Just curious, jg: The colors of the dead leaves in your photos are distinct. So are the green leaves. What happens if you simply find ‘segments’ that contain primarily the brownish-gold chroma range that’s characteristic of the dead leaves. Then do the same for bright green leaves. Then compare area ratios.
If you can find the brown segments, etc, there are a lot of other tools at your disposal as well. OpenCV can easily get contours of those segments, so you can discard small ones as noise. Etc.