Tag Archives: Bark

Bark (Cork Cambium)

Weirdly the definition of bark seems to be variable depending on what book or article you read. As my main reference for this post I’m using Romero’s “Bark: Structure and Functional Ecology” accessible via a free account on JSTOR here.

According to Romero, bark is all the tissues on a tree outside the vascular cambium – that is everything from (and including) the phloem outwards. The inner bark is simply the phloem (both the conducting layer and the non-conducting layer). The outer bark collectively is known as the ‘ritidome’. This is where a diagram is needed! This is the best one I could find (from the University of Vigo website).


The ritidome contains another meristem within the tree – the cork cambium. The cork cambium (called the phellogen) works similarly to the vascular cambium – it has a layer of stem cells which create layers of differentiated cells. In the top diagram there is one phellogen, a pale beige line. On its left is the phelloderm – this layer is not always created depending on the species but if present it contains living cells. On its right is the phellem, or cork, this is the thickest layer and these cells become suberised/lignified (impregnated with suberin or lignin) so they become the corky bark texture we are familiar with. All of these ‘phello’ layers together are known as the periderm. A microscope image of these layers in an old Pinus sylvestris are shown in the image below:


What’s interesting is that multiple periderms can develop over the life of the tree. A new periderm will develop on the inside of the old one, pushing that periderm layer to the outside. These aren’t always continuous either, and are affected by the structure of rays and growth rings within the phloem, which is why old bark has more character. Periderms can be shed, or retained, depending on the species. The pattern of a tree’s bark is genetically determined by the structure of the phellem cells which are produced, and by the location of successive periderms. Smooth bark can come from a single periderm and continuous shedding, while rough bark is created when the periderm has structural fractures or constraints – for example due to the development of rays (radial lines of cells in the phloem).

See this image of old bark from ‘The Plant Stem – A Microscopic Aspect’ by Schweingruber & Börner. It shows how the bark splits apart as the xylem and sapwood layers expands from the inside of the tree.


Bark is made up of quite different materials from the wood or foliage of the tree, with considerably more mineral compounds (such as ash). Both the inner and outer bark contain so-called ‘extractives’ (organic substances which can be dissolved in solvents, such as polyphenols, alcohols, resin acids, vitamins, alkaloids, pigments including flavonoids, terpenes, steroids and essential oils) as well as suberin, lignin and cellulose. Bark chemistry in general is poisonous and indigestible, representing a good barrier to herbivores or insects. As the inner bark is living tissue, it can produce its own metabolites as a defence mechanism, whereas outer bark is dead tissue and relies on its physical structure and the substances impregnated into its cells to repel invaders.

Bark helps trees reduce water loss, prevents pathogens entering and provides a protective layer to protect the living tissue underneath from mechanical and heat/cold damage. It provides a flexible covering for the tree which can absorb the stresses of bending and twisting, and prevent cracking of the trunk.

As bonsai enthusiasts, bark is a key part of the look of our trees and we want to encourage interesting bark with good character. Since the cells of bark are renewing from the inside, the only way to modify the appearance and texture of bark in a natural way is to manipulate the periderm – as mentioned above, this causes fractures and divergence of the growth habit of the phellem. Harry Harrington has a video showing exactly this on a young black pine – he wires the tree so that the wire interrupts the shape of the periderm and forces the phellem to grow in a twisted habit.


I’m a bit nervous by the suggestion to leave the wire in as this seems like it would then cut through the phloem and ultimately the xylem. Whilst the twisted shape should leave continuous conducting cells for both, I’d be concerned at how much water and photosynthate conducting would be reduced. If possible to remove the wire I think I would.

Thickening the Trunk

The first quality of a good bonsai is a thick trunk with movement and mature bark. So what actually contributes to the growth of a tree trunk?

Two processes are involved. The first is the creation of new sapwood. Sapwood is the living wood towards the outside of a trunk which conducts water (Ennos, 2016). Sapwood formed in spring is called ‘earlywood’ and is optimized for water & nutrient transport to help the tree with its growth spurt. Latewood is designed for structural support and carbon storage.

Water and nutrients are conducted from the roots through xylem vessels. The mechanism by which they work is explained in xylem but for the purpose of this section it’s important to understand that the reason why trees add new xylem vessels is because as it adds biomass – new branches and leaves – more water is required. So – the more biomass is added in a given growing period – the more water is needed – the more xylem vessels are added to the trunk. Xylem vessels also become non-functional for reasons explained in embolisms, so trees need to replace them as well as adding to them due to new growth. 

New sapwood (with xylem vessels) is added around the previous sapwood, encircling the tree. How much of the girth of a tree increases each year is determined by the tree’s food supply (Trouet, 2020); this is a combination of the amount of rainfall and the energy from the sun during that year.

This studyref found that “low precipitation at the start or during the growing season was found to be a significant factor limiting radial growth” for a range of urban trees in the UK. According to Trouet, “alternating wet & dry years create wide and narrow rings respectively.” So low water levels lead to small rings and high water levels lead to large ones. The earlywood creates a larger ring than the latewood, since the xylem vessels are larger in earlywood (for water transport) and smaller in latewood (for structural strength) (Ennos, 2016).

What this means for bonsai is that watering your tree well is important while developing its trunk, whilst ensuring you have a well-drained growing medium to avoid creating anoxic conditions (lacking oxygen). If your medium is well-drained and you water thoroughly throughout the tree’s growing season (but particularly during earlywood development), you’ll boost your tree’s girth by creating wide ‘good times’ sapwood rings.

The other factor mentioned is energy from the sun. Energy from the sun is used by the tree in photosynthesis, which converts energy into a form that the tree can use to respire and grow. If there is more sun, more energy is available and the tree is able to create more xylem, buds, leaves and biomass. This isn’t a straightforward linear relationship however, as photosynthesis reaches a saturation point based on a number of limiting factors (more in the post about photosynthesis).

The key point here is that reducing the ability of the tree to capture and convert energy will affect its growth. If you reduce the foliage on your tree or cut it back in spring, you reduce its biomass, it can’t generate as much energy, and doesn’t need as much water, so doesn’t add as many xylem vessels as it would have nor as wide a ring of sapwood. This reduces the trunk thickening you can achieve in a given time period. 

It’s worth noting that the roots of a tree need to be capable of delivering the amount of water that its foliage and branches require. Optimising trunk thickness requires a dense canopy of leaves and branches, matched by roots capable of delivering the amount of water that they need. This is why many bonsai enthusiasts will start a tree off in the ground or in a large pot, allowing growth to drive the trunk size until it’s at the level required.

Attempting to restrict the roots and size of the tree too early (e.g. by putting it in a bonsai pot) will restrict trunk growth by reducing the water available to the tree and reducing the energy it can create by reducing its foliage.

Like people, trees are genetically programmed to have different maximum heights and lifespans. Some trees are slow-growing (such as Yew) and some are fast (such as Eucalyptus) so to an extent the amount of trunk thickening that is possible also depends on the species of tree.

Trees grow most vigorously when they are free from environmental stressors – such as drought, extreme cold, loss of leaves due to high winds, attack by insects or animals.  A stressed tree will grow a narrow ring. BUT stress in the form of wind can foster positive qualities in a trunk. Ennos (2016) says that trees exposed to high winds without a prevailing wind direction grow shorter, with thicker trunks & roots, and adjust their wood cells to spiral around the tree creating a twisting effect. It’s not just the trunk that is affected – apparently this results in smaller leaves and shoots as well. Get your bonsai a wind tunnel!

Another way to thicken a trunk is to grow a ground-level branch, as layers of xylem will be added around this branch as well as the truck, or to have a multi-stem tree, which operates on the same principle. You want to avoid having one too much above the ground though, as it might cause the dreaded reverse taper.

I mentioned two processes involved in secondary thickening – the second process is the effect of an increasing bark layer. In most cases this will be dwarfed by sapwood increases but nevertheless biomass is added as bark via the cork cambium, another secondary meristem on trees. Some trees which retain multiple periderms (layers of cork with their meristems) can develop very thick bark which does contribute to the overall trunk girth as well.