A field of grass might generate more mass of grass in a year than the equivalent addition of mass in a similar area of forest. It would depend a great deal on the specific species of grass and forest plants you are trying to compare. What is more important in the long run is the NETT production of oxygen. Oxygen is being produced and used up at the same time. What we need to consider is the overall change. Do we produce more than we use up ( a NETT GAIN) , or do we use up more than we produce? (A NETT LOSS) In order to see a NETT production of oxygen, we must also see a NETT production of carbon products ; noticeably wood. Wood represents the locking up of the Carbon extracted from CO2 in order to release oxygen. So forests produce lots of wood, they must also produce lots of oxygen ; which is true. Grass on the other hand produces no wood. Its carbon is turned into carbon products such as sugars, starches and cellulose. These are all good carbon products, and represent a production of oxygen, and they are all produced by the forest plants as well. The problem is in the next step what happens to the grass?
If it is left on the ground it rots, and uses up oxygen as the sugars and starches and cellulose rot and release CO2 again. By equivalence, the forest may lose all its leaves in fall. If the grass is eaten by a cow, then the cow uses oxygen to burn the grass as fuel, and produces CO2. Similarly, parts of the forest plants are eaten fruits berries leaves etc. Either way, the NETT production of oxygen in a field of grass is very small, because the carbon products are not as long lasting as wood is. This locking up of carbon is a hot topic at the moment, with terms like carbon banks and carbon sequestering and carbon trading. By locking carbon up, either in living forests or as underground reserves of CO2, we are helping to reduce CO2 in the atmosphere, and hopefully reducing the greenhouse effect which is helping to drive global warming. Industries which produce a lot of CO2 by burning coal and oil etc, can offset their emissions by investing in the planting of carbon bank forests. The effectiveness of this strategy is debated though. To offset the emissions resulting from the production and burning of 1 gallon of ethanol (biofuel) you would have to grow approximately 10 pounds of timber (not including leaves etc.) To make the offset effective, you have to grow 10 pounds of WOOD for EVERY gallon of ethanol. That is a 5000 lb tree for every car every year. If you keep using petrol or gasoline, the tree has to be even bigger! original post by: Nigel Skelton- Tennant Creek AUSTRALIA =====================================