You have found the answer!! You have an estimated green
weight for your tree. Answers are nice but the nice feeling only lasts a little
while. The real excitement is in the exploration and discovery. Scientists want
to jump right back into the science so they won't get bored. The scientists look
at their answer and try to find new questions to explore in them.
Can we find any question in our tree's green weight? First
what does green weight mean?
When trees are alive their cells, like yours, have water in
them. When the tree is cut down the wood dries out. Green weight is the weight
of the tree when it is alive.
Our bodies are mostly water. The average adult human body is about 50 to 65 percent water. A child’s body is
closer to 75 percent water. We have different body types. Some of us
have more muscle, others more fat. Some people look like they are mostly skin and
bones. That is why the percentage of water can vary so much. Trees also vary a
lot between species. Some trees are tall and thin like many pine trees. Other
trees develop broad crowns like oaks. After you get to know a type of tree, you
can often identify them from a distance by their shape. You can probably do the
same thing with your human friends.
The percentage of water in trees also varies. A tree can be more than one half water.
Scientists have measured the green and dry weight of many different kinds of
tree. The dry/green weight table
gives the moisture content for different kinds of trees.
To find the dry weight of your
tree, multiply its green weight by the dry/green ratio.
You can use your calculator
or our on line dry weight calculator.
Once again you have an answer. Really we have a whole table
full of answers. Can you find a relationship between the
dry weight of a tree and its percentage of water?
I made a spreadsheet with data from
the Ratio of dry weight to green weight page.
I sorted the data by percent water and made these graphs. There isn't an obvious
trend. Many questions don't give us the answers we expect. Since there is a
pattern we should ask why there isn't. If trees become completely saturated with
water they sink. The reason they float is that the heart wood is dead and the
cells have "air" in them. When these balloons fill with water the log
sinks. The percentage of water relates to the percentage of cell that have water
in them and how much water they contain. The dry weight of a tree is related to
the ratio of cell wall to empty space in the cell. In dry weight the dominant
factor seems to be the percentage of cell that are filled with water. There is a
slight trend for heavy trees i.e. trees with relatively thick cell walls to
interior ratios, to have less water. All other things being equal that makes
sense but most of the time percentage of cell that are filled with water aren't equal.
It is good for students to see that
scientific explorations don't always proceed directly to the expected results.
Even unexpected results are opportunities to gain insight.
Chemicals in trees
Trees are 50% to 53% cellulose. Cellulose is a polymer of
sugar. Many sugar molecules are stuck together in a network to form cellulose.
Animals can’t break the cellulose down into sugar again. Some bacteria can
break down cellulose. Cows and termites have these bacteria in their digestive
systems. That is why cows can eat grass or even paper.
Minerals make up 1% of trees. The minerals are the ash you have
after you burn wood. The minerals are elements like calcium and iron.
The rest of the tree is lignin and some cytoplasm
chemicals. Lignin is a group of protein chemicals that act like glue to help
hold the cell wall together.
When chemists analyze wood they find that 50% of a tree’s
dry weight is carbon. Find the weight of carbon in your
tree by multiplying the tree’s dry weight by .5.
Check results with CO2 calculator.
The tree got its carbon from CO2 in the air. We
can ask how much CO2 has the tree sequestered from the
atmosphere? Find the
weight of CO2 that your tree has sequestered.Check
results with CO2 calculator.
Cars produce CO2 by burning gasoline. For each
gallon of gasoline burned 20 pounds of CO2 is produced. When cars
burn gasoline they put CO2 into the air. When trees grow they take CO2
out to the air. Calculate how many gallons of gasoline
the carbon in your tree is equivalent to. Check
results with CO2 calculator.
People produce CO2 in a variety of ways. You can explore
several of these carbon pathways.
Now that you understand the algorithm, we can put all of
these calculations together in a computer model. Then you can explore the
relationship of human production of CO2 and trees' ability to sequester it and
the computer will do our calculations.
Find the miles per gallon for several cars, trucks and motorcycles.
The average household drives 20,895 miles per year.
Calculate the number of trees needed to sequester the CO2 produced by a family using these different vehicles.
http://www.epa.gov/globalwarming/tools/wc_assum.html
On average electricity production in the
US
creates 1.64 pounds of CO2/kWhr. Find the number of kilowatt hours
that the power plant can produce when it produces the CO2 your tree
has sequestered. Compare this to the number of kilowatt hours your family uses
in a month. How many months of electricity is your tree equivalent to. How
many kilowatt hours of electricity do the lights in your classroom consume in a
day, week, month, year? Read the wattage on one light bulb
or tube in your class. Multiply that by the number of bulbs in the class.
Multiply that by the hours per day the lights are on. Divide that by 1000 to
change from watts to kilowatts. (w X #bulbs X hours)/1000 Multiply this by days
per week the lights are on or days per month or days per year. How many kilowatt hours of electricity does your school
consume in a month, year? The students can start with the
values above and extrapolate to the school. They need to include electricity
used for computers, AC, etc. That list is inevitably incomplete, so getting a
copy of the electricity bill from the principal may be a better strategy. Still
that is an estimate because the usage varies from month to month. This presents
an opportunity to discuss evaluating different strategies. How many trees is this equivalent to?
Multiply you kWhr used by 1.64 pounds/kWhr and the kWhrs cancel.
Divide the pounds of CO2 your school's electricity produced by pounds per tree.
The pounds cancel and you have the number of trees. kWhr X
pounds/kWhr, pounds/(pounds/tree)=
trees
Natural gas produces .12 pounds of CO2/ft3. Fuel
oil produces 22.29 lbs of CO2/gal.
Is your home or school heated with
natural gas or fuel oil? If it is, how many trees would be needed to sequester
this CO2.
Same idea as electricity but substitute gallons of fuel
oil for kWhr and 22.29 pounds for 1.64 pounds.
We have calculated three different weights for your tree:
the weight above the ground, the weight of the roots, and the total weight to
the tree. Which of these should we use when we consider
the trees contribution to our air? Answers will vary. The
bottom line is we don't have enough information at this time to have a definitive
answer. That is a good thing because as scientists, we have a good question that
we can explore. Every year you tree absorbs carbon to
make its trunk larger and also to make new leaves. The carbon stays in the
trunk. The carbon in the leaves returns to the air when the leaves rot the next
year. Eventually your tree will die or get cut down. What
will happen to its carbon then? How can we evaluate a tree’s or forest’s
long term ability to counteract the CO2 we are putting into the air?
Again the answers will vary and we need to explore more
models before we can draw conclusions.
The module “Forests, Mining Carbon from the
Air” will help you explore these issues.
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