Most bonsai enthusiasts I know love a bit of Dremel action – a great way to add interest to a tree and to make it look like an old tree is to create deadwood like jin, shari & uro, or more informal natural-looking deadwood forms. For the art and craft of this you should look elsewhere – Will Baddeley at Wildwood Bonsai runs workshops on carving (and has a good example on a Prunus mume on his website). Let’s be clear that any carving you do to a tree is creating or shaping deadwood. You can’t carve a live branch and have it stay alive, at least not the part you carved.
But what does this physically do to a tree? That very much depends on where you are creating deadwood. If the part of tree you are carving is already dead, then carving it will not affect the tree (although, you then have deadwood to manage, see the end of this post). If you are carving live wood, there are some considerations.
Firstly, go back and remind yourself how xylem and phloem work. They transport water, nutrients and dissolved substances like plant growth regulators around the tree. Leaves ‘load’ photosynthates (sugars) into the phloem and roots load water into the xylem. The point here is that movement through these vascular tissues is required in order for water, photosynthates and nutrients to travel around the tree. If you remove these tissues by carving, it will affect at least some parts of the tree.
Xylem and phloem vessels are not usually just one layer wide and they don’t flow end to end like a pipe – there is movement between adjacent vessels and different ways for xylem and phloem sap to flow if areas are damaged. But if you carve away the entire phloem layer – which most likely you will when creating deadwood – that path for phloem sap is closed. Similarly for xylem – if you carve away active xylem vessels then water can’t flow that way any more. You need to understand what the effect of this will be for branches and foliage which you want to keep. If an area of the tree has its water source shut off – it will die. If an area of root has its photosynthate source cut off the same will happen.
Whilst its not true that trees have ‘veins’ exactly since they have multiple connected cells (more like a bundle) and not just one vessel like a vein, the bonsai parlance which refers to ‘live veins’ is approximately correct. If you can imagine a vascular bundle passing between leaves, trunk and roots, you’ll be able to work out what consequences any carving will have.
Apart from anything else, carving live wood will result in a wound, you can read about how trees deal with these in Repairing (?) damage.
Once you have deadwood, what does it mean for your tree? Well, dead wood which is exposed to the environment decomposes over time, through the action of so-called sapotrophic organisms (those that feed off dead organic matter). The decomposition of deadwood worldwide is a critical component of the global ecosystem, releasing nutrients and carbon into the soil and atmosphere.ref In the forest, fungi, bacteria, invertebrates (like beetles) and nemotodes are the organisms which decompose dead wood. Basidiomycota is the only type of fungi which is know to degrade lignin, a major component of woodref (the dreaded Honey Fungus or Armillaria is a member of the Basidiomycota family). Below is the mix of fungi and bacteria involved in decomposition of a European beech (Fagus sylvatica)-dominated temperate forest.ref
The rate of decay of deadwood in the forest is determined by environmental and genetic factors. Gymnosperms (conifers) resist decomposition due to the volatile compounds in their wood.ref Angiosperms which have distinct heartwood, including oak, take longer to decay for a similar reason – heartwood often contains substances which deter fungi and bacteria.ref A fun fact is that plants don’t excrete like animals do. Instead they store away toxic compounds in their vacuoles (fluid-filled spaces within plant cells which occupy up to 90% of the cell volume) (Hallé). Just some of the compounds stored in vacuoles include pigments in flower petals, latex, digitalis in foxglove, resins, alkaloids such as opium and the chemicals in garlic.ref So these compounds can have the effect of slowing down decomposition, by being extremely unpalatable to microbes.
From a bonsai point of view, you want to avoid your deadwood being colonised by sapotrophic organisms – or at least you want to slow this process down as much as possible.
One approach for keeping your deadwood fungi/bacteria load down is to apply some ‘extractives’ to the wood – extractives are volatile compounds found in heartwood and bark, which have anti-bacterial/fungal properties. There’s quite a good thesis online which identifies many extractives from a range of different trees – you could try turpentine for example, which is extracted from pine tree resin. I’d avoid putting this into the soil though, for fear of harming beneficial microbes in the rhizosphere.
One of the main accelerators of decay in young stumps is moisture content.ref This is a another key control you have to minimise decay in deadwood on your bonsai – keep it dry – or in scientific terms reduce its ‘wettability’. This can be achieved by applying something like linseed oil. Other substances I have heard applied to deadwood are superglue (it reduces wettability by creating an impermeable layer on the wood), and wood preservatives but most of these have chemicals I wouldn’t want washing into my bonsai soil.
So in summary – before you carve, work out what you’ll be doing to the phloem and xylem flows to avoid damaging areas of the tree which you want to keep alive, and once you have deadwood, keep it dry and repellent to microorganisms. And in order to help with wound healing, carving in warmer weather when the tree is in active growth gives it the best chance of defending against pathogens which try to enter via the wound.