Periodic flooding, an integral part of the dynamics of natural large river-riparian ecosystems, provides a vehicle for exchange of materials and energy between rivers and riparian forests. However, flood control has generally reduced the frequency and intensity of flooding by large rivers around the world, including the Rio Grande in central New Mexico, where flow modification has been accompanied by invasion of the riparian forest by exotic plants. For the past 25 years our studies have been guided by a central question. How have flood control and resulting changes in hydrologic conditions affected biodiversity and ecosystem structure and processes in the Middle Rio Grande riparian ecosystem? The goal of our research program has been two-fold: 1) to better understand the basic ecology of a large river-riparian ecosystem in a semiarid region and 2) to provide information useful to conservation and restoration work in the Middle Rio Grande. Our research methods have combined large-scale experiments, comparative observations, and extensive, long-term monitoring.

Results/Conclusions

Experimental flooding stimulated increased soil microbial biomass and elevated forest floor respiration by up to three orders of magnitude. Flooding, which doubled the activity of decomposer fungi, also increased rates of decomposition by leaf litter and woody debris. Experimental and natural flood sites indicate that the forested riparian floodplain acts as a sink for inorganic nitrogen and suspended solids but as a source for dissolved organic carbon. While having no observed impact on small mammal populations, flooding of the riparian forest shifts species composition of forest floor arthropods, particularly among carabid beetles. In the riparian forest of the Middle Rio Grande, flooding is associated with higher abundance and species richness of carabid beetles and a shift in dominance from xerophilous to hygrophilous species. Shifts in the composition of carabid beetles occurred within two years of experimental flooding on a formerly dry site. Our research has documented the role of flooding as a key organizer of riparian ecosystem structure and function and biodiversity in this semiarid region. In addition to improving our understanding of the basic ecology of the Rio Grande riparian system, our research has provided information helpful to conservation and restoration of this regionally significant ecosystem and has acted as a guide to further research.