icons.collapse : StringDefault: '<i class="gj-icon chevron-down" />'

    
 <div id="tree"></div>
 <script>
     var tree = $('#tree').tree({
         dataSource: '/Locations/Get',
         icons: {
             expand: '<i class="material-icons">add</i>',
             collapse: '<i class="material-icons">remove</i>'
         }
     });
 </script>