[shkkmo]

> over geological timeframes, the plant life they contain is the reason we have any oxygen to breath

About 1/2 of the oxygen produced is produced by Phytoplankton, which are not plants (though they are similar).

> And if the amazon currently amounts to about 20% of the plant life on earth now

The amazon accounts 16% of the photosynthesis on land.

> its not wrong to teach that it is doing 20% of the oxygen maintaining.

It is wrong. If you want to provide a nice round, reasonably accurate number you should use 10%.

> does not make a lie of the understanding and respect that forests are lungs of the planet.

Forests are important and and a significant part of the respiration of them planet, but they are less important than our oceans. Our oceans are the true lungs of the planet (edit: and they are not in great shape, atmospheric oxygen may not be an issue, but oxygenation of the ocean is more variable).