example.xml

<ground>
  <tree-species>
    <tree id="1"><name>Pine</name></tree>
    <tree id="2"><name>Birch</name></tree>
    <tree id="4"><name>Palmtree</name></tree>
  </tree-species>
  <forests>
    <forest id="1">
      <name>Red Forest</name>
      <trees>
        <tree refid="1"><branch direction="left"/><branch direction="south"/></tree>
        <tree refid="2"><branch direction="right"/><branch direction="south"/><branch direction="west"/></tree>
        <tree refid="1"><branch direction="southwest"/></tree>
      </trees>
    </forest>
    <forest id="2">
      <name>Dark Wood</name>
      <trees>
        <tree refid="2"><branch direction="right"/><branch direction="south"/><branch direction="west"/></tree>
        <tree refid="4"><branch direction="northwest"/></tree>
      </trees>
    </forest>
  </forests>
</ground>