Why do some trees grow fast and some slow?

Shi Jun

Hello everyone, I am Shi Jun, a Ph.D. in botany from the Chinese Academy of Sciences. Today I want to talk to you about why some trees grow fast and some grow slowly. Different plants have different choices. Some trees grow five or six meters a year, while others grow five or six centimeters a year. Why is there such a big difference?

Let's first talk about why trees grow tall - in fact, it is to compete for sunlight. For example, in the hot and humid tropical rain forest, there is no shortage of water or temperature, but sunlight has become a precious resource that many plants compete for. As long as you walk into the rain forest, you will find that the light suddenly becomes much darker. Generally speaking, after 4 pm, it is like sunset in the rain forest.

In the Xishuangbanna tropical rain forest, the radiation in the canopy can reach 158.92 watts per square meter, while the radiation intensity on the ground is only 12 watts per square meter. The sunlight is intercepted by layers of plants before it reaches the ground. In order to get more sunlight, plants have to do everything they can to grow taller, so many plants living here are much taller than we can imagine. For example, teak can reach 40 meters, and parashorea chinensis can reach 60 meters. This is a world of competition for light, so the big trees in the forest basically grow straight up.

Of course, under sufficient sunlight, plants will be different. For example, on the African grasslands, the height of the trees is usually only about 10 meters. There are few such trees that reach the sky. Their shape is more like a big umbrella. In fact, by opening the leaves, you can absorb a lot of sunlight, because there is no very strong competition between trees here. Everyone is very far away, so there is no need to grow very tall to get enough sunlight. So being tall actually has high returns. Is growing taller quickly the best choice? In fact, not necessarily. What we did not expect is that both fast-growing trees and slow-growing trees appear in tropical rainforests. This is actually a different competition strategy for different plants. Some plants choose to accumulate and then develop. For example, the more familiar Dalbergia odorifera, it doesn’t matter if you are not familiar with this name. Let’s change its name. Its name is Huanghuali. Under good water and fertilizer conditions, Huanghuali grows about two centimeters a year, gradually increasing, that is, the diameter of its trunk closest to the ground increases by 3 centimeters. It usually takes 6 to 7 years of growth before core wood is produced. What is core wood? As the name implies, it is the wood in the center of the tree. In fact, they are aging. The xylem is located between the xylem and the sapwood. These cells used to be the pipelines for transporting water to the tree. During the growth of the tree, the pipelines in the center of the tree gradually lost their functions. These core woods that have lost their functions will gradually be filled with oil, gum, tannin and other colored substances, which will make the color of the new wood in the area darker and darker, making it clearly different from the surrounding sapwood. As for those surrounding woods that still maintain activity and can transport water to the tree, they are called sapwood. Because they are not filled with secondary metabolites, the color appears lighter and the wood is softer. Everyone, please note that whether the wood is hard enough is actually directly related to the secondary ecological metabolism and the amount of secondary metabolites filled.

Of course, some plants choose to grow quickly. For example, there is a plant called balsa wood in the rainforest. It can grow 5 to 6 meters a year. The growth rate is beyond imagination. But their material is very loose. The wood is very light. How light is it? An adult can hug a two-meter-long tree trunk and run. Can you imagine how light it is? But very light wood also has its role. Before the emergence of synthetic materials, we mainly used this kind of wood to make model airplanes, and this kind of wood can even affect the direction of the war. Unexpectedly, during the Second World War, Britain produced an airplane called the Mosquito bomber, and its main material was balsa wood. The wings are lighter, and the airplane can fly very fast. Driven by the same engine, the wood-light airplane can fly very fast, which can take the lead. Second, airplanes made of wood are particularly easy to repair, which is easier than metal repairs. You can find a carpenter to fill it with this wood, and it will be repaired quickly. The damaged airplane can return to the battlefield quickly. So from this perspective, balsa wood has made an outstanding contribution to the victory over German fascism. So in this sense, every tree has its own value.

I would also like to remind everyone that no matter whether it is a heavy tree or a light tree, everyone thinks that it is a plant that can continue to grow, so we have an illusion that wood can be produced infinitely and seems to be a renewable resource. Logically, there is nothing wrong with it, but it is wrong to add a limitation - for example, under the conditions of limited human historical development, this wood is not a renewable resource. For example, it takes about 300 years for a red sandalwood to grow to be able to make furniture, about 500 years for a huanghuali tree to grow to be able to make furniture, and more than 800 years for a parashorea chinensis to grow to adulthood. So when we were repairing the Forbidden City, it was really difficult to find a golden nanmu tree. So we preserved this big tree, which not only preserved the material of furniture, but also a living history book.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Shi Jun

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.