Forests,

Mining Carbon from the Air

Trees grow by taking CO₂ from the air and water and minerals from the soil. They use sunlight for energy to change these raw materials into wood and leaves. The tree’s growth depends on the type of tree, rainfall, soil conditions, latitude, elevation, competition from other trees etc. Trees also grow slowly. Some trees grow for centuries or even thousands of years. To find how fast trees sequester CO_2, we need to look at different combinations of these factors. We would need hundreds of test plots. Since trees grow slowly we would need to wait years to see the results.

 Fortunately many scientists have been experimenting with trees. One scientist looked at the effect of rainfall, a second scientist looked at soil conditions, etc. Each scientist makes an equation to predict the effect of his/her factor. If we put these equations together we can predict how different kinds of forests grow. There are many equations. Doing all of the math would take too long and be too confusing. If we put the equations together in a computer program, the computer will do the math for us. We call this kind of program a model.  Doing science using these models is called Computational Science.

We will us a forest model made by Wageningen University and Research Center. The model simulates a hectare plot. A hectare is about the size of 2 football fields. After all of the variables are put in the model we run it. The model reports on the carbon in the forest for the next 300 years. There are 2 sets of data. One set gives the carbon stored in different parts of the forest. One part of the forest is the roots of the trees. These are called the stocks. The second data set gives the carbon that is moving into and out of each part. These are called flows.

 The questions we want to investigate are:

1. Where is the carbon stored in the forest?

2. How do these carbon reservoirs change over time?

3. What can we do to increase the carbon stored in the reservoirs?

Ask your teacher if you will be using CO2fix, This is the model made by Wageningen University and Research Center, or the Excel version.

In CO2fix open the model named WUR-ex1. When WUR-ex1 opens you see the carbon stock table. This is the run for the forest plot. Each row is one year. How many years does this run model the forest? 300 years What parts of the tree does the model monitor? Stems (trunk), foliage, branches, roots. What is the total dry mass of the trees when the simulation starts? .64 Mg/ha Mg is a metric ton or a million grams What is the total mass of the trees at the end of the simulation? 433.52 Mg/ha

Copy the Biomass dry weight column into an excel spread. Graph the column to see how the mass changes over time.

What percentage of the Biomass is carbon? 50%

 Find the column that shows the total carbon in the soil. Copy this column to your spreadsheet and graph it.

Which year has the most carbon in the soil? Year 0 or at the beginning of the experiment. Explain why the soils carbon dips then increases then decreases again. The carbon is constantly decomposing and changing back into CO₂. In the first years the trees are small and produce very few needles and debris. The carbon in the soil is disappearing faster than it is being replaced. As the trees mature they produce more debris and the amount in the soil increases. When the trees reach maturity their production of needles and debris levels off and starts to decline slightly with increasing age. The soil C  level mirrors this gradual decline.

Explore the data on your own to answer these questions.

Which year has the largest amount of carbon in the forest? 121 How much carbon is in the forest this year? 390.29 MgC/ha

 Which year is the most carbon being sequestered? Look at the flows chart. Copy the atmosphere increments and losses columns onto a spreadsheet. In a third column calculate the difference by subtracting the losses from the increments. In year 20 the atmosphere lost 6.91 MgC/ha. The forest stored this 6.91 Mg. It is important that the students differentiate between the total carbon stored during a year and the change in carbon during that year.

Which year does the forest start producing more CO₂ than it is sequestering? year 122

After the year when the largest amount of carbon is in the forest, what is the relationship between the forest and the atmosphere? The atmosphere starts putting more C0₂ into the atmosphere than it is removing. Is the forest removing or adding CO₂ to the atmosphere. Adding The trees are getting bigger and older why doesn’t the forest contain more CO2? Tree growth slows but doesn't stop. There is more damage to the trees increasing the litter on the soil thus increasing the C0₂ the soil produces. Some trees die and slowly rot to produce more CO₂.

Which age range of forest would we need to help balance the CO2 that we are adding to the atmosphere? Young forests. With this kind of tree, Norway Spruce, 6 < forest > 121 can sequester carbon. Between 12 and 50 years old is the best.

If you want the forests to help deal with our CO2 responsibilities what recommendations would you make for our
national forest policy?  We should consider harvesting some of our forests to keep them young. We haven't explored the effect of harvesting forests on the carbon balance so we can't draw conclusions yet but we should look into it.

What are the product values in the model? (all 0) This forest isn’t harvested. It is managed like a wilderness area. Lumber is removed from most of our forests. This model allows us to simulate harvesting of products from the forest.

The authors of the model suggest you practice thinning your forest by following these directions.

Open ‘WUR-ex2.co2’  you will start to manage the forest and produce wood products

 Go to the biomass parameterisation module, and choose the ‘thinning harvest’ tag

Parameterise two thinnings (year 30 and 60) and one final felling at age 100. Fraction removed is resp 0.25, 0.25, and 1.

Allocate of first thinning, .1 to logwood, and .7 to pulp-pap

Allocate of second thinning, .3 to logwood, and .6 to pulp-pap

Allocate of final felling, .7 to logwood, and .2 to pulp-pap. Leave the rest of the cells as they are, i.e. all branches are allocated to slash. 

Now choose ‘Apply’ and ‘OK’

 Go to ‘products parameterisation module’. This has now been parameterised for you. Try to get an idea what has been inserted for products.

  What fraction of products in long term use will be recycled at the end of their life?

What fraction of recycled products in long term use, will go to products with a medium term use?

 Now choose ‘Apply’ and ‘OK’

Under the icon ‘View options’ choose the ‘products’. Analyse what you see.

 Go back to the ‘view carbon stocks table’, and copy the column with the total products carbon stock into an excel sheet.

Graph this column. Explain the curve i.e. why does it spike up and then curve back down again over and over again? When the forest is harvested the carbon is removed to make paper or lumber. Over time the paper etc, is used and discarded to return to C0₂ in the air. Some of the product is recycled. Over time less is available to be recycled and also to change back to C0₂ Is there an overall trend? Some of the product remains from the previous harvest so the subsequent product levels are larger than the initial ones.

 On the stocks table copy the column with total atmosphere C. Graph the atmospheric and products C together. What patterns do you see. Both are variable, so how can you find a pattern in or trend that they share? The products increase at the same time the atmospheric C0₂ increases and they decrease together. Stated another way, as the products increase the sequestered C0₂ decreases. Sequestered C0₂ and products are approximate mirror images of each other. The atmospheric levels rebound from the rush of a harvest more gradually.

Both have minimum values at 100 year periods. If you graph these minimum values you will see a relationship.

What should we conclude about the forests ability to permanently sequester our carbon? It may be able to hold the carbon but the rate on additional sequestration declines with time.

If you are using the downloaded model you can ask your students to change the run from 300 years to 500 or 600 years to see if the trend continues.

Suggest ways to change our forest management strategy or product recycling policy. For example, should we harvest on a different schedule, harvest part of the top of the trees into pulp, have a recycling campaign. You need to justify the practicality of your policy changes. 

Many different options are available to the students. One of the most effective changes is to change the final product values. Click on the chair. Change the Energy values to .1 instead of .4, .7, .6. That changes the final disposition of part of the products from burning for energy to landfill.