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Invasive Plants and River Ecosystems


The character of an ecosystem is strongly altered when the dominant form of vegetation changes. This is particularly true when the new form of vegetation brings new pathways for the cycling of nutrients. The emergence of Russian olive, a nitrogen fixing plant that can directly utilize gaseous nitrogen through interactions with microbes in their roots, as a dominant plant in the riparian zones of rivers is one good example of such a change. River corridors throughout the western United States are increasingly invaded by this tree, and this tree has the potential to strongly alter nutrient cycling in these ecosystems.

Three graduate students at the University of New Mexico being supported by the Integrative Graduate Education and Research Traineeship (IGERT) through the National Science Foundation (Mary Harner, Chelsea Crenshaw, and Jennifer Follstad Shah) grew curious about the role of Russian olive along the corridor of the Rio Grande in central New Mexico. The graduate students linked up with a visiting international scientist (Dr. Manuela Abelho from Portugal) and two additional colleagues (Martina Stursova and Robert L. Sinsabaugh) to design an experiment on the decomposition dynamics of leaf litter from this non-native nitrogen-fixing tree species when compared to native cottonwoods. The hypothesis was that the added nitrogen from the Russian olive would increase the decomposition rates in soils where the tree had established.

The leaves of the Russian olive trees were placed in the Rio Grande, on the surface soils of the riparian zone, and buried within the riparian soils at seven locations along the river. The decomposition of the Russian olive and cottonwood species was similar in the river, but the decomposition of the nitrogen-rich Russian olive was higher in both the surface and subsurface soil environments. Moreover, the Russian olive was a significant source of nitrogen to the soil and a large portion of this new nitrogen went into the decomposition of the native cottonwood litter. This interaction in nutrient cycling between native and non-native species is highlighted in an article in press in journal Ecological Applications (Harner et al. 2009). Nitrogen-rich litter from this newly introduced species increased soil fertility and enhanced the decomposition of the native tree species at times when the riparian soils were moist.

Address Goals

The research encompassed by this journal article illustrates signature characteristics of our Freshwater Sciences IGERT program. It integrates our interdisciplinary themes, hydrology, ecology, and geochemistry, and also addresses issues critical to our understanding of invasive plant species that replace native species in vulnerable river riparian zones and floodplains. The research advances our knowledge of the fate of nitrogen associated with nitrogen-fixing riparian invasives and how the establishment of these plants may fundamentally alter overall nitrogen cycling dynamics in river-floodplain ecosystems. The possible effects of nitrogen input from invasive plants on nutrient exchange between rivers and their floodplains, including nutrient retention capabilities, is an especially important component of the research relevant to river management strategies. The interaction of our IGERT students with a scientist from Portugal cultivated an international collaboration that expands the impact of the research and will help foster future research efforts.