EDIT

        '#title' => t('Classification'),

        '#weight' => 1,

        '#type' => 'select',

        '#options' => cdm_get_taxontrees_as_options(TRUE),

        '#description' => t('A filter to limit the search to a specific classification. Choosing <em>--- ALL ---</em> will disable this filter.'),

      );

    $form_params['area'] = implode(',', $area_uuids);

  }

  // Simple search will not submit a 'tree' query parameter,

  // so we add it here from what is stored in the session unless

  // SIMPLE_SEARCH_IGNORE_CLASSIFICATION is checked in the settings.

  //   $form_params['ignore_classification'] =  NULL;

  // }

  return $form_params;

}