Wood structure - sapwood, heartwood, annual rings, pores, resin

Sapwood, heartwood, latewood, earlywood, annual ring are names used quite often in articles, but which I have not talked about much. I know, you can work wood without knowing scientific names. But knowing the terms helps us to have a common language. The above names are common and define the structure of wood, regardless of species. Depending on these elements we know that we are dealing with harder or softer wood, with a more pronounced or duller pattern, with pores or without.

Information deciphered in the tree cutting section

The structure of the wood is revealed as soon as the tree has been cut. Looking into the cross-section of most trees, you can see a lighter area towards the outside of the tree and a darker area towards the inside. These are alburn and heartwood, sapwood and heartwood, for those who want to search for information on foreign sites.

Sapwood and heartwood

The difference between them is huge. Sapwood is physiologically active wood, i.e. wood made up of cells that transport water with mineral salts, nourishing the tree. It is therefore less dense and has a high water content. Wood from this area is very little used and has a very low outdoor resistance. The heartwood, however, is hard and dense and consists of inactive cells. It is the area responsible for the tree's strength and not for feeding it. This is basically the wood used for furniture, construction and other wooden objects.

The areas do not have the same size throughout the life of the tree. At the beginning, when the tree needs a lot of food to grow, the sapwood zone is very large and the heartwood zone is small. This is why young trees break more easily and it is not recommended to cut them down for wood processing. As time goes by the proportion changes, some of the cells cease their feeding activity and move into the resistance structure. This is why trees are cut at maturity, when the heartwood area is at its maximum size.

There are not always differences in colour between sapwood and heartwood. In this case identification is made by density and water content. Species with clearly demarcated areas: nuc, oak, frasin, ulm, acacia, cherry, larch. Species with areas not delimited by colour: fag (the red heart of the beech is a false heartwood), plop, you, birch, fir, spruce.

Bark and marrow

On the outside of the tree are bark. It is very clearly separated from the rest of the tree, having different structure, texture, colour and appearance. It has an area of living cells in contact with the tree and an area of dead cells on the outside. It is a protective layer similar to human skin, covering the whole tree, but the one on the trunk is different in thickness and appearance from the one on the branches. It contains tannins, resins and other substances that repel pests and repair wounds.

The pith is in the centre, surrounded by the heartwood. It is brownish in colour, different from the heartwood. It is made up of dead cells and is loose and without resistance. It is not used at all for the production of wooden objects. After maturity this area begins to grow, reducing the tree's strength. Inside, a loose, hollow area is formed, with no role in supporting the trunk. This is why, over time, the trees become increasingly hollow until they are easily blown down by winds and storms. Like humans, trees grow old and die. The age at which this happens differs from species to species. Some species are very long-lived, reaching 1000, even 2000 years, such as the olive tree or 1000 years for the oak, others are shorter-lived, 200-300 years (beech), even 100 years (poplar).

Annual rings

I don't think it's a secret anymore that the age of a tree can be identified by the annual rings we discover on the trunk cut not far above the ground. They are specific to species growing in the temperate zone, when the growth and development periods of plants are well defined. Annual rings are concentric circles that run from the centre of the tree to the bark and are regular in outline in most species. However, there are also situations where the outline is finely wavy (arinThe rings are not visible in all species, but that does not mean they do not exist.

Each ring has two zones. The lighter, more tanned area is called early wood. It is the wood deposited in the spring, when food supply is high and growth is rapid. The darker and denser area is called late wood. It is the wood deposited in summer, when the food supply is lower (it doesn't rain as much) and the laying of the cells is more thorough.

Visible structural elements in woodworking

When the wood is processed, other specific elements appear: pores, medullary rays, resin channels, resin pockets. These elements help to identify the species and form the natural pattern so specific to wood.

Porii

They are basically sectioned wood cells, appearing as small holes in the wood structure. From the outset, the existence of pores tells us that the wood is hardwood. Softwood has no pores. The transport of food in resinous plants is through very thin vessels called tracheids, with food entering the cell by osmosis. When these vessels are cross-sectioned, there are none of the holes typical of hardwoods.

The pores are larger and more numerous in early wood and smaller and fewer in late wood. When the wood is cut tangentially to the annual ring, wood with large pores forms specific patterns called fladers, or cathedral cut, in English. When the cut is parallel to the annual rings (radial), they are seen as parallel lines.

Pores determine the ability of wood to absorb water. The more and larger they are, the more the wood will tend to absorb. If such wood is used outdoors it is best to protect it from moisture and not to come into direct contact with the ground. Wood species with large pores: oak, ash, walnut, chestnut.

Spinal Rays

They are tissues made up of radially arranged cells that start from the marrow and reach the shell. In cross-section they appear as lines or bands of varying thickness, straight or curved, with a gloss and colour different from that of the surrounding wood. In cross-section they may be interrupted, but are always continuous through the wood to the bark. When the cut is radial, the medullary rays appear as bands of different lengths and heights, with a particular lustre. This sheen has led to the name mirrors, an aspect appreciated in paltin or oak.

Resiniferous channels

These are places where resin is stored in the tree. Like medullary rays, they link the pith to the bark. Their size and location differ from species to species. In cross-section they can also be seen with the naked eye, being whitish in colour, and in longitudinal section they appear as yellow-brown or whitish broken lines.

Channels are specific to softwoods, but they are not found in all species. They are present in pine, spruce and larch, but not in fir or yew. However, by accident, resin pockets may also occur in fir wood. Resin is used by the tree to heal wounds. If the tree has been damaged during growth and internal wounds appear, the resin under the bark migrates to the wound and heals it.

I hope the information helps you to understand why wood looks or behaves a certain way or why trees are only felled when mature. If you think it's useful information for others, you can share it. And if you have any queries or questions, you can leave them in the space below.