What process is responsible for returning water vapour into air from plants?
i think that would be 'condensation'
Rather simplified: The trees take up water (and nutrients) from the soil through their roots. Then, by a process known as transpiration (in which plants release water vapour from their leaves during photosynthesis), water vapour is released from the leaves. This is likely to create a localised climate and create the wet environment of a rain forest.