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62 F. ZAMORA-ARROYO ET AL Discussion As is the case along many western rivers, the stretch of the Colorado River between Morelos Dam and the Hardy River is dominated by the exotic shrub, T. ramosissima. Although often considered an undesirable species, T. ramosissima can fulfil valuable ecological functions in riparian ecosystems (Stromberg, 1998b). This study shows that with the resumption of pulse floods following the filling of Lake Powell, native trees have also reestablished along this river stretch. The tree cohorts appear to be related to the 1981-1986, 1993 and 1997-1999 releases of water from the United States to the floodplain below Morelos Dam. These releases were related to strong ENSO cycles and are expected to continue into the future, whenever precipitation in the watershed exceeds storage capacity in the reservoir system (J. Harkens, River Operations Manager, BOR, Boulder City, Nevada, pers. comm.). Native trees, including many over 6 m height, now account for 20% of the species composition in this river stretch, whereas they remain rare on the U.S. stretch of river above Morelos Dam. Native trees are less salt-tolerant than T. ramosissima (Glenn el ai., 1998). The results support the pulse- flood hypotheses for the establishment of native trees, which states that occasional overbank flooding is necessary to wash salts from the banks to allow mesophytic species to germinate (Briggs, 1996; Poff el ai., 1997; Busch & Smith, 1995). Otherwise, backside become too saline for all but the most salt-tolerant plant species. Floods also serve to deposit bare mineral soil needed for germination of native trees and they moisten the soil at the appropriate time, when seeds are viable. Thus, the winter and spring timing of releases to the delta were fortuitous. On the United States stretch of river, by contrast, overbank flooding is now rare (Ohmart el ai., 1988). The carrying capacity of the river channel is large, as most of the diversions take place near the northerly international border. Furthermore, the flood- plain has been worked to prevent flooding of private property in many locations. Ii Therefore, excess releases that reach the delta remain channelized until below Morelos - I Dam, and do not germinate extensive new cohorts of native trees on the United States' ._~_ 1 The February-AprllI997, release of 3xl09m3 at 8o-120m3s-1 was sufficient to bring the river out of its channel to inundate most of the floodplain, and water exited to the Gulf of California and Laguna Salada. We conclude that this flow rate and volume is sufficient to inundate at least the northern portion of the floodplain (the cotton- wood-willow zone) sufficient to allow the establishment of new stands of native trees. The 1993 release was a single event of approximately 3-months duration in winter and spring yet it produced the largest cohort of native tree~, so we conclude that (,' a 3-month spring release is sufficient to germinate tree seedlings. The 1993 cohort t,:,'",' of trees were still abundant in 1999 despite lack of flows from 1994 to 1997, showing l that trees can survive at least 4 years between floods. In years without floods, native trees can exist on alluvial water tables (Seaforth el ai., 2000; Springer el ai., 1999). Depth to ~ ground-water is no greater than 1-2 m along this stretch of riparian corridor, even in ~ years without surface flow (Coition National del Aquas, Mexicali, Mexico, unpublished ~, ground-water maps, 1995-1998). Nevertheless, the increase in total vegetation cover ~n response to multiple years of flooding shows that surface flows also play a role. In controlling vegetation cover. Their role in recharging the ground-water or moderating its salinity is unknown. A hydrological model of the delta. floodplain is needed. . Deliberate floods have been tested as management tools In the Grand Canyon (Collier el ai., 1996) and Rio Grande (Molls et ai., 1998), but up to now the ecological effects of waste spills into the delta have not been recognized. Recently, we observed that native trees have also regenerated on the Gila River (southernmost tributary of the Colorado River), apparently as a result of flood releases from Pa~nted Rock Dam during following 1993 and 1997 ENSO events (P. Nagler, unpublished results of an aerial survey of the Colorado River and tributaries, April 2000). The delta REGENERATION OF TREES IN RESPONSE TO FLOOD RELEASES '(j~ floods appear also to stimulate the shrimp catch in the upper Gulf of Californill (Galindo-Beet el ai., 2000). Large, infrequent disturbances such as these releases need to be included in ecosystem management plans (Dale et aI., 1998). The future of the regenerated delta ecosystem is in doubt. In Mexico, plans are underway to further channelize the river to remove obstructions to future releases (Valdes-Casillas, 1998). In the United States, the criteria for declaring surplus flows are being revised to attempt to retain more of the flood water for human use (NagleI' el at., 2000). Nevertheless, results show that the delta of an arid river can retain natural ecosystem functions that have disappeared from upstream, regulated stretches, and that water availability may actually increase as the dam systems mature (fill). Hence, delta regions of arid rivers should be targets for conservation actions to maintain riparian biodiversity . c.) C':,) c:,..." ~l i"''':' N References Briggs, M, (1996). Riparian Recovery in Arid Lands, Strategies and References. Tucson: University of Arizona Press 159 pp, Busch, D. & Smith, S. (1995). Mechanisms associated with decline of woody species in riparian ecosystems of the southwestern US. Ecological Monographs, 65: 347-370. Collier, M., Webb, R & Andrews, E. (1997). Experimental flooding in Grand Canyon. Scientific American, 276: 82-89. Cook, C. & Bonham, C. (1977). Techniquesfor Vegetation Measurements and Analysis fora Pre- and Post-mining Inventory. Colorado State University Range Science Department Science Series No. 28, 99 pp. Curtis,J. & Cottam, G. (1962). Plant Ecology Workbook, Minneapolis, Minnesota: Burgess Pub\. Co. 193 pp, Dale, V" Lugo, A. MacMahon, J. & Pickett, S. (1998). Ecosystem management in the context of large, infrequent disturbances. Ecosystems, 1: 546-557. Dynesius, M. & Nilsson, C. (1994). Fragmentation and flow regulation of river systems in the northern 3rd of the world. Science, 266: 753-762. Felger, R, Broyles, B., Wilson, M. & Nabhan, G. (1997). The binational Sonoran Desert biosphere network and its plant life (with plant checklistand index). Journal of tM Southwest, 39: 411-566. Galindo-Beet, R, Glenn, E. Page, H., Fitzsimmons, K., Gallindo-Bect, L., Hernandez-Ayon, j., Petty, R, Garcia-Hernandez, J. & Moore, D. (2000). Penaeid shrimp landings in the upper Gulf of California in relation to Colorado River freshwater discharge Fishery Bulletin, 98: 222-225. Glantz, M. (1996). Cumnts of change: El Nino's Impact on Climate and Society, U.K., Cambridge University Press 194 pp. Glenn, E., Lee, RC., Felger, R & Zengel, S. (1996). Effects of water management on the wetlands of the Colorado River Delta, Mexico. Conservation Biology, 10: 1175-1186. Glenn, E., Tanner, R., Mendez, S., Kehret, T., Moore, D., Garcia, J. & Valdes, C. (1998). Growth rates, salt tolerance and water use characteristics of native and invasive riparian plants from the delta of the Colorado River, Mexico Journal of Arid ETI'lIironments, 40: 281-294. Glenn, E., Garcia,J., Tanner, R., Congdon, C. & Luecke, D. (1999). Status of wetlands supported by agricultural drainage water in the Colorado River delta, Mexico HortScience, 34: 39-45, Li, Z. & Kafatosm M. (2000). lnterannual variability of vegetation in the United States and its relation to E1 Nino/Southern Oscillation Remote Sensing of Environment, 71: 239-247. Molls, M., Crawford, C., Ellis, L., Valett, H. & Dahm, C. (1998), Managed flooding for riparian ecosystem restoration - managed flooding reorganizes riparian forest ecosystems along the middle Rio Grande in New Mexico Bioscience, 48: 749-756. Morrison, J., Postel, S. & Gleick, p, (1996). The Sustainable Use of Water in the Lower Colorado River Basin, Oakland, California USA: Pacific lnstitute. Nagler, P., Gillon, K., Pitt, J., Snape, W. & Glenn, E. (2000). Application of the endangered species act across international borders: the case of the Colorado River delta, Mexico, Environ- mental ScUnce and Policy, 3: 11-15. Nagler, p" Glenn, E. & Huete, A. (2001). Assessment of spectral vegetation indices for a riparian ecosystem in the Colorado River delta. Journal of Arid ETI'lIironments, 49: 91-110. doi: 10. 1 oo6/iare,200 1 ,0844.