If you want to add accurate collision properties to your trees, here's some tips to do just that... quickly, easily, and perfectly centered every time.
1) Set your tree on the terrain, open up it's properties box and set it to no collide -facing- as Phin noted above.
example
2) Create an object box (right click and select 'insert') open up it's properties box and resize it to 2x2x20 for the average tree. Real big tree's require 3x3x20 and little guys are 1x1x20
example
3) Left click on the object box ('box' will appear) and copy it (Ctrl+C or right click and select 'Copy') Now left click on your tree bin and drag it to where you want it to be (note; you
must move the tree bin some so that traxx centers the bin to your cursor point) now just hold down 'Ctrl' and hit the V key to paste the object box perfectly centered to the trees trunk.
example ...note, I raised the height value to 50 for that example for a visual.
=============
Bonus Tip... after you set your first tree down and have it's properties set to NCF, copy and paste that one around your track, then come back and paste the object box in them all or just the one's nearest the track.