Bark is similar in many ways to our own skin and is essential for a tree’s survival. And a whole host of other species make use of this niche in the forest ecosystem. The visible part keeps moisture in and keeps infection out. But there’s a lot more to bark than first meets the eye.

 

What is bark?

Imagine for a moment you are wearing X-ray glasses and you can look just below the surface of a tree’s trunk. Here we see a layer of living tissue. Zooming in really close, this tissue is like a bundle of straws packed together. What we are seeing is the tree’s plumbing, conductive ‘pipes’ for transporting fluids. This tissue comes in two main forms.

The first layer we see is the phloem. Phloem is right below the surface bark and carries sugars from the leaves down to the rest of the tree. Further in is more ‘plumbing’ called the xylem or sapwood. These tubes carry water and minerals the opposite way, up to the leaves.

Sandwiched between these two layers is the cambium. The cambium’s job is to produce cells. On the inside it creates more xylem and on the outside it creates more phloem.

Xylem cells die quickly. They actually have to so they can fully play their role as pipes. After a while though, they become blocked and are replaced by newer xylem. This surrounds the old layer, which is why a tree’s girth expands each year. The blocked tubes become the tough heartwood of the tree. Heartwood gives the tree ‘backbone’ and is good at resisting rot and insect attack.

As phloem dies it is pressed outwards and becomes part of the bark. Many trees also have a cork cambium layer, outside the first one. Its job is to produce cork, which also forms a major part of bark.

So taking off our X-ray glasses, the main point is that everything outside the main cambium layer is the bark.

 

Bark as protection

The outer cork protects the tree from the elements – from scorching by the sun or drying by wind. It also helps to ward off fungal infection, insect attack, and the attention of hungry birds and mammals.

The bark of different trees has evolved to withstand the environment in which each species occurs. Scots pine bark offers protection from fire. In prehistoric times, wildfires would very occasionally sweep through areas of pine woodland. The thick, plated bark of Scots pines would help many of the older trees to survive.

The white bark of silver birch reflects sunlight and protects the tree from getting damaged by ultraviolet rays. Birch seeds can travel long distances and birch can easily find itself without the shelter of companions so this protection is important.

Many trees have chemicals within their bark that ward off fungi and insects. Scots pine has sticky resin and oak bark contains a lot of tannins, chemicals that taste off-putting and are also toxic in high doses.

Many trees including silver birch get rougher as they get older, which makes it harder for animals to damage the bark.

 

Bark as a habitat

Cracks in bark provide great habitat. The deep fissures and crevices in the bark of an old oak or Scots pine are a haven for many species of insects and spiders. These invertebrates attract birds such as treecreepers and crested tits.

Even after a tree has died, bark can be a home for all sorts of wildlife. Bats sometimes roost beneath loose bark and a multitude of invertebrates also live out their lives in this hidden world.

In the Caledonian Forest, some of the most obvious life on bark takes the form of lichens and small plants. Plants that live on trees, without actually causing them any harm, are called epiphytes. The texture of bark influences which epiphytes live upon it. In an old pinewood it is common to see many other plants such as blaeberry growing in the thick crevices of Scots pine bark.

The texture of bark, and thus the lichen communities, can change during the lifetime of a tree. Young hazel has fairly smooth bark, and so attracts lichens that prefer this texture, particularly the script lichens. (These lichens are distinguishable by the tiny ‘squiggles’ on their surface). Bark often gets rougher as the tree ages. It then becomes more suitable for other species, including the leafy, frogskin-like lungwort. The lichen community can also vary on different parts of the same tree. Aspen bark has smooth and rough areas, each supporting different species.

Chemistry can be as important as texture when it comes to bark as a habitat. Aspen bark is not as acidic as that of some other trees such as pine and birch. This means that it can support species of plants and lichen that might not otherwise be present in a pinewood. (Interestingly aspen can also photosynthesise through its bark!)

 

Food for wildlife

Bark does a great job of protecting the tree. Even so, there are some very determined creatures that are keen to get to the nutritious cambium, or the wood beneath it. Many mammals eat bark, and by looking at the height and details of the damage, we can find out what mammals are present in an area.

As their name suggests, bark beetles are among the insects that use bark. The larvae burrow down to get to the cambium and each beetle species makes distinctive galleries, or passages in the wood. They can then carry in fungal spores that the bark would usually repel, which is how Dutch elm disease is spread.

If bark is damaged around the circumference of the trunk, the tree is in real trouble. The phloem can no longer do its job of transporting sugars, and the tree may die. Voles often eat the bark at the base of young trees, killing young saplings. Deer also strip bark (as well as damaging it by ‘fraying’ their antlers on it to shed the velvet coating). The bark of trees including aspen and willow is an important food source for the European beaver.

While all this bark feeding can be destructive to individual trees, it is worth taking a step back. From an ecological perspective it shows how bark can support a wide range of different species. Also, when a tree is killed or harmed by bark damage, valuable dead wood habitat can be created for fungi, insects and many other organisms.

Bark’s main purpose is to protect the tree. But when we take a closer look we can see how every surface, nook and cranny in the woods can provide food and shelter for myriad living things. In this way bark also helps increase the biodiversity in a forest.

 

Written by Dan Puplett

 

Sources and further reading

Brown, R.W., Lawrence, M.J. & Pope, J. (2004). Animals – Tracks, Trails and Signs. Hamlyn: London.

Mitchell, A. (1982). Trees of Britain and Northern Europe. Collins: London.

Steven, H.M. & Carlisle, A. (1959). The Native Pinewoods of Scotland. Oliver & Boyd: Edinburgh.

Street, L. & S. (2002) The importance of Aspens for lichen. In: Cosgrove, P & Amphlett, A. (eds.). The Biodiversity and Management of Aspen Woodlands: Proceedings of a one-day conference held in Kingussie, Scotland, on 25th May 2001. The Cairngorms Local Biodiversity Action Plan: Grantown-on Spey.

Tudge, C. (2005). The Secret Life of Trees. Penguin: London.

Wohlleben, P. (2017). The Hidden Life of Trees. William Collins: London.

http://publicationslist.org/data/pfern/ref-25/Fernandes%20et%20al.%20FEM%202008.pdf (Accessed June 23rd 2020)

https://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2435.12372 (Accessed June 23rd 2020)