Establishment Patterns of Native Populus and Salix in the Presence of Invasive Nonnative Tamarix

Despite growing concern and research interest in plant invasions, we know very little about ecological processes involved during invasion episodes. Understanding these dynamics is especially important for habitats subject to reestablishment of historical disturbance regimes; because many invasive species respond positively to disturbance, interactions between colonizing natives and nonnatives at the seedling stage may determine success of invasions. In the American Southwest, reduced flood disturbance in this century has been associated with the invasion of nonnative Tamarix ramosissima, with Tamarix appearing to displace native flood-dependent species of Populus and Salix. We studied two riparian communities of these woody species that had established after over-bank flooding was allowed to occur in two protected areas in New Mexico. Growth and mortality were monitored for each individual within 1-m2 plots of seedling communities within these sites for 2–4 yr. Abiotic conditions measured for each plot included soil salinity, texture, and nitrogen and phosphorus concentration. Although Tamarix seedling densities were an order of magnitude greater than those of the native species at initial establishment, mortality of Tamarix was also much greater than for Populus or Salix and occurred primarily in plots where native species were the most dense. Both natives were taller and grew faster above ground than Tamarix in the first 4 yr, and growth of Tamarix was negatively correlated with neighbor densities. In contrast, Populus and Salix growth and survival were never correlated with Tamarix densities. For all species, none of the abiotic variables measured was as important as neighbor density for explaining growth and survival patterns. These results have positive implications for managers who wish to reestablish the native dominants of riparian forests through reinstatement of flooding. Although Tamarix can respond positively to flooding, a range of conditions clearly exists in which Populus and Salix establishment can be stimulated by flooding, even when Tamarix seedlings are present. Furthermore, the presence of such natives even at the seedling stage may help to reduce densities of invasive Tamarix seedlings.