I got some answers on this.
It could be that you hit an internal memory limit in ArcScene.
Can you check only very few ( e.g. 5 ) trees and check if ArcScene displays them validly ? If they're not displayed nicely, can you open them in ArcGlobe ?
If also ArcGlobe does not show the trees nicely, it may be an exporter bug.
Either way, let me know.