you might be able to do it by changing the texture filter before loading the model.
ClearTextureFilters
AddTextureFilter "",1+2+8 ;color, alpha, and mipmapped
Tree1=LoadMesh( "models\Tree1.x" )
ScaleEntity Tree1,.1,.1,.1
EntityType Tree1,type_scenery
TranslateEntity Tree1, 0,0,0
after loading all models that you want to use alpha on, you should clear aout the texture filters again and set it back to 1+8.
ClearTextureFilters
AddTextureFilter "",1+2+8 ;color, alpha, and mipmapped
Tree1=LoadMesh( "models\Tree1.x" )
ScaleEntity Tree1,.1,.1,.1
EntityType Tree1,type_scenery
TranslateEntity Tree1, 0,0,0
after loading all models that you want to use alpha on, you should clear aout the texture filters again and set it back to 1+8.