Oxygen Levels in a Forest Garden

I've read that air above the ocean or in a forest contains significantly more oxygen than the average 20%. This is part of the reason we feel refreshed after visiting the beach for instance. Has anyone measured this in a mature Forest Garden?

Well, the forests make that oxygen from CO2, but the CO2 levels in the atmosphere are about 500 times lower than the oxygen levels. To significantly increase the O2 levels the forests would need some mechanism that brings in new CO2 without also causing the created oxygen to disperse.

Thanks John. The mechanism might be the wind break. I would expect the oxygen levels to be different in an open field, a regular forest, and a forest garden. It may not be significant though.