Wood Defects: Young Heartwood and Growing Tensions Explained

Young Heartwood

Young heartwood is a defect that occurs in wood from trees that have had a strong initial growth rate in diameter. This rapid growth results from favorable ecological conditions or growing up in a forest of low mass density (low density of trees per hectare or defective). This rapid growth results in wood that differs from normal wood of a particular species, with the following characteristics:

• In the late wood, there is a lower proportion of cells with thick walls.
• The apparent specific gravity is lower.
• The fibers are shorter.
• Tangential and radial contractions are lower, while the longitudinal contraction is higher, this being 10 times more intense than in normal wood.
• Sawn pieces that contain young heartwood tend to warp during drying.

As for the case of spiral grain that sometimes goes along with youthful heartwood, the appearance of this defect can be reduced by silvicultural measures and technology.

Growing Tensions

Growing tensions are a very important structural defect, affecting a considerable extent of some timber harvesting. The stresses of growth can be detected in all forest species as a result of maturation of tissues, but while in conifers (Gymnosperms) it is of low intensity, some hardwoods (angiosperms) have been measured to have an average stress of 100 kg / cm² (mainly in the genera Fagus, Eucalyptus, and Populus), with an extreme value of 285 kg / cm in Eucalyptus regnans.

There are a number of methods designed to reduce the effects of growing tensions, methods listed below:

a) Slotted

It consists of making a slit along the circumference of the roll, about 20 cm from the head and with a depth approximately equal to half the radius of the roll at that point. The slot should be done before practicing any transverse subdivision or refresher of the head. For example, suppose a roll of 8 m long, projecting subdivided into two 4 m each, there will be two slots in the entire circumference of the roll at that level. If the tree contains important growth tensions that could have been expressed in cracks in the heads of the trunks of 4 m, the prior of the slots will have avoided or at least diminished it. This method has been tested initially in Australia and then tested in different countries (including Uruguay, in Eucalyptus poles) with promising results.

Soaking the heads of the rolls by spraying water

With this method, an intensity reduction of tensions between 15 and 20% has been found, and achieved a redistribution of the same (homogenized in the transverse plane) and a reduction in the demonstration of warping during sawing.

Immersion in hot water

The basis of this method is that growth stresses are released when the woody tissues are softened by hot water. Treatment can last between 24 and 48 hours, letting it cool for about 7 hours. There have been successful results in relation to the tensions, although there is a danger that cracks occur in the heads of the rolls due to excessive temperatures.

Special Needs Sawing

It consists of carrying out simultaneous parallel cuts through the use of double or quadruple saws or circular saws or multiple alternatives.

To complete this topic, research is being carried out to find methods to select trees with reduced growth strains, which undoubtedly will be the long-term solution.