Pretty basic compared to Xfrog, TPF and SpeedTree but FREE!
http://3darcstudio.com/en/downloads-files/sketchup-plugins/206-3d-tree-maker-sketchup-plugin (http://3darcstudio.com/en/downloads-files/sketchup-plugins/206-3d-tree-maker-sketchup-plugin)
Pretty sure export will require Sketchup's Pro Version (£384.00 ouch)