Ye olde trees-vs-CO2 question.

Factual Questions
1

Those green campaign ads half-forcingly ask us to plant some trees to counter the increasing CO2 level in the air.

However, I remember that my teacher in high school told the whole class about plant’s photosynthesis: plants absorb CO2 and sunlight to “cook” their food in the day, and, in the night, they inhale oxygen and exhale CO2 as any other living things.

My question is, if plants do need oxygen, will all our efforts to plant them is fruitless? Or worse, do their night cycle actually offset their day cycle, making them as evil as we, humans, ozone-wise?

…or, maybe my teacher was a bit tipsy then?

please, quench this humble mind’s thirst of truth.

2

Some of the CO2 absorbed by a tree is re-emitted at night when the tree needs energy. However, a significant amount becomes part of the tree permanently. Every single bit of carbon in a plant was taken from the atmosphere. On balance, a growing plant will absorb more carbon dioxide than it emits.

3

if plants were not a net producer of oxygen we would be breathing something else or not breathing at all.

4

Also, ozone has nothing to do with it.

5

Plants don’t exclusively respire at night - they consume oxygen during the day too, but with photosynthesis going on they are net producers of oxygen while the sun is shining.

Lazy has it right: all the wood in a tree (well the vast, vast majority of the carbon) comes from the CO2 they use during photosynthesis.

6

As has been said, much of the dry weight of a tree is carbon it has removed from the atmosphere by the process of photosynthesis and converted to cellulose, lignin, and other compounds. As long as it continues to grow, a tree will be a net carbon sink with respect to the atmosphere.

7

A high school teacher said this?

8

Out of curiosity, about what percentage of the CO2 a tree absorbs is used for energy production versus sequestered. I would imagine it depends on the plant but I’m interested.

9

The figure you’re looking for is known as gross primary productivity, which is the total amount of energy produced in photosynthesis. Subtract the plant’s respiration, and you get net primary productivity, which is a measure of how much energy (and carbon) it stores.

Here’s some lecture notes from a biology course that explains the topic further.

So a large majority of fixed carbon remains fixed, and only a small fraction is immediately burned to meet the plant’s needs.