What is the newly discovered new wood "middle wood"? How is it better than hardwood and softwood?

Tuchong Creative

In daily life, softwood and hardwood each have their uses. Recently, scientists have discovered a new type of wood in between - "middle wood". It not only combines the advantages of hardwood and softwood, but also greatly enhances the ability of trees to store carbon.

Let me first tell you a counterintuitive piece of knowledge. We usually think that strong wood comes from the snowy forest areas of Northeast China. In fact, truly high-grade hard wood, especially various types of mahogany, mainly comes from tropical rainforests.

The difference between hardwood and softwood is not that hardwood is hard and softwood is soft, but it is based on many factors such as their growing environment and wood structure. They are two common ways to classify trees.

Softwoods usually come from coniferous trees, and hardwoods usually come from broadleaf trees. Generally speaking, softwoods are usually less dense and softer than hardwoods, but there are many exceptions. For example, Douglas fir has the same density as some hardwoods, and balsa, which is a hardwood, is as light and soft as foam.

Hardwood and softwood each have their own advantages and disadvantages. For example, softwood usually has a smooth texture. Most of the wood used for building structure frames comes from softwood, which is relatively abundant and cheap, easy to process, has a small shrinkage and swelling rate, and is not easy to crack, but its hardness and wear resistance are disadvantages. Hardwood is just the opposite. Its material is denser, its texture is richer, it has a beautiful appearance, and high wear resistance. It is particularly suitable for making furniture, floors and fine wood products. However, hardwood is often easy to deform and crack. Friends who have used mahogany furniture in the north must have a deep understanding of this.

The basic difference between hardwood and softwood lies in the microstructure of the wood.

The structure of cork is relatively simple, mainly composed of larger longitudinal cells (tracheids) and a small number of radial cells (xylem rays). The vertically distributed tracheids determine the basic structure of cork, and the fibers covering the xylem cells are also relatively thick.

The structure of hardwood is more complex, with wood rays accounting for a larger proportion of the wood, which greatly affects the texture of the wood. In addition, the longitudinal cell distribution in hardwood is more complex, with large diameter vessels to transport water and inorganic salts, and the surrounding finer wood fibers give the tree stronger structural strength and durability.

The researchers found that hardwood trees (such as oak or birch) typically have giant fibrils with a diameter of about 15 nanometers; while softwood trees such as pine or spruce have giant fibrils with a diameter of up to 25 nanometers or more. This difference determines the material difference of the wood.

In addition, there is another huge difference between hardwood and softwood, that is, softwood trees tend to be more efficient in absorbing and fixing carbon dioxide. This ability is considered an important option for responding to and solving global climate change.

So, is there a kind of tree that can combine the advantages of both and avoid the disadvantages?

The two existing species of the genus Tulip, namely the North American tulip tree and the tulip tree, have giant fibrils of about 20 nanometers, which is between softwood and hardwood. This makes the tulip tree have the dual characteristics of traditional hardwood and softwood, with considerable wood hardness and the strong carbon dioxide absorption and carbon fixation ability of softwood trees.

Between 30 and 50 million years ago, when the tulip tree diverged from its Magnoliaceae ancestor, the Earth experienced a drop in carbon dioxide concentration, with atmospheric carbon dioxide concentration dropping sharply from around 1000 ppm to around 320 ppm. Under these conditions, the tulip tree, which has a strong carbon fixation capacity, had a considerable advantage in this change.

Today, the tulip tree, which combines the characteristics of traditional softwood and hardwood, not only provides us with a good wood choice, but also provides a very important afforestation option for carbon neutrality.

Many friends are concerned about the quality of wood. In addition to hardwood and cork, what other factors are related to wood quality?

The quality of wood is closely related to the type of tree. Generally speaking, except for special uses, such as making thermos stoppers, the soft cork layer of cork oak is selected, and papermaking can use the flexible fiber-rich bark of paper mulberry. In most cases, we usually choose wood with high hardness, high strength and high heartwood to make furniture, wardrobes and other household items.

The so-called heartwood, as the name suggests, refers to the wood in the center of the tree. They are aged xylem, located between the pith and sapwood. These cells were once the conduits for transporting water to the branches and leaves of the tree. During the growth of the tree, the conduits in the center of the tree gradually lose their drainage function. These heartwood cells that gradually lose their drainage function will darken in color due to filling with oil, gum, tannin or other substances, making the boundary between the sapwood and heartwood of some woods clear. As for those sapwoods that still maintain activity and are not filled with secondary metabolites, they will appear softer.

It is precisely because of the existence of secondary metabolites that wood has certain special properties. However, it often takes time to acquire such properties.

The preciousness of Huanghuali lies in the accumulation of time. Although after fifty years, Huanghuali has grown to the thickness of a bowl, its heartwood is probably not as thick as a rolling pin. The reason why the heartwood is precious is that secondary metabolites accumulate in the process of tree growth, filling up the pipes that were once used to transport water. Only then can such wood have considerable hardness, color and fragrance, and become a good wood. Take Dalbergia odorifera as an example. Under good water and fertilizer conditions, it can usually grow about 2 cm in height and 3 cm in diameter each year. It takes 6-7 years of growth to produce heartwood. Under wild growth conditions, the accumulation rate of heartwood is even slower, and it may even take 10 years to produce heartwood. From this perspective, although mahogany trees have strong vitality, it is difficult for them to produce a large amount of wood resources in the foreseeable time period. In this sense, mahogany is actually a scarce and "non-renewable" resource.

Our development of medium-wood species such as the tulip tree today will provide a new option for our future timber selection and afforestation.

This article is a work supported by the Science Popularization China Creation Cultivation Program

Author: Shi Jun, Ph.D. in Botany, Chinese Academy of Sciences

Reviewer: Director of Science and Technology Department of Martha China Forestry Society, Senior Engineer

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.