Why species such as acacia, chestnut or larch are more hardy outdoors than beech, birch or poplar

Durability is the property of wood to resist for as long as possible the attack of fungi and insects that have got into the wood due to excess moisture. Durability is therefore equivalent to outdoor resistance, as excess moisture is specific to this environment. The outdoor resistance of a wood can be natural or due to treatments it has previously undergone. Impregnation with special substances, under specific conditions, can transform a less durable wood into a very durable one. Kebony and Accoya are such examples. Coating wood with exterior resistant oils, varnishes or paints can also increase the wood's resistance to moisture and other atmospheric factors. But it is difficult to cover the wood completely so that water cannot find a way in. That's why it's very important that the wood used is naturally highly durable. Find out why some species of wood are more durable than others and can be recommended for outdoor use below.

Standard EN 350:2016 classifies wood species according to durability

The wood holds up quite well in the air or fully submerged in water. In direct contact with the ground, however, its strength is greatly reduced. Moisture and air create a breeding ground for fungi and insects that attack and destroy wood. How quickly it is attacked and destroyed depends on the characteristics of the species.

European Standard EN 350:2016 - Durability of wood and wood-based products - Testing and classification of the durability to biological agents of wood and wood-based materials - -combined the old EN 350-1 and EN 350-2 standards into one document. There are 4 categories of durability for wood, depending on resistance to different factors: wood-rotting fungi, wood-boring insects, termites and marine organisms.

Depending on the resistance to fungi that cause rotting, wood is divided into 5 durability classes. DC 1 is the most durable grade, with a strength of more than 25 years, and DC 5 is the perishable grade, with a strength of less than 5 years (for some countries less than 1 year). Find here classes with examples of wood species for each. The strength refers to heartwood, the sapwood of all species is classified in DC 5.

Why heartwood is stronger on the outside than sapwood

The difference in strength between sapwood and heartwood is their ability to absorb water, i.e. how quickly the fungi can get inside. When very young, the tree draws its food from the ground to grow by forming new wood cells. As it grows, the old cells die, lose their capacity to absorb, and gradually strengthen and take on a supporting role. Cell death occurs differently from species to species (e.g. 2-3 years in acacia, 12-13 years in ash).

As new cells form in the area between the bark and the trunk, dead cells accumulate inwards, forming heartwood. The new cells form a ring between the bark and the heartwood, an area called alburn, responsible for feeding the whole tree. When the tree is young and growing, the sapwood area is wide, with little heartwood. As it approaches maturity, the heartwood part grows, with the sapwood part becoming increasingly narrow.

The loss of feeding capacity makes the heartwood absorb less water and so the fungi that cause decay have a harder time reaching it. It is the part of the wood that is used to make furniture and other objects because of its much better mechanical strength and workability than sapwood.

However, according to the standard classification, heartwood can last from less than 5 years to more than 25 years, depending on the species. What causes this difference? The presence inside the vessels of formations and substances that block the entry of water, and thus the fungi that cause decay.

Tiles, resins, gums, tannin - formations and substances that determine the durability of wood

Over time, some wood species have evolved and developed defences to protect themselves from attack by fungi or to repair wounds caused by insects and animals. As sapwood cells cease their feeding activity, becoming so-called dead heartwood cells, parts of them turn into fungicidal compounds or simply into formations blocking the entry of water into heartwood vessels. The latter are called tile and are outgrowths of parenchyma cells (those that form the woody mass).

Tyloses are like air bubbles that attach themselves to the vessels of deciduous heartwood (in the pores) or in the tracheids of the softwood. They make it much harder for water to get in and out. The more there are, the more the wood is protected, i.e. water does not get in, and so the fungi that cause decay are kept away. Acacia has its pots blocked with such tiles, its resistance to the outside is very high. The percentage of tiles is also very high in chestnut, which is also known for its outdoor resistance. Unlike acacia and chestnut, beech, poplar, willow and birch have free-standing pots, with water free to enter.

How important the presence of tiles is can be seen in oak. White oak (Quercus alba) has tiles in its pores, while red oak (Quercus rubra) does not. As a result, white oak is listed as durable wood (DC 2), while red oak has low durability (DC 4).

Another effect of the presence of tiles is that the wood dries longer. Tiles accumulate in the cut area when the wood is cut, preventing water from escaping. Chestnut, with the pots almost blocked by the tiles, dries much more slowly, while it takes up to a year for the log to reach the equilibrium moisture content of 30%.

Resins, gums and other substances that protect the wood from fungi, insects and termites are stored in the tiles. Depending on their nature, wood gives off a specific smell. The resins in resinous wood have the ability to repair wounds caused by animals or various weather phenomena.

The study of wood composition and microscopic structure can give us important information on the behaviour of the species under outdoor conditions. However, it should not be forgotten that natural durability depends on the geographical region from which the tree originates, growing conditions, age and other such variables.

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