Your model seems to work fine for me. Here is the script I used to test it out:
Code:
import viz
viz.go()
viz.add('tut_ground.wrl')
model = viz.addChild('testtree.lwo')
model.drawOrder(-1) #Ensure model is rendered to depth buffer before other models
model.disable(viz.COLOR_WRITE)
import vizcam
cam = vizcam.PivotNavigate()
cam.centerNode(model)
cam.setDistance(model.getBoundingSphere().radius*5)
cam.rotateUp(10)