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REGULATED RIVERS: RESEARCH & MANAGEMENT, VOL. 11, 275-281 (1995) EFFECTS OF AN EXPERIMENTAL FLOOD ON LITTER DYNAMICS IN THE MIDDLE RIO GRANDE RIPARIAN ECOSYSTEM MANUEL C. MOLLES, JR., CLIFFORD S. CRAWFORD AND LISA M. ELLIS Department of Biology, University of New Mexico, Albuquerque, NM 87131, USA ABSTRACT The objectives of the study were to measure the effects of an experimental flood on : (1) leaf litter production by the riparian forest; (2) leaf decomposition; and (3) the amount of forest floor organic matter. The study area is located at Bosque del Apache National Wildlife Refuge, approximately 160 km south of Albuquerque, New Mexico. Experimental and reference study sites of about 3.1 ha were both located in cottonwood (Populus fremontii) riparian forests that had not been flooded for approximately 50 years. The experimental and control study sites were sampled for two years before flooding the experimental site. The experimental flood peaked on about 31 May, the average date of peak flow in the 100 year hydrograph for the USGS gauging station on the Rio Grande at Embudo, New Mexico. Litter fall was lower at the flood site than at the control site, whereas the rate of leaf decomposition was higher at the flood site during the period of inundation. However, the flood did not produce a measurable response in standing stock of forest floor litter. This pool of carbon may require the cumulative effects of several years of flooding to show a measurable response. KEY WORDS: forest floor litter; manipulative flooding; Rio Grande; riparian ecosystems INTRODUCTION Periodic flooding is an integral part of large river-floodplain ecosystems and is an important mechanism for the exchange of materials and energy between rivers and riparian forests (Junk et al., 1989; Sparks, et al., 1990; National Research Council, 1992). However, flood control has generally reduced the frequency and intensity of flooding by many large rivers. For instance, the Rio Grande in central New Mexico was histori- cally characterized by spring flooding during snowmelt in the upper basin and summer flooding in response to thunderstorms. Now, however, dams have eliminated most floods in the upper and middle Rio Grande. Flood control and other water management practices such as channelization and levee building have greatly altered various aspects of the floodplain ecosystem (Campbell and Dick-Peddie, 1964; Howe and Knopf, 1991). One observable feature of the present riparian ecosystems of the middle Rio Grande is an accumula- tion of large amounts of leaf litter and woody debris. We propose that the build-up of this material is at least partly due to the absence of substantial flooding during the past 50 years. This study focuses on the effects of the first of a series of planned, manipulative floods on the production, decomposition and storage of plant litter in a riparian ecosystem. Plant litter dynamics reflect major ecosys- tem processes including: (1) primary production; (2) decomposition; and (3) nutrient cycling. The limited research that has been done on the litter dynamics of riparian zones has produced diverse results (Malan- son, 1993). Peterson and Rolfe (1982) reported nearly identical rates of litter production in riparian and upland sites, whereas Shure and Gottschalk (1985) measured lower litter production in their wettest riparian sites. Similarly, whereas Shure et al. (1986) reported higher rates of decomposition in riparian zones than in drier upland areas, Chauvet (1988) encountered higher rates of decomposition in their drier study sites. Our specific objectives were to measure the effects of an experimental flood on: (1) leaf litter production by the riparian forest; (2) leaf decomposition; and (3) the amount of forest floor organic matter. We predict that annual flooding (1) decreases litter production by the riparian forest, (2) increases rate of decomposition and (3) decreases the amount of litter stored on the riparian forest floor. CCC 0886-9375/95/070275-07 Received 15 July 1994 © 1995 by John Wiley & Sons, Ltd. Accepted 9 February 1995