A study of birch and fir trees in Canadian forests determined that carbon flow between these species moves one way in spring and the other way in summer.

In spring the leafless birch receives carbon from the fir trees. In summer, when the birch has fully leafed out and the canopy shades the fir trees (reducing photosynthesis) the flow of carbon switches back to deliver sugars to the firs. After the birch loses it's leaves in the autumn the flow switches again and moves carbon from fir to birch.

It's a continual flow downhill from abundance to scarcity.