Note that carbon-dioxide is only a trace gas and so photosynthesis is not going to have any impact on a gas that makes up 21% of the atmosphere. However, the increased carbon-dioxide is aiding plant growth around the world, which may in turn reduce the amount of carbon-dioxide in the air. The reduced carbon-dioxide will slow plant growth. It's a rather negative feedback loop that will tend to stability.
RE: Why do we plant trees when most of the oxygen comes from the ocean