Where do trees get their mass from?

Digging up the dirt on how trees grow.

Have you ever wondered where trees get their mass from? One of the more common answers, as seen in the video published in 2012 is that the mass (increasingly bigger size) of a tree comes from the soil. Which makes sense, right? After all, we are taught that plants need soil (enhanced “dirt”) to grow. According to Michigan State University Extension, problems typically arise when asked to explain why there isn’t a big hole around a tree. If the tree is using soil, then there must be less soil around it. But studies show virtually no difference in the amount of soil in a pot when a seed is planted from the amount of soil in the same pot when the plant from that seed is harvested. So where does the mass come from?

The mass of a tree is primarily carbon. The carbon comes from carbon dioxide used during photosynthesis. During photosynthesis, plants convert the sun’s energy into chemical energy which is captured within the bonds of carbon molecules built from atmospheric carbon dioxide and water. Yes, the carbon from carbon dioxide in the air we breathe out ends up in “food” molecules (called glucose) each of which contains 6 carbon atoms (and 12 hydrogen atoms and 6 oxygen atoms).

However, there is a negative side as well. Plants use the energy in some of the carbon molecules they make for the activities to keep themselves alive and to reproduce. This process is called cellular respiration, which all living things do. But there are still carbon molecules (glucose) left over. These left-over glucose molecules are used to form the complex structures of plants, such as leaves, stems, branches and roots as well as fruits, seeds, nuts or vegetables. Each year trees use the left-over carbon molecules to add to themselves, making themselves bigger in mass (size).

Voila! Most of the mass of trees is carbon. The processes involved are all pretty complicated and we can thank several Nobel Laureates for working out the details.

It is also important to note, the soil acts as an anchor for the plant through its roots as well as providing the plant with water and small amounts of nutrients that plants need, but the soil itself is not used.

To learn more about the ways 4-H youth can explore more about their environment, visit the science and technology page.

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