Photo:
Jan Pisek

A change in the leaf inclination angle can change ability of trees to absorb carbon

In order to control global warming, it is essential to know how much forests emit or absorb carbon. At present, however, it is evaluated relatively one-sidedly and superficially. A study led by Tartu Observatory associate professor in remote sensing of landscapes Jan Pisek indicates that small things like the leaf inclination angle can have a significant impact on climate models and should be measured as accurately as possible. Novaator interviewed Pisek´s co-author Steffen Manfred Noe, professor at the Estonian University of Life Sciences.

According to the research, it is vital to study the leaf inclination because it is indirectly related to the ability of trees to absorb carbon. "We see that a huge part of how light passes through the tree system is somehow related to the distribution of tree leaves across the diverse canopy," Noe says. The position of tree leaves can vary depending on the different stages of the tree life, types of trees and seasons.

Noe was one of the authors of a new scientific article indicating that leaf inclination is essential in modelling various natural processes. Together with his colleagues, he assessed the characteristics of the leaf angle of the five most common broad-leaved tree species in Europe. The authors stated that in models such as calculating the annual carbon balance of the forest, for a more accurate results, it would be worthwhile always to prefer leaf inclination data collected locally rather than predetermined standards.

Not all leaves grow in a warm place

"The leaf inclination is important if we want to observe solar radiation as the energy source that triggers photosynthesis," says Steffen Manfred Noe. "It all depends on how the energy from the sun is distributed daily and how effectively it is delivered to the plant," Noe points out.

According to the professor, when looking at any tree, a person will inevitably see that it has leaves with very different angles of inclination. "Therefore, it would be important in the studies to look at the leaf inclination angle distribution as well. We shouldn't just look at one leaf, but at a larger scale," he notes.

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In the distribution of leaves, it is essential to note what factors affect their position: for example, is it a tree that spreads alone in the middle of the field or is it a tree squeezed between its congeners in the forest? "When a tree is inside the forest, only a small part of it receives maximum energy from the sun," describes the professor.

Secondly, according to him, monitoring the radiative transfer in the canopy is very important. "This means that the radiation extends through the entire canopy: in addition to leaves under direct sunlight, the tree also has leaves in the shade," Noe explains.

This may raise the question of how a leaf that remains in the shadow of the top leaves, or understory, can survive without much direct sunlight. They still live. For example, if the wind moves the canopy, a tiny spot of light always comes down, with the maximum amount of solar radiation inside," the professor points out. Although such a beam of light lasts for a short time, the lower leaves are adapted to cope with it.

According to Noe, studying all this in the laboratory and based on one leaf is convenient. The researcher can pick a plant, choose a suitable leaf, and place it at the best possible angle concerning light. It can be done to test how photosynthesis proceeds under ideal conditions. "However, when we look at nature, we have to accept that not all leaves are in an ideal position," Noe says.

Here we can use another research option – remote sensing. "The effect of the leaf position has already been considered automatically. When we look at the photosynthesis from a satellite or aircraft unit, we can't see one leaf," says the professor. However, remote sensing uses, among other things, spectral analysis, which makes it possible to look through the canopy. 

"In spectral analysis, we can see how some of the light that has reached the plant disappears or absorbs, but the other part does not disappear and is reflected back," Noe explains. In other words, spectral analysis indirectly looks at the tree canopy photosynthesis. Energy, which is not reflected back, was therefore used in some biological processes. 

Breathing and clouds

According to Steffen Manfred Noe, it is crucial to investigate what is happening in the tree canopy because, in addition to photosynthesis, other vital processes take place there. During photosynthesis, a plant captures carbon, water and solar energy from the atmosphere to excrete oxygen; without the sun, it releases carbon or breathes. "The forest decomposition process works in the same way. All the leaves that fall begin to decompose. It's also breathing, just soil breathing," the professor adds.

Another lesser-known canopy process, he said, is cloud formation. For all vegetation, it is important to release water vapor into the air to get nutrients from the soil. For example, Estonia has about four to six times more leaf area than land. This means that, at least in the summer, the transpiration is greater than the evaporation," Noe says.

Chemical molecules that humans recognize as forest odor can also escape from the vegetation. They produce aerosols in the air. When they reach the right height, they condense and create a visible cloud. Clouds begin to function as a kind of sunscreen: protect the forest from excessive sunlight. That's how forest ecosystems and climate are interlinked," Noe explains.

A more precise knowledge of the leaf inclination helped to make more accurate climate predictions, for example, about photosynthesis. "Currently, there is a great political interest in how much carbon we can absorb from the air to vegetation and store somewhere for a longer period. Storing it in trees is one possibility here," he said.

Secondly, leaf inclination could be used in modelling radiative transfer. It is currently based on a simple budget logic. "We can observe how much fuel has been sold, burned, and carbon released into the air. This is set in contrast to the annual growth in forests. Comparing them reveals the carbon balance," he describes. Based on the results of the simplified system, policy decisions are made, and carbon emission allowances are determined.

However, the professor sees that Europe is slowly moving from a simplified system to an evidence-based system. With global warming, for example, it may happen that forests that grow in drier years no longer absorb carbon but instead release it into the atmosphere — an evidence-based system would consider this.

On the technical side, information on the leaf inclination is currently difficult to retrieve. However, Noe sees that with the development of digital technologies, it will become easier over time. "For example, we can now grow a virtual plant on a computer that already looks quite truthful. This allows the leaf inclination distribution to be reconstituted by tree type and season," Noe notes. 

According to him, the main thing is that the assessment system for the processes of the substance circulation in trees should be as flexible as possible. This is mainly important from a strategic perspective: How to manage in the future? How can we deal with climate change? Sometimes it requires us to look at a little bit more detail.

 

Steffen Manfred Noe and colleagues write about their work in the Agricultural and Forest Meteorology journal.

This article was published in Novaator.

This study was supported from Estonian Research Council Grants PUT1355, PRG 1405, and Mobilitas Pluss MOBERC11.

 

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