many different species.

Here are some examples from our botanic garden. Caucasian wingnut (Pterocarya fraxinifolia), a very large tree at Brera, has a deeply furrowed bark. Close up it looks like the sort of landscape of canyons and ridges you can see from a plane (perhaps even a spaceship). In stark contrast the bark of the nettle tree (Celtis australis), grey and smooth, looks much like that of the beech, but here and there a wide open eye stares at you alarmingly (this is most probably a scar left by an old branch). Similar ‘eyes’ are found on the trunk of Firmiana platanifolia (a tree from the Far East). Its bark is grey and smooth too, but is also lined with vertical streaks of a vivid green. They indicate spots where the outer coating of bark is very thin so that the underlying green tissue shows through. In the date-plum (Diospyros lotus) the bark is divided into quadrangular tablets, quite like a bar of chocolate. Another interesting bark is that of the Japanese larch (Larix kaempferi) that flakes off in large rectangular scales, each one composed of many layers, grey on the outside and reddish-brown on the inside. Last but not least in this short compilation is the plane tree (Platanus sp.) whose trunk, seen from a distance, looks rather like a military uniform. Everybody is familiar with it, but take a closer look and you will discover an enormous variety of patterns and hues reminiscent of abstract painting (Paul Klee perhaps?) Pure beauty!

The best time for looking at bark is in winter, when the sunshine can fall unhindered on the trunk and when there are fewer other features to distract the eye. Different details may be highlighted according to the direction of the light, just as on the stage of a theatre.

Bark should be appreciated not only with our eyes, but with our other senses. Touch is especially important. Bark may be smooth or rough (there are different types of roughness) and these tactile properties may differ as you slide your hands down the trunk’s surface, encountering fractures or lenticels, scales or swellings...

Other sensations may be interesting. The bark of Firmiana (the one with the green stripes) feels distinctly cooler than that of any other tree, probably because the insulating layer is thin and heat flows more easily from our hands to the trunk. The thick bark of Pterocarya fraxinifolia or Dyospiros lotus feels much warmer.
Even the sense of smell may be involved. Trunks and branches of plane trees, poplars, and walnut trees emit very particular fragrances, especially after rain. These are very small details, but becoming familiar with them can be a first step towards familiarity with the wider natural world.

Each tree’s bark is a microcosm, a miniature ecosystem. For some organisms it provides shelter, for others a resting place, for others a means of transit. You find a solitary spider, a swarm of ants marching in a column, or the silvery trace of a snail. In February the dark bark of some trees, especially the large lime tree (Tilia tomentosa), accumulates enough solar radiation to warm up quite a bit and then large groups of firebugs (Pyrrhocoris apterus) appear on it, to warm up in their turn. If they were cats or dogs instead of insects I would say ‘lazily basking in the sunshine’. Seen from afar they appear like bloodstains.

Children greatly appreciate these signs of animal life, as they do all the other minute details of the bark. Our botanic garden has organized specially for primary school children a bark observatory, using the stereomicroscope. With the aid of this instrument their imaginative fantasies can roam in a fantastic landscape.

One final consideration. Our modern world is not a very good place to live in, and this is partly due to important people who lead the political or economic destiny of nations. Mostly male, middle-aged, well-dressed and ultra serious, they seem to have few occasions for relaxation or amusement. Could they not have some fun with a little programme on bark? Gazing into a deep crevice with a small red bug crawling along inside, or feeling a wonderful sense of relaxation while gently caressing the cool bark of Firmiana? ‘Botanic fun for busy VIPs’ – our little garden of Brera is too small and uninfluential to launch such a scheme. But perhaps a larger garden could try. Any offers?


Un parcours novateur à travers le jardin botanique de Brera, de l’Université de Milan en Italie, invite aussi bien les jeunes que les anciens, quelle que soit la saison, à suivre une piste dans le vieux jardin pour découvrir le monde merveilleux de l’écorce.  L’écorce est la peau de l’arbre.  Elle peut être lisse, craquelée, fissurée, sillonnée, ridée, etc., mais elle a sa propre trame, sa propre couleur et un motif particulier constitué de plaques, d’ébréchures, de taches et de rides. Chaque arbre a sa propre peau pour se protéger, qui permet également de l’identifier et de le distinguer. Sur l’écorce vivent nombre d’habitants, tels que des champignons et des lichens. Bien d’autres êtres vivants y résident ou y passent, notamment des escargots, araignées, insectes et fourmis – un microcosme à découvrir.  Chaque visiteur peut apporter un stéréomicroscope lui permettant de réaliser ses recherches sur les arbres de manière approfondie. Les guides aident également le public à observer l’écorce et à faire fonctionner leur imagination pour visualiser des mondes extraordinaires, des paysages lunaires et des histoires fantastiques.  Ce parcours est un voyage entre l’art et les sciences, menant à découvrir l’arbre et sa morphologie.


El jardín Botánico Brera de la Universidad de Milán, Italia, invita a los jóvenes y los no tan jóvenes a recorrer un sendero. Cualquiera que sea la estación, el sendero en el jardín antiguo es para descubrir el maravilloso mundo de la corteza. Siendo que la corteza es la piel de los árboles. Esta puede ser lisa, arrugada, en lajas horizontales, estriada, partida, etc. pero tiene su trama, color y diseño particular a manera de placas, lajas, pecas y surcos. Cada árbol tiene su propia piel que lo protege, la cual también lo identifica y distingue entre otros. En la corteza también hay un sinnúmero de organismos vivos como lo son hongos, líquenes. También muchos otros que viven o pasan temporadas en la misma, como son el caso de los caracoles, arañas, insectos y hormigas, esto es todo un microcosmos a descubrir. Cada visitante puede llevar consigo un pequeño estereo microscopio el cual lo habilita a llevar a cavo su propia y meticulosa investigación. También se cuenta con guías que ayudan y orientan al publico a mirar las diferentes cortezas y usar su creatividad para visualizar mundos que se tornan maravillosos a los ojos, imaginándo paisajes lunares o construyendo historias fantásticas. El sendero se puede considerar un viaje entre el arte y las ciencias, todo esto para descubrir lo fascinante de un árbol y su morfología.

Claudio Longo
Professor of Botany
Università degli Studi di Milano
Biology Department
Via Celoria, 26
20100 Milano


Università degli Studi di Milano
Museo Astronomico - Orto Botanico di Brera
Via Brera, 28
20121 Milano