January 10, 2022

Imagine you aren’t sitting at your computer or looking down at your phone reading this article. Instead, try to picture yourself for a moment walking through the woods. What kinds of trees do you see around you? Are they tall? Are they short? Do they all look the same, or are there some clear differences distinguishing some trees from others? Depending on where you are from or where you live and the particular kinds of goals governing authorities prioritized throughout history, your visualization of a forest could very well be different from someone else’s.

Regionally, no two forests are the same, and this is especially true when it comes to their varying degrees of diversity. Some forests, and generally the most commonly thought of,  host a wide variety of tree species while others like the well-known pine forests in Israel mostly exhibit one dominant species above the rest. However, it turns out that this difference is important as evidenced by a new Israeli study which found a direct relationship between the diverse presence of tree species in forests and capturing carbon dioxide from the atmosphere. The more diverse a forest’s trees are, the more efficient and capable it is at absorbing and storing CO2.

Funded by the JNF and led by a doctoral student Ido Rog from Dr. Tamir Klein’s laboratory from the Department of Plant and Environmental Sciences at the Weizmann Institute of Science, researchers collected a wealth of ecological and physiological data from 25 trees belonging to five different species all within a 10 dunam (nearly 2.5 acre) area from the Yishai Forest near Beit Shemesh.

After inputting the data into a computer model, the researchers were able to determine the amount of water the wood of the trees utilize as well as the amount of carbon stored within. Compared to tree specimens belonging to a forest hosting one abundantly dominant species as a control group, trees from diverse-heavy forests were found to store about 130 grams (32 percent) more CO2 per square meter per year or 32 percent more CO2

Due to the current climate crisis, this ecosystem service has never before been more urgently crucial than it is now. Because they absorb almost a third of all CO2 emitted into the atmosphere, the ecological role forests play remains one of the most important tools for reducing the effects of modern-day global warming.

Trees Can Be Territorial Too

According to the researchers, diverse forests are more capable of capturing and storing CO2 due to the more effective way in which they utilize water resources. The researchers’ findings even show that diverse forests are able to utilize 40-80 millimeters more water per meter per year than non-diverse forests. Because of this, trees from diverse forests are better able to perform photosynthesis, an energy cultivation process key to any plant’s survival.

According to the study, one factor that allows trees from diversity-packed forests to make better use of water stems from “territorial division”—the range of depths different roots from different tree species typically occupy. Some trees’ roots naturally grow downward to exploit water present in deeper layers of soil and others have evolved to grow more horizontally to take advantage of water present in shallow layers of soil. Therefore, in a non-diverse forest, the dominant group of trees compete with each other for the same supply of water due to the similar depth-occupying space their roots all share. This means less water to go around, and thus less energy to perform photosynthesis effectively. However, when there is diversity amongst tree species, all their roots collectively occupy different depths of soil, thereby reducing competition for water, increasing water uptake, and enabling higher functioning photosynthesis throughout the forest.

Optimal water utilization is also in part due to the combined growth of shorter, shade-preferring trees, such as elm and oak, under tall trees like pine and cypress. Not only does this results in an even fuller coverage at the surface, but the combined presence of both shorter younger trees and taller mature trees reduces competition between trees of the same species as well.

“When trees thrive in a good water economy, they open their leaves’ stomata—the small nostril-like openings on the leaf, and they pull in CO2 into them which turns into sugar (glucose) during the process of photosynthesis,” Rog says. “When this happens, water vapor escapes from the stomata into the air and the plant loses water. Therefore, when the plant is experiencing a water shortage, it will prefer to close these openings.”

Different Priorities Back Then

Despite the advantages a diverse forest has over a non-diverse forest, some of Israel’s unfortunately are reminiscent of the latter because of the initial planting policy Israel established in its first decades of statehood.

“The purpose of planting in the forest was to maximize the productivity of the forest in order to yield as much wood material for construction as possible in the shortest possible time,” Rog explains. “They did not deal with the forest’s resistance to fires, or its efficiency in terms of carbon storage or any other ecological qualities.”

Dr. Gilad Ostrovsky, head of the JNF’s Forestry Division and its chief forester, confirms Rog’s remarks. “Speaking of the areas planted in the ’50s and ’60s, most of the forests were definitely made up of one dominant species,” he says.

However, Ostrovsky says that in 1990 there was a change in the KKL-JNF’s planting strategy.

“The composition of planting has shifted from relying on coniferous trees, such as the Jerusalem pine (Aleppo pine), to planting broad-leaf trees such as oak, elm, or carob,” he says. “In fact, there is a very large diversity.”

According to him, part of the diversity stems from natural processes and not solely from deliberate planting.

“After the establishment of the state, the massive deforestation carried out by villagers for the purposes of heating and cooking ceased, and recovery of the oppressed Mediterranean forest began. Even the single species pine forests naturally diversified.”

If It’s Good For the Environment, It’s Good For Us

It is important to emphasize that besides storing carbon, a diverse forest also has other benefits. According to new but yet-to-be-published studies, diversity in forest tree species can support a healthy variety of bacteria and fungi which effectively increase the chances of survival of insect populations. In addition, other studies conducted around the world have linked tree diversity in forests to better resistance to natural disturbances like fires.

“These findings must be taken into account, and we are already seeing how they affect decision making policies,” says Rog. “For example, the new forests planted on Mount Gilboa are much more diverse.”

Beyond that, a diverse forest offers a scenic advantage and recreational outlets for those enchanted by the outdoors as it enables someone to use and enjoy the forest regardless of the season—even when some of the species are shedding their leaves or are not blooming.

“The aim is to think about how the forest can contribute to human well-being, both as a highly ecologically functioning carbon sink and a place of leisure,” Rog concludes.

The article was made by ZAVIT – Science and the Environment News Agency