I would rather suggest not to use Alpha for trees. IMHO you should use DXT3 textures in masked mode. There are plugins to export DDS/DXT for Photoshop and Gimp, even some free standalone converters.
Using the alpha sorting code Stevie mentioned means also speed loss, and you have to include every alphaed mesh in the sorting update function.
Using the alpha sorting code Stevie mentioned means also speed loss, and you have to include every alphaed mesh in the sorting update function.