1. Introduction The Rio Grande’s flows are about half of what they were 60 years ago, and half of the wetlands in the Middle Rio Grande have been lost in just 50 years (Crawford et al. 1993). Overbank flooding in late spring/early summer due to snowmelt runoff occurs in a very limited area only in years with above-average snowfall. The last major floods in the Middle Rio Grande that produced large-scale cottonwood establishment occurred in the springs of 1941 and 1942. Cottonwood germination, which requires sand bars and adequate moisture from high river flows (Braatne et al. 1996), has declined substantially. Results of experimental flooding show that flooding promotes leaf and wood decomposition, reduces woody debris biomass, increases bacterial and fungal activity, reduces density and patchiness of vegetation, promotes growth of mature cottonwoods, reduces abundance of Armadillidium vulgare, an exotic isopod, and increases abundance of Gryllus alogus, a native cricket (Molles et al. 1998). Without changes in water management, the Rio Grande bosque (riparian forest) will be dominated by exotic species, namely salt cedar (Tamarix ramosissima) and Russian Olive (Elaeagnus angustifolia) over the next 50 to 100 years (Howe and Knopf 1991). Restoration of the native bosque along the hydrologically altered and highly controlled Rio Grande would require instituting managed late spring/early summer floods in years with good water availability that coincide with the historical peak in snowmelt discharge. Acknowledgements UNM Hydrogeoecology Group NSF Grant DEB-9903973 Middle Rio Grande Conservancy District Bosque del Apache National Wildlife Refuge Belen Consolidated Schools City of Albuquerque Open Space Division New Mexico State Land Office Rio Grande Nature Center Contacts Jennifer F. Schuetz 505-277-5732 jschuetz@sevilleta.unm.edu Manuel C. Molles, Jr. 505-277-3050 molles@sevilleta.unm.edu Cliff N. Dahm 505-277-2850 cdahm@sevilleta.unm.edu Cliff S. Crawford 505-277-3411 ccbosque@juno.com Abstract Over the past fifty years, volume and timing of the Rio Grande’s flow, including the annual flood pulse, have been altered due to damming and diversion of the river. As a result, the river is largely isolated from its riparian forest, or bosque, and the native cottonwood forest is aging, is not regenerating, and is being invaded by exotic plant species. Restoration of the native bosque may require the use of managed floods; however, there is limited scientific data to assess the implications of this management activity. To provide information about ecological implications of overbank flooding and restoring hydrologic connectivity between the river and its floodplain, we are investigating how 4 flooded and 4 nonflooded sites within a 160 km stretch of the Middle Rio Grande differ ecologically. Ecological indices of the cottonwood canopy, forest floor, soils, and groundwater were chosen to provide a picture of riparian ecosystem health. These indices include: tree and shrub species composition and abundance in litterfall, carbon to nitrogen ratio in litterfall, reproductive status or flowering of trees, nutrient characteristics of new leaf tissue, abundance and type of ground-dwelling arthropods, leaf decomposition rates, soil net nitrogen mineralization, soil moisture, forest floor respiration, root growth and biomass, soil texture and chemical characteristics, groundwater levels, and groundwater chemistry. We are just completing our first full field season measuring these variables. Results will help us inform policy-makers on potential tools, such as instituting managed late spring/early summer floods in years with adequate water availability, to help restore the native bosque and the hydrologic connectivity between the river and its floodplain. Subsystems Under Study Cottonwood Canopy Forest Floor Soils/Vadose Zone Alluvial Aquifer leaf litter woody debris water table Conceptual Model of Project = groundwater well = pressure transducer to monitor groundwater levels = water content reflectometer (soil moisture at top 30 cm of soil) = 4 pitfall traps – focus study on isopods, crickets, carabid and tenebrionid beetles = neutron probe access tube (soil moisture from 25 cm to 1 m below surface at 15 cm increments) = tubes to take images of roots = litterfall tub = soil respiration measurement PT CO 2 = cottonwood leaf decomposition study = soil samples for net nitrogen mineralization Study Site Layout Literature Cited Braatne, J.H., S.B. Rood and P.E. Heilman. 1996. Life history, ecology and conservation of riparian cottonwoods in North America. In Biology of Populus and its Implications for Management and Conservation. Edited by R.F. Stettler, H.D. Bradshaw, Jr., P.E. Heilman and T.M. Hinckley. NRC Research Press, Ottawa, Ontario, Canada. pp. 57-85. Crawford, C.S., A.S. Culley, R. Leutheuser, M.S. Sifuentes, L.H. White, and J.P. Wilber. 1993. Middle Rio Grande Ecosystem: Bosque Biological Management Plan. U.S. Fish and Wildlife Service, District 2, Albuquerque, New Mexico, 291 pp. Howe, W.H., and F.L. Knopf. 1991. On the imminent decline of the Rio Grande cottonwoods in central New Mexico. Southwestern Naturalist 36: 218-224. Molles, M.C., Jr., C.S. Crawford, L.M. Ellis, H.M. Valett and C.N. Dahm. 1998. Managed Flooding for Riparian Ecosystem Restoration. BioScience 48(9):749-756. Preliminary Results In addition to the parameters shown above, the following analyses are being conducted: Diameter at breast height At 1 flood and 1 nonflood site, 2 wells 6 m deep and one 12 m deep: DO, pH, cations (Fe, Mn, Mg, Ca, K, Na, Si), anions (F, Cl, NO 3, SO 4, Br, PO 4 ), alkalinity, H 2 S monthly during summer Soil conductivity, organic matter, particle size, extractable bases, clay mineralogy, calcium carbonate, chloride and pH in these pits C, N and P in surface- active arthropods Reproductive status and defoliation status DO, temperature, pH, alkalinity, ammonium, phosphate, sulfate, chloride, nitrate, sodium, potassium, iron, manganese, magnesium and calcium 2 soil pits at 4 sites to designate soil horizons and provide a detailed description of each horizon Standing stock of litter and coarse woody debris C, N and P in mature, active cottonwood leaves before and after spring flooding and in senesced cottonwood leaves from December of each year Alluvial AquiferSoils/Vadose ZoneForest FloorCottonwood Canopy 40m PT Rio Grande CO 2 N Not to scale Preliminary Conclusions Flood sites seem to have higher soil moisture than nonflood sites, and flood sites appear to be more connected to river flow, as indicated by the increase in soil moisture as a result of the flood pulse and monsoon rains. Cottonwood leaves dominate leaf fall in flood and nonflood sites. Litterfall data suggest that litter from exotic plants is more prevalent at nonflood sites. More wood appears to be falling from the canopy at nonflood sites compared to flood sites. Beetle composition appears to be dominated by carabids, particularly mesophilic species, at flood sites, while tenebrionids and other xerophilic beetles dominate collections at nonflood sites. Exotics Natives Exotics