Assignment levels of oxygen emission are not precise and different methods can give different results. That said, it is well documented that oxygen release is proportional to the overall leaf mass, also known technically as Leaf Area Index.
There is general agreement that:
- Pines are at the bottom of the list in terms of oxygen release because they have a low Leaf Area Index.
- Oak and aspen are intermediate in terms of oxygen release.
- Douglas-fir, spruce, true fir, beech, and maple are toward the top of the list for oxygen release.