It does a bit of both, alot depending on the density of the tree. Still however, it does improve in situations where either aborption or diffusion is needed. Plants help with slap echo as well. Still it depends.
For instance I find dipoles and planar types don't do as well with plants behind them, nor any sound treatment, save some diffusion. Yet it's most often the opposit with dirrect radiating traditional speaker designs.
It seems my experience with plants has also been that they work at least better sometimes in smaller rooms, where absorption only would be too much, and diffusion better. Here plants often work cheeply if need be.
Like anything, you need to experiment to find out in your room.
For instance I find dipoles and planar types don't do as well with plants behind them, nor any sound treatment, save some diffusion. Yet it's most often the opposit with dirrect radiating traditional speaker designs.
It seems my experience with plants has also been that they work at least better sometimes in smaller rooms, where absorption only would be too much, and diffusion better. Here plants often work cheeply if need be.
Like anything, you need to experiment to find out in your room.