Trees and CO₂

Trees and Carbon Cycle

Trees are considered a powerful means in mitigating the climate warming by consuming CO₂ from the atmosphere. I try to make this idea more precise.

Trees (as all green plants) get their living energy from the photosynthesis. The basic reaction is:

6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂
[Carbon dioxide] + [Water] + [light energy] → [glucose] + [oxygen]

Glucose is a simple sugar. Trees use a part of the glucose to get the energy for their living. This process is called respiration and is carried out according to the following oxydation reaction:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + Energy

Let me note that when the wood is burnt about 1% remains as ash which contains about 50% calcium oxide (CaO), 15% potassium oxide K₂ (german "Kaliumoxid") and about 20% other oxids. The rest is calcium carbonate (CaCO3), other carbonates, minerals and metals. Ash is basic (alkaline) with pH ~ 12.

We notice that the respiration is the reverse of photosynthesis, so the consumed CO₂ is released back to the atmosphere. Fortunately, not all sugar is used for the respiration. A part of the glucose is not used or is transformed into cellulose and lignin which build the wood of the tree. This remaining biomass of the stem, leaves and roots keep carbon in form of stable organic carbon molecules. It is estimated that

50% of dry wood = carbon.

The carbon (C) stored in the tree is obviously called "tree carbon storage". As the tree grows and gains mass more and more carbon is stored in the tree. About 70% of the carbon is stored in aboveground wood and 30% is stored in the roots. The carbon added to the tree per year (or other time unit) is refered as "carbon sequestration". For example, measurements show the following approximate values for an oak:

C-Sequestration	29 kg/year
Lifespan	100 years
C-Storage (100-old oak)	2900 kg

When the tree dies the wood decays by oxydation processes and all the stored carbon is relaesed to the atmosphere. So finallly, no carbon is permanently removed. This is the well-known carbon cycle.

When the tree is harvested and the wood is fired for energy the carbon is also returned to the atmosphere, but we gain thermal energy. So, burning wood for energy is considered CO₂-neutral process. The problem is that heating with wood is rather inefficient and needs special infrastructure for preparation, transport and burning.

Harvesting wood for timber is enviromenal-friendly because the timber keeps the carbon. But even timber will once be destroyed and return the stored carbon to the environment.

Links

• Trees and Carbon Dioxide (by R. P. Bouchard, 2018)
• Carbon cycle (University of Helsinki)
• Burning Trees for Energy Harms the Climate (C. Hanson and J. Ranganathan, 2017)
• CO2 and Trees (Botanical Society of America)
• Carbon cycle in trees
• Carbon Cycle (NASA)
• Carbon cycle
• Carbon and Other Cycles (Philippe Ciais et.al., IPCC - United Nations, 2018)
• Carbon Cycle (G. A. McKinley, Columbia University)
• Carbon Cycle
• Carbon Cycle
• Forests as Carbon Sinks (2017)

Comments

New Comment