define a new material (for example "mat_tree") and in the effect string write:

"technique t{ pass p{ zwriteenable = 1;}}"

OR - if you don't need "smooth" alphablending:

"technique t{ pass p{ alphatestenable = 1;alphablendenable = 0;}}"

and assign this material to the tree(s). Look up "material" in the manual if you need more help of how to create them.