Root patterns and hydrogeomorphic niches of riparian plants in the American Southwest

Root depth influences plant distribution and function but has been little studied in dryland riparian zones. Using values from literature and excavations, this study examined rooting patterns for 125 riparian species of southwestern USA and asked if maximum root depth varies with 1) moisture affinity, 2) growth habit, 3) flowering season, and 4) exotic vs. native status. Root depth varied with all factors analyzed. Hydroriparian species had shallower roots than mesoriparian and xeroriparian species. Annuals had shallower roots than perennials, and shrubs had deeper roots than trees. Hydroriparian perennials and cool-season annuals had the shallowest roots; xeroriparian shrubs had the deepest. Plants varied widely in root depth as well as in root length (near surface laterals) and root:shoot ratios. This wide range is typical of ecosystems that have many water sources, with each rooting pattern being adaptive in a different hydrogeomorphic setting. Exotic species showed trait divergence as evidenced by greater rooting depths. There were many data gaps, with several species having no rooting data. There is a need for site-based studies of rooting patterns in dryland riparian zones to more accurately document hydrogeomorphic niches and predict changes in plant distributions.