Not sure how much this helps... here's a group of stuff I have. I haven't gone through it all and cleaned it for usability or anything...
I don't really have materials for landscaping objects... I tend to just make a material with a basic color and usually for bark just put a basic bump map (noise) on it for roughness, and leave the leaves to green with no texture/bump maps.
This first zip has most of the trees I use... I think I got 'em from Mav.
I used to have some others I'd accumulated but... now I can't seem to find them... if I come across them I'll put 'em back in here.