Is your tree model centered in MED? If it's offcenter, odd things can happen such as improper collision. A centered object is defined as where the furthest edges of the model are equidistant from the given axis. If you have an X coordinate of -16 on the far left and +16 on the far right, the model is centered on the X axis. If you have a Z coordinate of 48 at the highest and -8 at the lowest, it is not centered but moving it to 28 at the highest and -28 at the lowest will. The same goes for the Y axis.

Edit: version 3.6? There is no such version. Some examples are 6.11.4, 6.20.2, and 6.40.5. Knowing the edition also helps as some editions don't support certain features (like dynamic shadows).