It has been way too many years ago, but I think I remember from biology class that plants absorb CO2 during the day and expell it at night.
Is this right?
What is the net, and how does it play into greenhouse gasses?
There was a recent GQ thread (or maybe Cecil’s column?) on this in the last month or so, and one response said that actually trees produce no net gain or loss of CO2/oxygen usually since oxygen used to generate sugars is eventually consumed once a tree starts breaking down these molecules. The only time a forest produces oxygen is when it’s growing.
G’day
The net effect is that the plants’ tissues are made largely of organic compounds such as cellulose and lignin, and that the carbon contained in these compounds has come almost entirely from CO2 absorbed from the air. A plant absorbs more carbon than it emits while it is living and growing. But that carbon is released after the plant dies, as its tissues burn and decay.
So while a forest is growing, getting thicker, and building up humus in its soil it sequesters carbon, consuming more CO2 than it produces. When the forest reaches a steady state it achieves a dynamic equilibrium, releasing as much CO2 from catabolism, decay, and combustion as is it absorbs in anabolism. When the forest is cleared and burned, as its soils are cultivated and oxidised, and as the building, furntiure and paper products made of its timber are gradually burned or decay, the carbon is released again as CO2.
Some time ago I worked on a report by by the [Australlian] Bureau of Transport and Communications Economics in which we estimated how many trees would have to be grown each year to sequester the carbon released by annual fuel consumption of a car. The answer was surprisingly modest, but few people appreciated the point that more trees would have to be planted year after year, and that as they reached maturity they would have to be replaced by new trees, and that the new trees would count against the original year’s emissionis, not against new emissions in the year in which the replacement trees were planted. Each year’s carbon emissions have to be balanced by a permanent land use change of graxing land to plantation. And yet the cost estimates were based on a certain real price of timber, which obviously could not be sustained in the face of indefinitely continuing expansion of the plantations.
So. Reafforestation and the growth of a forest fix carbon. The steady state of a forest in which young trees grow only as old trees die stores carbon, but does not fix more. Basically, a square kilometre of forest store a certain amount of carbon, but over a long term it releases as much as it absorbs.
Does that answer your question?
Regards
This is true as a sort of net overview, but it isn’t that they stop producing CO[sub]2[/sub] during the day and start producing Oxygen - it is just that their output of CO[sub]2[/sub] is outweighed by their uptake of the same when they are exposed to light and actively photosynthesising.
The short, oversimplified version of the answer is that if it’s light out, plants turn CO2 from the air into sugar and O2. The sugar is used for two things: keeping the plant alive, which turns sugar and O2 into CO2, and making the plant bigger, which turns sugar into wood.
So if the plant is getting bigger, it absorbs more CO2 than it releases. If the plant is getting smaller (e.g. rotting away after death) then it releases more CO2 than it absorbs.
And that can more-or-less be extrapolated to forests.
Yeah, it does. But I have one more. How many trees/car/year did your study conclude?
We concluded that pine plantation that is in a steady state, with logging and replanting proceeding uniformly, stores an average of about 219 tonnes of carbon per hectare (including standing timber, decaying humus, and off-site wood and timber products. That works out (if memory serves) to having to plant about an extra nine Pinus radiata per car per year.
The study is Bureau of Transport and Communications Economics 1996 Working Paper 23: Trees and Greenhouse: costs of sequestering Australian transport emissions. ISBN 0 642 24591 6.
Regards,
Agback