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 delApache NWR

Cochiti Reservoir

Elephant Butte Reservoir

100 km

New Mexico

BelenLos Lunas

Bernardo

Nonflooded sitesFlooded sites

12

45

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

H0: 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

H0: 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

H0: 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

H0: C:N & C:P ratios in new leaf tissue at flooded sites < at nonflooded sites

H0: 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

H0: 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

H0: litterfall at flooded sites > at nonflooded sites

H0: C:N in litterfall at flooded sites < at nonflooded sites

Record percentage of flowering trees to determine reproductive status

H0: flowering at flooded sites > at nonflooded sites

Record the defoliation status of trees

H0: 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

H0: abundance of carabids at flooded sites > at nonflooded sites

H0: abundance of tenebrionids at flooded sites < at nonflooded sites

H0: abundance of native crickets at flooded sites > at nonflooded sites

H0: abundance of isopods at flooded sites < at nonflooded sites

Measure soil respiration at 10 locations using a soil respiration system unit

H0: 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

H0: leaf decomposition at flooded sites > at nonflooded sites

Measure standing stock of litter and coarse woody debris in August each year

H0: 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

H0: variation in water table depth at flooded sites > at nonflooded sites

H0: 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, NO3, SO4, Br, PO4), alkalinity, H2S 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 litterwoody 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

CO2

= decomposition study

= soil samples for net nitrogen mineralization

Study Site Layout

40m

40mPTPT

Rio Grande

NNot to scale

CO2 CO2

CO2 CO2

CO2 CO2

CO2 CO2

CO2 CO2