Composition of a trunk
Trees are living beings, very complex and perfectly adapted. They consist of different parts. Like our skin, which forms a protective barrier against external aggression, wood protects itself with its own skin: the bark. This will prevent most of the external attacks.
It will develop as the trunk grows to keep this envelope intact throughout the life of the tree. Inside the tree, by simplifying we find 2 parts that are more or less distinct depending on the species: the sapwood and the heart (also called heartwood).
A tree grows over the years, developing from the heart outwards. It is traditional to be able to count age by counting its rings, in a perpendicular section. Each ring corresponds to one year of growth.
Sapwood
The outer, newer area of the tree is called the sapwood. It is the one which has just grown and which carries the nutrients, which allows them to be exchanged between the roots, the leaves, photosynthesis, and which carries the sap.
Only the sapwood is attacked by wood-eating insects and fungi. It is in this part that there are sugars and nutrients essential for the development of these parasites.
This sapwood often has a different color from the heart of the tree, which is lighter. This is generally eliminated when preparing the wood into trays or beams because of this color difference. (Certain woods have no visual differentiation such as spruce, fir, ash, beech, plane tree, maple, birch, poplar, linden. The color is uniformly white, and therefore used as is, without elimination of the sapwood which cannot simply be distinguished for elimination.)
The Heartwood
The heartwood is the internal part of the wood, corresponding to the oldest growth zones, which no longer contain living cells. It is a hard, compact, dense, dry and rot-proof wood resulting from a progressive transformation of the sapwood, an evolution which is accompanied by the disappearance of cellular starch and deposit of various substances (tannins, resins) on the cell walls, which makes this tissue much more resistant.
This central region of the trunk is almost entirely dead wood in which there is practically no water exchange, while the sapwood is a living, active wood whose conductive vessels carry raw sap. We now understand better why certain trees, although they seem rotten from the inside, continue to grow and do not collapse.
Burl
Often seen as a rounded growth on a tree trunk or large limb, a burl results from a tree being subjected to some form of stress. It could be an injury, a virus or a fungus. It is an anarchic growth of cells, growing in all directions, creating a very complex pattern but very different from the rest of the trunk.
The burl is used in the form of a sheet of veneer or marquetry by cabinetmakers who appreciate its richness. The most popular provide very grained and very uneven woods.
Note, however, that this harder part of the wood is more difficult to work.
Some woods, despite their protection, are subject to attacks by parasites. We find these attacks in the fibers of the wood, either the color of it has changed, or the structure itself has been transformed.
Often the wood becomes darker and this creates incredible patterns, extraordinary for sculpture.
Here, a piece of heated lime, where the mushrooms have created these very particular gray/black patterns.
