This code works (but produces the transparency issue):
private void loadGround() {
ground = Object3D.mergeAll(Loader.loadOBJ("NewLevel/Forest.obj", "NewLevel/Forest.mtl", 1f));
ground.setTransparency(100);
ground.setCollisionMode(Object3D.COLLISION_CHECK_OTHERS);
ground.rotateAxis(ground.getXAxis(), (float)Math.toRadians(180));
ground.translate(0, 30f, 0);
ground.setCollisionOptimization(true);
ground.addCollisionListener(this);
theWorld.addObject(ground);
skyBox = new SkyBox(800);
if (buffer.usesRenderer(IRenderer.RENDERER_OPENGL)) {
skyBox.compile();
ground.compileAndStrip();
}
theWorld.buildAllObjects();
ground.enableLazyTransformations();
}