Hi,

 I have seen a new plug-in, which is a beta for now, that creates trees and plants. It seems to have a lot of control and it is integrated in XSI 5.1 but will be in 6 soon. There is a free to download version to test out.

http://www.simartom.com/

otherwise there is the hair instancing but it is a long process and you have to paint a lot of weightmap create the geo of the trees and so on. This seems to be one good plugin...

BloomPix Studios
Sergio Gardella
www.bloompixstudios.com
3dsergio@gmail.com
Barcelona, SPAIN