Biological degradation of wood is a very complex process that depends on many factors. To test the natural durability of wood against wood-eating fungi, wood samples are typically exposed to them.
At the end of the exposure, the loss of mass caused by the attack of the fungi allows to appreciate the natural durability of the wood.
The loss of mass divides the species into different classes: class 1, very durable to class 5, non-durable. Sapwood is always considered as non-durable whatever the species considered. Therefore, the durability tests only cover the heartwood of the wood...
The median values of mass loss caused by the Coriolus fungus are shown in the figure below.
Analysis of median mass loss values shows that juvenile wood (located near the pith) is less resistant than mature wood (located near the bark) to attack by lignivorous fungi.
Several studies show that the difference in durability observed between juvenile and mature wood can be attributed to the strong variations in extract content, absent in the sapwood, less in the juvenile wood, and increasing towards the outer areas of the heartwood. These chemical substances make the wood resistant to degradation.
In addition, the natural durability of the heartwood increases as one approaches the sapwood. In Robinia, it is the polyphenols (more specifically flavonoids ) that give the wood its natural durability.
Overall, Robinia wood is assigned to class 1 (very durable), although the more sensitive juvenile wood is mainly found in class 2 (durable). However, the proportion of juvenile wood in the log is significantly lower than that of mature wood. This is all the more true in the context of Romanian forests where Robinia grows rapidly.