Re: New tree vs mulched mature tree. Net environmental benefit?

Stuart,

Where to begin? Yes, there probably is some size where a tree optimally produces oxygen, but you're concerned about the wrong thing. The Earth has plenty of oxygen, but the concentration of carbon dioxide is increasing and that increase is closely correlated to rising temperatures globally.

One of the reasons that big trees produce proportionately less oxygen than smaller trees is that most of the biomass of large trees is wood. Trees are gigantic reservoirs of stored carbon; the bigger and older the tree, the more carbon is sequestered. One big worry is that respiration rates increase faster with temperature than photosynthetic rates, so that as climates warm, trees will begin releasing more carbon than they store, and this leads to increased mortality.

Just as with logging, clearing forests for agriculture, and forest fires, this leads to a lot of carbon being released from these biological reservoirs in a short period of time, and more carbon dioxide leads to higher temperatures, and more tree death; you can see the outcome of this path.

So rather than being concerned about maximizing oxygen production, you should simply plant more trees and reforest cut areas, to sequester more carbon. To add to your concern, high latitude areas have a lot of carbon sequestered in peat and boreal forest organic matter, which decomposes very slowly because it's frozen much of the year. But as it warms, more of this carbon will be released, and you have much the same problem as with tropical forests.