Date: Fri, 8 Jul 1994 11:56:00 PDT
Subject: Re: Oxygen and trees
Sender: "dprice@nofc.forestry.ca" <dprice@nofc.forestry.ca>
To: CLEADEM <CLEADEM@galaxy.gov.bc.ca>
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Posting-date: Fri, 8 Jul 1994 00:00:00 PDT
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Hi Carole
You may also want to circulate my response to Dunn,
and his reply to me!....
Regards
Dave
====================================================
Hi Christopher
My source of information is the Intergovernmental
Panel on Climate Change, Scientific Assessment, 1990,
but the carbon storage data must be available from
many sources. (Scientific American has published
several articles that would provide information on
this question over the last few years). Fig 1.1
of the IPCC Scientific Assessment indicates that land
biota take up about 100 Gt (i.e. billion tonnes) of
carbon annually, and release about 102 Gt (including
soils). I.e. there is a net uptake of about 2 Gt/yr,
of which most would appear to occur mainly in the
northern hemisphere (boreal forest and tundra
ecosystems). This would imply a net release of oxygen
of about 32/12 * 2 = 5 Gt per year.
Now all we have to do is divide by the number of trees
on the planet! Your guess is as good as mine. It has
to be at least 5 billion - probably 500 billion, for
0.01 t/yr.
A better way to approach this is to look in the
physiology literature. We would say a typical
forest might manage to take up 0.5 mg C02/m2/s under
optimal conditions. If a typical tree occupies 10m2,
then that would be 5mg per second, or 432 g per day.
This would give about 150 kg per year. Now we have
to allow for respiration losses, tree age and the
fact that optimal conditions don't normally occur.
Maybe only 10% of this number - which is about 15 kg
per year. Multiply by 32/44 (ratio of O2/C02 molecular
weights) to get about 12 kg or 0.012 t O2 per year.
On balance, my guess is: of the order of 0.01 t O2
per year for the "average" tree.
I too was interested in this question, and after a
discussion some time ago with a colleague, confirmed
his calculations with some of my own. I am afraid
you might be disappointed (so might the press!)
Figure 1.1. shows that total global storage of carbon
in biomass and soils in organic form (i.e., not including
coal, oil and gas reserves, nor limestones, nor carbonates
in sea water or deposited on the sea bed) is estimated as
approximately 2000 Gt. In comparison, the atmospheric
carbon content (i.e. mainly CO2), is currently (1990)
about 750 Gt. This means that if all the available
surficial organic material were completely oxidised to CO2,
the atmospheric carbon content would increase by about a
factor of 4, from its current content of around 350 ppm to
say, 1400 ppm.
Since the oxidation of one molecule of carbon to CO2 ties
up precisely one molecule of oxygen, the increase in CO2
would be matched by the decrease in O2 content. The
increase in CO2 from 350 to 1400 ppm is an increase from
0.035 to 0.140 % CO2. Since the oxygen content is about
20.5 % (or so), it would decrease by an equivalent amount -
say to 20.4 %.
In other words, trees, and vegetation in general, really
cannot have any significant impact on the atmospheric
oxygen content. Even if every last scrap of biota and
soil carbon were oxidised, (excluding you and I, so to
speak), the air would still be perfectly breathable. Of
course, the climate might be unbearably hot, and we
might not have anything to eat, but these are other issues!
Don't get me wrong. The global vegetation is important
to the global climate and our well-being for many reasons,
which include, maintaining the atmospheric CO2 concentration
(to some extent), but I don't believe keeping the O2
concentration up is one of them.
Dave Price
Ecosystems Modelling Group
Canadian Forest Service, NOFC
EDMONTON, Alberta
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