The thing that makes a tree realistic is the amount is the amount of daylight you can see through the branches/foliage too. This is nt a vey good shot of an Oak I made for my old O gauge layout, but I think you get the idea.
Its the old wireframe method with a 50/50 pva/plaster coating on the skeleton. The foliage is woodland scenics material, which is ideal. The trick is to almost pull the stuff apart, creating an impression of airy leaf cover. If you just put clumps on willy nilly that's just what it looks like. Take time to tease the stuff apart.
Just my 2p