Using JXB to serialize a TreeModel to XML

I want to serialise my DefaultTreeModel to XML via JXB and have written a simple application which works when the object being marshalled is a LinkedList but when I convert it to a DefaultTreeModel, all I get in the output is <treeModel/>. Have used the debugger to make sure the tree is populated just before the XML is written. Does anyone have a clue why this doesn't work?

@XmlRootElement
public class Zoo {
private DefaultTreeModel treeModel;

public Zoo() {
    DefaultMutableTreeNode node = new DefaultMutableTreeNode("test node");
    this.treeModel = new DefaultTreeModel(node);
}

public void add( Animal a ) {
        DefaultMutableTreeNode node = new DefaultMutableTreeNode(a);
        this.treeModel.insertNodeInto(node, (DefaultMutableTreeNode)    this.treeModel.getRoot(), ((DefaultMutableTreeNode) this.treeModel.getRoot()).getChildCount());
}

@XmlElement
public DefaultTreeModel getTreeModel() {
    return this.treeModel;
}

public void setTreeModel(DefaultTreeModel treeModel) {
    this.treeModel = treeModel;
}
}

public class JXBTest {

/**
 * @param args the command line arguments
 */
public static void main(String[] args) {
    Zoo z = new Zoo();
    Cat c = new Cat();
    c.setName("Pussy");
    c.setColour("Blue");
    z.add( c );
    c = new Cat();
    c.setName("Meow");
    c.setColour("Green");
    z.add( c );
    Dog d = new Dog();
    d.setPaws(4);
    z.add(d);

    File f = new File("/tmp/animals.xml");
    try {
            JAXBContext context = JAXBContext.newInstance( Zoo.class );
            Marshaller m = context.createMarshaller();
            m.setProperty( Marshaller.JAXB_FORMATTED_OUTPUT, true);

            m.marshal( z, f );
        } catch( Exception ex ) {
            ex.printStackTrace();
        }

}

}

The output was:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<zoo>
    <treeModel/>
</zoo>