Dont use it for trees, as then u cannot walk through the leaves. As all brushes will be clipbrushes. (If i am not totally wrong)
Edit your et_entitys.def file found in the /script dir.
Replace the misc_model stuff with the code below. What u need to replace is the stuff between the /* and */
Then u will have a checkbox for spawnflags 2 and some more instructions on how to rotate models.
/*QUAKED misc_model (1 0 0) (-16 -16 -16) (16 16 16) - SOLID
-------- KEYS --------
"angle" sets the angle to the degree (0-360)
"angles" set model rotation on all axis (0 90 360)
"model" arbitrary .md3 file to display
"modelscale" scale multiplier (defaults to 1x)
"modelscale_vec" scale multiplier (defaults to 1 1 1, scales each axis as requested)
"modelscale_vec" Set scale per-axis. Overrides "modelscale", so if you have both, the "modelscale" is ignored
-------- SPAWNFLAGS --------
Spawnflags 2 will make it solid
-------- NOTES --------
Model that will be compiled into the map.
*/