I've used bits of pine cone for scenic terrain, and was very disappointed by the results. I just couldn't get it to stay put... it didn't help that I did not prime/paint it, but left it natural- so it looked "too" real compared with everything else!
I also have a model with a large real rock on its base. It was OK at first... but now that the rock is painted to look like a rock (funny about that...) it actually blends in and looks a LOT better!
Just figured I'd share in case you decide to go the "natural" route... it's not as easy as it looks! I think it's actually easier to use the "fake" stuff... at least if you want it to blend in with everything else

. Especially if you're using twigs, I would think you would need to prime/paint it to get it to fit in well. I'm not sure if something like that will take the paint well, though...