No plugin, but I recently discovered this standalone app (for Windows):
http://mongoose.anim8or.org/treemaker/index.html
It exports trees in 3DS and OBJ format. The LWO exporter seems broken (exports crash both AC3D and LightWave on import for me)?
Here's another link to a discussion regarding another one --- not as friendly/intuitive, but should work on all platforms:
http://www.ac3d.org/forum/showthread.php?t=3120
Dennis