I think that doing trees with "leaves on" is one of the things that gives most nightmares to some diorama builders as it is very difficult to make them look properly. Don't worry much about it as you'll find better ways in the next future if you keep on experimenting. Also have a look on google and try to find a favourite method. This one you have tried here comes from a smaller scale (trains) I think and it works at that scale, but it looks a little bit messy at 28/32mm scale. To finish the look of these trees with that method you need to hairspray the "candy-floss" and then immediately sprinkle lots of green flock (or whatever colour you prefer) on top of it. Better try this on a fresh tree than on this diorama or you'll mess up this one a lot!!