You can set the C1TreeExtender control so that the open nodes will automatically collapse when another node is opened.
Complete these steps:
<ul>
<li class="folder"><a><span>Folder 1</span></a>
<ul>
<li class="folder"><a><span>Folder 1.1</span></a>
<ul>
<li class="file"><a><span>File 1.1</span></a></li>
<li class="file"><a><span>File 1.2</span></a></li>
<li class="file"><a><span>File 1.3</span></a></li>
<li class="file"><a><span>File 1.4</span></a></li>
<li class="file"><a><span>File 1.5</span></a></li>
</ul>
</li>
<li class="file"><a><span>File 1.2</span></a></li>
<li class="file"><a><span>File 1.3</span></a></li>
<li class="file"><a><span>File 1.4</span></a></li>
<li class="file"><a><span>File 1.5</span></a></li>
</ul>
</li>
<li class="folder"><a><span>Folder 2</span></a>
<ul>
<li class="file"><a><span>File 2.1</span></a></li>
<li class="file"><a><span>File 2.2</span></a></li>
<li class="file"><a><span>File 2.3</span></a></li>
<li class="file"><a><span>File 2.4</span></a></li>
<li class="file"><a><span>File 2.5</span></a></li>
</ul>
</li>
<li class="folder"><a><span>Folder 3</span></a>
<ul>
<li class="file"><a><span>File 3.1</span></a></li>
<li class="file"><a><span>File 3.2</span></a></li>
<li class="file"><a><span>File 3.3</span></a></li>
<li class="file"><a><span>File 3.4</span></a></li>
<li class="file"><a><span>File 3.5</span></a></li>
</ul>
</li>
</ul>