Here’s a video on modeling cypress trees / swamp trees.
trees
Making Stands For 10mm Trees
Tim’s Wargaming Blog has an illustrated tutorial on making stands for 10mm trees.
Modeling Forest For Lord of the Rings
The video that follows is full of inspiration and instruction for making forest terrain In this instance, the idea is to create “Lord of the Rings” style terrain.
Making Trees From Twist Ties
Creative Twilight has a nice tutorial on making trees from twist ties and joint compound.