Erik,
Look in the aisle of your local grocery market in the dried spices section. You'd be amazed what you'll find there that will work for leaves. One that I like to use if Fancy Majorum. They look really good for 1/35 leaves. To change the color, try soaking them in a different colored dye, let them dry and away you go. They are normally a light green in color, with a smattering of yellow so they work well for just about any season. The only draw back to using dried spices is your diorama might smell good enough to eat!

Barring that, Silver Birch seeds are nice too, but can be expensive to buy. I have a tree in my back yard that supply's me with plenty. I'd be happy to send you a bag of them if you are interested. I get all the leaves/ground cover I can handle from this tree free for the taking. A method I use for getting the leaves on the tree is to spray hair spray all over the branch areas. Then, using a large paper bag, hold the tree in it, roots point towards the opening, and sprinkle my leaves on. You can then close the bag around the roots, and holding onto the whole thing, shake to get the leaves spread around a little more. Let it set a bit, and repeat as needed to fill things out. Once its to my liking, I will seal it with a flat overspray. Dump the remianing leave from the bag to save for another project.

Smaller branches can be added from small roots from other plants to help bulk things out too. There are a lot of ways to make trees, this is but one way I make mine. Hope this helps. "Q"