Natural Science Forum / Biology / Biology / November 2004

You can work it out easier in terms of the energy you burn. The oxygen
produced by producing X kcal of energy in glucose is exactly the same
as the amount of oxygen consumed in metabolizing those X kcal of
sugars.

Your metabolic rate is about 2000 kcal/day or over  700,000 kcal/year.
The question is: how much "tree" (or other plant matter) is required
to produce 730,000 kcal/year in new growth through photosynthesis?
That is exactly the definition of primary productivity.

A tropical rain forest or wetland might have a primary productivity of
9000 kcal/yr per square meter. (see
http://www.physicalgeography.net/fundamentals/9l.html
for example).  A temperate forest yields only 6000 kcal/yr per square
meter. In other words, it takes over 100 square meters of temperate
deciduous forest to generate enough energy through photosynthesis to
supply your metabolism.

How many trees are there in a typical temperate deciduous forest in a
10 meter by 10 meter square?  That is your answer.

Grassland has a lower primary productivity so it takes more area to
support you.  Note:  if you feed the plant growth to an animal and
then eat the animal (if you get a lot of your calories from meat) it
takes perhaps ten times the amount of area in trees to support you
since 90% of the energy consumed by your food animal is lost in its
metabolism.

You may be surprised to learn just how much plant matter it takes to
support animal life.  That is why a balanced aquarium needs an awful
lot of plants for just a few small fish.  That is also why when you
look at forests or fields you see an awful lot of plant matter but
very few or very tiny animals.  Aquatic ecosystems are supported by an
enormous amount of algae growing in the surface waters that we
ordinarily don't even see.

I am assuming, of course, that this is a legitimate question of your
own and not a homework assignment!

This is overly simplistic.  Most photsynthetically generated oxygen derives not
from trees but from aquatic vegetation.  So your "opposite" if there wewre one
is a three dimensional volume of algae populated water.

There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy