Dr. Doug Pollard at this Centre has provided a response to the questions raised by Christopher Dunn that should be of interest to others as well. He can be reached directly on the network at: dpollard@A1.PFC.Forestry.CA if you have any follow up questions for him. __________________________________________________________________________ |Jim Arnott |TEL: +1 (604) 363-0701 | |Research Scientist |FAX: +1 (604) 363-0775 | |Pacific Forestry Centre |INTERNET: jarnott@A1.PFC.Forestry.CA| |506 W. Burnside Rd., | | |Victoria, B.C. V8Z 1M5 | | |Canada | | |____________________________________|____________________________________| ****************************************************************************** The answer is as simple or as complicated as you wish to make it. Lets try a simple approach: Since the basic photosynthetic reaction, the starting point for practically all organic matter, releases a molecule of oxygen for every atom of carbon absorbed, all you have to do is consider what the net carbon change is in the system under consideration. A tree that has attained a dry weight of 240 kilograms contains approximately 120 kg of carbon, and has thus generated 320 kg of oxygen (your basic chemistry). What happened en route (respiration, litter fall, predation etc) is irrelevant. The question of how much oxygen is released must be framed in terms of the current state of the system (in this case a living tree) and how long it took to reach it from an earlier state (maybe 25 years from a seed). The issue becomes complicated because at sometime the tree ceases to be. If burned to ash, the net oxygen released over its lifetime (say 320 kg) is now converted to carbon dioxide. Thus all the carbon dioxide that was absorbed from the atmosphere is now back there! A tree that dies, falls, and rots in place does the same thing but over a long period, with a half life for stored carbon of perhaps 25 years. One can only get meaningful figures by considering the matter on a large landscape level over a long period of time. This was done for carbon in the conversion of old growth to second growth forests in the Pacific Northwest (Harmon et al Science 247 699-702, 1990): an estimated 1.5-1.8 Gt of carbon was released (using 4.0-4.8 Gt oxygen in the process). All this is very nice, but the fact is that oxygen concentration has not budged from its historic level because it is so abundant in the atmosphere, which is heavily buffered by dissolved oxygen in the oceans. Conclusion: oxygen released by plants is not an issue of concern to today's society. Many other things are, including atmospheric carbon dioxide.
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