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 require sand bars and adequate moisture from high river flows (Braatne et al. 1996), has declined substantially. 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 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. 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. Acknowledgements UNM Hydrogeoecology Group NSF Grant DEB-9903973 Middle Rio Grande Conservancy District Bosque del Apache National Wildlife Refuge 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 60 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 exotics. Restoration of native bosque requires the use of managed floods; however, there is limited scientific data to assess 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 regularly flooded and 4 regularly 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 ecosystem condition. These indices include: litterfall, carbon to nitrogen ratio in litterfall, reproductive status/flowering of trees, nutrient characteristics of new leaf tissue, abundance and type of ground-dwelling arthropods, leaf decomposition, net nitrogen mineralization, soil moisture, forest floor respiration, root growth and biomass, soil texture and chemical characteristics, groundwater levels, and groundwater chemistry. Our first full field season measuring these variables will occur in 2001. Albuquerque Rio Grande Sevilleta NWR Bosque del Apache NWR Cochiti Reservoir Elephant Butte Reservoir 100 km New Mexico Belen Los Lunas Bernardo Nonflooded sites Flooded sites 1 2 4 5 7 3 8 6 Study Sites Soils/Vadose Zone Measure soil moisture every hour using a water content reflectometer situated vertically in the top 30 cm of the soil near the center well  H 0 : soil moisture at flooded sites > at nonflooded sites Use a neutron probe to measure soil moisture throughout the top 1 m of soil (at 15 cm increments) at 10 locations every 2 weeks  H 0 : soil moisture at flooded sites > at nonflooded sites Sample soils 4 times a year to determine potential net nitrogen mineralization between the north and center well, west and center, east and center, and south and center wells  H 0 : net nitrogen mineralization at flooded sites > at nonflooded sites Excavate 2 soil pits at 4 sites to designate soil horizons and provide a detailed description of each horizon Take soil samples from these pits to measure soil conductivity, organic matter, particle size, extractable bases, clay mineralogy, calcium carbonate, chloride and pH Flooded cottonwood sites with native understory of willow and other species, relatively healthy cottonwoods, and a small quantity of coarse woody debris and litter on forest floor Nonflooded cottonwood sites with non-native understory of salt cedar and Russian Olive, older cottonwoods, and a large quantity of coarse woody debris and litter on forest floor Cottonwood Canopy Collect leaves from the canopy before and after flooding to measure C:N, C:P and ratio of mass to leaf area  H 0 : C:N & C:P ratios in new leaf tissue at flooded sites < at nonflooded sites  H 0 : mass to leaf area ratio at flooded sites < at nonflooded sites Take images of roots 5 times a year using 10 clear plastic tubes, 1 m in length, installed at a 45 degree angle  H 0 : root growth at flooded sites > at nonflooded sites Collect litterfall each month from 10 locations and in December measure C:N and C:P in senesced leaves  H 0 : litterfall at flooded sites > at nonflooded sites  H 0 : C:N in litterfall at flooded sites < at nonflooded sites Record percentage of flowering trees to determine reproductive status  H 0 : flowering at flooded sites > at nonflooded sites Record the defoliation status of trees  H 0 : defoliation at flooded sites < at nonflooded sites Cottonwood leaf Forest Floor Collect surface arthropods near each well 4 times a year – focus on carabid beetles, tenebrionid beetles, crickets and terrestrial isopods  H 0 : abundance of carabids at flooded sites > at nonflooded sites  H 0 : abundance of tenebrionids at flooded sites < at nonflooded sites  H 0 : abundance of native crickets at flooded sites > at nonflooded sites  H 0 : abundance of isopods at flooded sites < at nonflooded sites Measure soil respiration at 10 locations using a soil respiration system unit  H 0 : forest floor respiration at flooded sites > at nonflooded sites Study leaf decomposition using 25 mesh bags filled with 5 grams of cottonwood leaves – collect 5 bags 5 times a year  H 0 : leaf decomposition at flooded sites > at nonflooded sites Measure standing stock of litter and coarse woody debris in August each year  H 0 : standing stock of litter & coarse woody debris at flooded sites < at nonflooded sites Measure C:N and C:P in arthropods to study food web dynamics Galerita janus (carabid) collected from flooded sites Alluvial Aquifer All sites have 5 wells that are 1.5 m below the baseflow water table with screen length of 1 m. at 2 of these wells: measure water table depth every 30 min. year-round using pressure transducers at 3 of these wells: measure groundwater depth every 2 weeks using a beeper  H 0 : variation in water table depth at flooded sites > at nonflooded sites  H 0 : water table elevation at flooded sites > at nonflooded sites During summer at wells without pressure transducers, analyze: DO, temperature, pH, alkalinity, ammonium, phosphate, sulfate, chloride, nitrate, sodium, potassium, iron, manganese, magnesium and calcium In addition, at 1 flooded and 1 nonflooded site, we have 2 wells 6 m deep and one 12 m deep: Measure 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 months Use a multilevel sampler (MLS) to analyze major cations and anions, DO, pH, DOC, ammonium, organic acids, and methane. After snowmelt, will conduct analyses for inorganic N and P. Seasonally, will study stable isotopes of O and H of water and C of methane. Conduct a particle size analysis on sediment cores and iron extraction from the sediment leaf litter woody debris water table Subsystems Under Study Cottonwood Canopy Forest Floor Soils/Vadose Zone Alluvial Aquifer Hypotheses and Methods = groundwater well = pressure transducer = water content reflectometer = group of four pitfall traps = neutron probe access tube = minirhizotron tubes = litterfall tub = soil respiration measurement PT CO 2 = decomposition study = soil samples for net nitrogen mineralization Study Site Layout 40m PT Rio Grande N Not to scale CO 2