WarFactory has a tutorial on making African Acacia Trees.
Modeling
Basing Figures With Baking Soda
A mix of baking soda and super glue can be used to make finely textured bases.
Using Sculptamold To Make Wargames Terrain
I have used this stuff for years. It’s amazing.
Making Pine Trees
The All Hell Let Loose Blog has an article on making 6mm pine trees.
Modeling Trees
Here’s some advice on modeling trees from air conditioning filters.