Ahhh... maybe I can help, mdbruffy... : )

It might be a bit pricey ($29.00) considering you only want one tree; but Xfrog's Weeping Willow at Turbosquid includes three versions of a 3D Willow tree, in the following formats:

3D Studio ( .3ds ) (can import to Poser)
Bryce 4 (.br4, .obp)
Lightwave ( .lwo, .lw, .lws ) (can import to Poser)
Maya 5 ( .ma, .mb )
OBJ ( .obj ) (can import to Poser)
Cinema 4D ( .c4d )
3ds Max ( .max )
Vue 4 vob (.vue)

Just a note that the full DVDs of these trees (Xfrog's Basic package) contain 3 versions of 20 species, for a total of 60 3D models for $149.00. So if you use vegetation a lot, and can afford it, you'll only pay $2.48 for one model with the full package.

Hope this is of some use to you!!  : )