Understanding Trees

Three hundred and seventy years ago a Belgian alchemist and physician was exploring ideas that there were other gases in the atmosphere, as well as the idea that the substance which makes up plants and trees did not come from the soil, which was thought at the time.

van Helmond

Jan Baptist van Helmont (1580 – 1644)

This man, Jan Baptista van Helmont, was the first to do these experiments on a willow tree in a pot over a 5 year period. He took accurate readings of the mass of the soil, the tree and the water added, using modern experimental ideas of science introduced by the likes of Galileo, William Harvey and Francis Bacon.

Weeping Willow

Weeping Willow

After 5 years he checked the mass of the soil and found it had not changed by much, and so it could not explain the mass of the willow tree. He therefore assumed incorrectly that all the mass came from the water added during its lifespan. It was only until 160 years had passed that Nicolas-Théodore de Saussure, a Swiss chemist, repeated van Helmont’s experiments along with analysis of the gases given off by the tree, and concluded that carbon dioxide contributed significantly in explaining the mass of the tree.

Wood cross section

From carbon dioxide to carbon in wood…

Today we know that about 98 % of the mass of a tree is made up of carbon, hydrogen, oxygen, nitrogen, phosphorous and sulphur, with the first three of those elements contributing mostly to its mass (dried wood is about 50 % carbon).

What this means, is that simply by using a small amount of surface area of arable land, and by adding water, we have a mechanism of removing carbon from the atmosphere. And this resulting creation not only provides a habitat for a multitude of life ranging from the microscopic to larger mammals, it also provides us with oxygen and is most pleasant in appearance. And I have not even mentioned fruit trees here.

I challenge you to think of a similar mechanism which can remove carbon from the atmosphere with the same cost, efficiency and environmental impact.

The Natural Powerhouse

While humans have proudly developed their own power sources using mostly fossil fuels, uranium and rivers, photosynthesis in nature captures approximately 6 times as much energy consumed by modern civilisation. This capture rate is about 100 terawatts, gained from our closest star, the Sun.

The Sun

Trees love the Sun (Soho Extreme Ultraviolet Imaging Telescope, EIT Consortium)

How is this done? The leaves of trees and plants have tiny organelles containing chlorophyll which absorb mostly blue and red light, so we see them as green. An idea of this global activity can be observed in the world map showing concentration of chlorophyll in the sea (from phytoplankton), and vegetation concentration on land.

Chlorophyll Map

Chlorophyll Map of Earth. Sea shows chlorophyll concentration and land shows relative vegetation index.

This energy captured promotes, through the production of electrons, the reaction between carbon dioxide from the atmosphere and water to produce sugar and oxygen. In trees, the water comes from the roots transported via the xylem (see previous blog) and reacts with carbon dioxide which enters through tiny holes in the leaf, or stomata (see pic).

Tomato leaf Stoma

Leaf Stoma

The sugars produced are then transported via the phloem to the roots. The basic reaction is 6CO2 + 6H2O –> C6H12O6 (sugar) + 6O2, meaning for each part of carbon dioxide reacting, an equivalent amount of oxygen is produced.

In this way a mature tree can provide enough oxygen for 2 people to live per year, while in total, photosynthetic organisms convert about 100-115 petagrams (15 zero’s!) of carbon into biomass per year.

The competition for light in the forest is intense and sometimes trees are growing too close to each other, thereby undermining each other’s health. Shade tolerance is therefore a key competitive advantage. However, with human activity, competition for light is the least of a tree’s worries. Trees have evolved at different latitudes in different ways to capture this light. At the equator where the sun is overhead all year round, trees have broad canopies. While at higher latitudes trees generally have narrow and extended crowns to capture light at lower angles, e.g. conifers.

There is thus perhaps a simple way of being more conscious of trees and having them in our awareness, if on a sunny day we can occasionally be mindful when we enjoy a deep, luxurious breath of air, and think about that oxygen that sustains us.