I think spruce branches and conifers in general must work better than broadleaf for brash mats and tracks. The fc article says as much, the problem I've found and I'm felling birch, alder, hazel and chestnut is that the branches tend to wander off into 3 planes instead of lying flat(er) in 2 as with most conifer brash. Think how big a pile of broadleaf brash gets if you just dump it on top of each other, not exactly easy to drive over! I'm experimenting by chopping my piles of brash with the chainsaw so they are in small pieces (and about 10% of the volume!). I've got a nice mixed aggregate of chopped up brash which I'm hoping will make a fine track, once I've spread it out.
Rich