Outer Bark: Its structure and function

This second blog on bark, building up to the Bushcraft Show, looks at the outer bark of trees and its structure and function.

Just looking at a selection of common British trees species in detail it never ceases to amaze just how variable the outer bark of different species of tree can be. The appearance and even the feel of the bark can be a key identification feature, especially in winter when the leaves are absent.  The pictures below demonstrate the diversity of outer bark.

willow

birch

hawthorn

hornbeam

crab apple

sycamore

blackthorn

pine

larch

elder

ash

But is means of  identifying trees aside, what is the outer bark for?  Bark is to trees what our skin is to us it is the protective outer layer of the tree that is comprised of several different layers and is generally considered to be all the layers outside of the vascular cambium layer (see diagram below).

In this blog we are focusing exclusively on the outer bark, also called the periderm (and sometimes the rhytidome) it serves to protect the tree from impacts as well extremes of temperature from weather and fire and also protects against other external agents. It is made up of three layers.

The inner most layer is the phelloderm or cork skin which separates the outer bark from the inner bark.

Moving out from this is the phellogen, sometimes called the cork cambrium which is responsible for laying down more cork cells which make up the outer layer or phellem.

The outer most part consists of dead cork cells.  These cork cells are hollow and it is this feature which helps insulate the layers underneath from extremes of temperature in some cases protecting the tissue below from scorching during forest fires.  These hollow cells also act as shock protection cushioning impacts.  The walls of these cork cells  contain a waxy compound called suberin  which helps to waterproof the bark and gives it its greyish appearance. This not only stops water getting in but also prevents water loss from the stem.

The outer bark also contains protective chemicals which help protect the layers inside from attack from insects, fungi and bacteria. We have already looked at one group of these chemicals, tannins, in a previous blog , but other chemicals include things like salicin (a precursor to aspirin) which is in willow and poplar bark,  alkaloids like quinnine and curare,  and plant glycosides.  All of these, as well as being actually toxic to a greater or lesser degree, tend to be very bitter compounds and so deter things eating the bark. Other tree barks contain oils such as the oil found in birch bark, which have been demonstrated to have anti bacterial and anti fungal properties.

There are different terms to describe how the outer  bark appears which is dictated by how the pelliderm is laid down as the tree grows.   This can vary from the trunk through to the branches and also with the age of the tree.

Smooth bark is found in trees such as beech, the bark simply grows with the tree and expands it is usually relatively thin.

In other trees the outer bark is said to be discontinuous. As the tree grows a new layer of cork cells are laid down beneath the outer ones and in so doing cut the outer cells off from water causing them to die. The way this is achieved can happen in different ways resulting in different appearances on the tree;-

Ring Bark is where a new continuous layer is put down each year resulting in concentric layers of outer bark.

Peeling or Exfoliating Bark, is where the layers are laid down unevenly resulting in alternating thick and thin layers, these thin layers can give way resulting in the bark separating which gives rise to large sheets coming away, revealing smooth, new bark below it.  Sometimes this type can be divided up into those where the layers come away in large sheets and  is called paper bark. The appropriately named Paper Bark Acacia is an example.

 

When these layers are thin they can break up into strands it is sometimes known as shaggy or string bark, like our own Silver Birch

 

Scaly Bark is when the bark grows in discontinous, successive overlapping layers. Some conifers like cedars and cypresses fit this description.

 

Persistant or Fissured Bark is where the bark cracks and becomes deeply fissured over time, oak being a good example.

 

Furrowed Cork bark is where the bark is deeply furrowed as accumulations of dead cork cells build up in ridges. The field maple is a good eaxample.

These dead layers as we have discussed are water resistant, but they are also impervious to gases. The living tissue of the trees within are constantly metabolising and so need oxygen and have to get rid of carbon dioxide. To achieve this there are lens shaped pores throughout the bark that allow the trees to breath through their bark.  These vary in size, being tiny in some trees but much larger in others and clearly visible to the naked eye.  Those horizontal lines on birch trees and cherry trees are the lenticels.

These pores can get clogged by pollutants, algae, lichens etc. so some trees shed their outer bark to basically unblock their lenticels.  Again the faithful old silver birch is a good example, as anyone who has got a fire steel will tell you.

Finally as can be seen from the pictures above, outer bark provides an excellent host for mosses, lichens and algae.

In the next blog on bark we will move down a layer and look at the pholem or inner bark.

Kev Palmer

 

References: 

Second Skin; Everday and Sacred Uses of Bark Worldwide – Royal Albert Memorial Museum & Art Gallery Exeter

http://www.botgard.ucla.edu/html/botanytextbooks/generalbotany/barkfeatures/index.html