Density

Why does the annual ring width and plant location's geographic position very strongly on density.The reason for the large variations in density within a single species is primarily wood anatomical building and tree growth rate. The thick-walled summer wood is three times heavier than the thinner walled spring wood. The width of the darker band summer wood is basically the same regardless of the tree's growth rate, at the same latitude and site quality. The width of the darker band also decreases summer wood darker the farther north you go.

At the same ring width, the farther north you go, the smaller the density is. As the annual ring width increases at one and the same geographical area reduces the density. Because the width of annual rings naturally recedes from the marrow out against cambium (growth zone) so does the density in principle from the marrow against cambium. It is just the pith wood that has a higher density. The fibers are there short and small and have relatively large fiber wall portion.

As the average density is highest at 1 to 2 mm ring width. At very tight annual rings, the density is extremely low. Such wood has very thin fiber walls. Similarly, the density decreases when the growth rings are wider than 2 mm. At the same ring width, the density thus varies within wide limits.

The following factors affect the density:

  • Temperature: Warmer climate gives heavier wood than colder climate at a given ring width.
  • Humidity: Compared with wood from wetter climate provides timber from a timber drier climate with less sommarvedsandel. This means that wood from drier climate is easier at a certain ring width.
  • Ground conditions: Suitable soil conditions for a species gives heavier wood than soils with less suitable conditions.
  • Stock Density: Dense stands provide lumber with thin rings and a large percentage summerwood. It gives trees with heavy timber.
  • Positions of the tree: At the same ring width, wood in the lower part of the trunk is heavier than wood higher up in the tree; and wood in the outer part of the stem is heavier than wood in the inner part of the trunk. This is true except for the five to ten growth rings closest to the pith, where the density is relatively high. This is because the tree produces wood with high density in the parts that are most strained under wind load.
  • Fertilization: In a very weak ground, with nitrogen deficiency, fertilization gives a significant increase in growth, but that the density decreases.
  • Genetic characteristics: Trees grown in the same stock in the same growing conditions, but with different genetic characteristics, may have significantly different densities at the same annual ring width. Variations of up to 25 percent have been measured.

The wood density is relevant to various characteristics of wood. The uptake of water is such slower and lower at high density than at low. This variation is important for the resistance to microbial degradation. Decay fungi as well as blue-stain fungi must have access to free water in the wood for their growth. This means that the moisture content must be over 30% of these fungal groups could attack the timber.

Spruce timber with great summer wood and thus high density may be considered to have good resistance to microbial attack if used above ground. Frodvuxet spruce timber with low density contrast is considered to have a greater ability to absorb water and is therefore more likely to be attacked by microorganisms. Pine wood block easily takes up water regardless of density and therefore more easily attacked by microorganisms. To cope pine sapwood wood against rot must be pressure impregnated timber, whether it is above ground or below ground. Since pine sapwood with low density lighter take up water is easier to push than impregnate pine sapwood with high density. The higher uptake of impregnation liquid means better protection against microbial attack.

The amount of shrinkage or swelling at a change in moisture content below the fiber saturation point is proportional to the density. Along the fibers decreases shrinkage with increased density and increases in radial and tangential direction.

Density in different parts of a tree
Density in different parts of a tree

 
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