old thread but top in google. I post the solution that worked for me, since I had the same prob today:
i_node_key = cl_alv_tree_base=>c_virtual_root_node.
TRY.
CALL METHOD i_prod_tree->get_subtree
EXPORTING
i_node_key = i_node_key
IMPORTING
et_subtree_nodes = et_subtree_nodes.
CATCH cx_root.
EXIT.
ENDTRY.
Top node seems to be
cl_alv_tree_base=>c_virtual_root_node.. From there you can get the subtree.