Detecting Trail in Forest Images
It may not be sufficient by itself, but since one of the problems has to do with lighting variations, a shadow removal pre-processing step may help.  The technique I'm thinking of is described in the following paper:


  "On the Removal of Shadows From Images", G. D. Finlayson, S. D.
  Hordley, C. Lu, and M. S. Drew, IEEE Pattern Analysis and Machine
  Intelligence (PAMI), Vol.28, No.1, Jan, 2006, pp.59-68.
  http://www.cs.sfu.ca/~mark/ftp/Pami06/pami06.pdf


The first part of the process produces an illumination invariant grayscale image, which is probably what you want in this case.  Here's a shot of the example they give in the paper:


(source: datageist.com)  

In fact, taking the process one step further to produce a "chromaticity" image may be what you need to cleanly distinguish between the live and dead leaves.  Once again, an example from the paper:


(source: datageist.com)  

The catch, however, is that the camera needs to be calibrated first.  Assuming that's possible, some combination of the representations they describe will probably make the other methods you're using more effective.