You can press [i] to activate spray mode when you have the tree selected. Left mouse will then spray LOTS of trees down. Currently writing a small addition to this feature which establishes a MAX density so don't simply overlap trees within a small confined space and create a slow slow level in the process.
PC SPECS: Windows 7 Ultimate 64-bit, Intel Core i7 920 (PASSMARK:5008), NVIDIA Geforce 9600 GT GPU (PASSMARK:752) , 6GB RAM