This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. Riparian Vegetation on the Rio Saota Cruz, Sonora Gilberto Solis-Garza1 and Philip Jenkins2 Abstract.-Though riparian areas are infrequent, they have economic, ecological and social significance because they provide useful products; water, forage, firewood, protection to wildlife, increase in water quality and quantity, and erosion control. How­ ever, the ability of a given site to provide this range of products is dependent upon the quality of the vegetation present. Studies related to the abiotic factors and biotic factors and to survey the number of species present in the Rio Santa Cruz (RSC) are few. The objectives of this paper is to create a species lists based on existing vegetation in the RSC. The vegetation diversity was obtained from 10 representative sites along the river, each 20 x 50 meters. A total of 149 species in 104 genera and 38 families were found. The most diverse families were Asteraceae, Poaceae and Fabaceae. The largest genera were Machaeranthera, Eragrostis and Ipomea. Herbaceous species averaged 70.5%, grasses 18.1 %, trees 7.4%, and shrubs 4.0%. Cottonwood (Populus fremontii) and Goodding willow (Salix gooddingii) were the most frequent tree species on the sites. Preliminary investigations indicated that 40.2% of the cottonwood trees are 10 or more decimeters in basal diameter basal (15 em above the ground). This can be due to perturbations that impact the younger plants. Loss of seed viabil­ ity, intolerance of lowering of water tables, and livestock grazing are examples. Resumen.-Las areas riparias tienen gran importancia econ6mica, ecol6gica y social ya que proporcionan agua, forraje, lena, protecci6n a la fauna silvestre, incremento en la calidad y cantidad de agua, reducci6n de erosion, etc. Sin embargo, Ia calidad de los productos obtenidos dependera de las especies de la vegetaci6n presente. Estudios relacionados a determinar la vegetaci6n presente en el Rio Santa Cruz, Sonora (RSC) son escasos. El objetivo del presente estudio es elaborar un listado de especies basado en la vegetaci6n actual del RSC. La diversidad de la vegetaci6n se obtuvo a traves de 10 sitios de 20 x 50 metros localizados a lo largo del cauce del RSC. Se identificaron 149 1 Research-professor, Departmento de Investigaciones Cientificas y Tecnologicas Universidad de Sonora, Hermosillo, Sonora 2 Assistant Curator, Herbarium, Department of Ecology and Evolutionary Biology, Univer­ sity of Arizona Tucson, Arizona 100 USDA Forest Service Proceedings RMRS-P-5. 1998 especies pertenecientes a 104 generos y 38 familias. Las familias con mayor numero de generos son: Asteraceae, Poaceae y Leguminosae siendo los mas representativos: Machaeranthera, Eragrostis e Ipomea. Las especies identificadas son herbaceas anuales (70.5%), pastos (18.1 %), arboles (7.4%) y arbustos (4.0%). El alamo y el sauz son las especies arb6reas mas frecuentes. Estudios preliminares indican que el40.2% de los individuos de la especie alamo son mayores a 10 em. de dbh. Lo anterior, debido al impacto de las perturbaciones a las plantulas, perdidad de viabilidad de la semilla, y baja tolerancia ala falta de humedad, pastoreo, etc. INTRODUCTION Studies of Sonoran riparian areas are scarce. A survey of the literature revealed that no vegetative work has been previously done in Santa Cruz River basin in Sonora. Elsewhere in Sonora, Gentry (1942) discusses ripar­ ian vegetation along the Rio Mayo, White (1948) surveyed the region of the Rio Bavispe, Garza (1985) studied the area of the Sierra de los Ajos, and Solis and collaborators (1993) surveyed the Rio de los Ajos. Just north of the border, Stromberg et al. (1993) studied the affects of sewage affluent on vegetation in Nogales, Arizona. The Santa Cruz River, part of the Colorado River system, arises in the San Rafael Valley of Arizona, taking its water from the Patagonia Moun­ tains on the west, the Canelo hills on the north, and the Huachuca moun­ tains on the east. Miller Peak in the Huachucas is the highest point in the drainage at 2885 m. After it arises in the San Rafael Valley at about 1600 m, the river flows south into Sonora. It continues south as far as the ex­ hydrometric station called El Cajon, thence southeast to the town of San Lazaro, where it turns west. It then turns north and reenters Arizona east of the city of Nogales (Sonora and Arizona). The length of its course in Sonora is 56 kilometers. The channel continues northward through Tucson to meet the Gila River southeast of Phoenix, Arizona. The basin is located in north-central Sonora, between the coordinates 31 o 08' and 31 o 20' North latitude and 110° 30' and 111 o 00' West longitude. The riverbed is localized between 31 o 09' and 31 o 20' North and 110° 50' West. In Sonora, the valley is delimited by Sierra El Chivato on the east and Sierra El Pinito on the west. Sierra San Antonio lies between the south-flowing and north-flowing parts of the valley in Sonora (Figure 1). The area falls almost entirely within the municipios of Nogales and Santa Cruz. Through much of its course in Arizona it flows only in time of flood, but in Sonora there are lengths of the stream with enough water to support USDA Forest Service Proceedings RMRS-P-5. 1998 101 Cuenca del Rio Santa Cruz: Figure 1. Map of the upper Santa Cruz drainage ~ showing the locations of the ten sites. lii=O-=o:zO -==-==1 0 km N riparian vegetation and three species of fish. It is a principle source of water for Nogales. Without doubt, it is an area of great importance for ecological and human resources, and it is a fragile and rare commodity in this arid part of the continent. Among the other functions of the river and the associated vegetation, it sustains life by giving water, plus it controls erosion and purifies the water by filtering contaminants. Riparian areas provide food and shelter for up to 75% of the native wildlife for at least some part of their life cycles (ASU-COLEF 1995). Materials and Methods The object of this study was to begin a list of plant species that grow adjacent to the Santa Cruz by creating study plots and collecting voucher specimens from them that could be accurately identified later in a her­ barium. In order to sample the changes in vegetation along the gradient of the stream, a number of sites had to be chosen. A preliminary survey of the area was conducted to select the collection sites. The sites were chosen by visual observation based on the criteria of relative homogeneity of the dominant species present and the estimated biomass (the areas with greater biomass were opted for). In addition, the presence trees and shrubs that are considered obligate riparian species (as defined by Dick-Peddie and Hubbard, 1977) was a prerequisite. Ten sites were chosen, each 20 x 50 meters, running parallel to the course of the river (Figure 1 and Table 1). They were surveyed and col­ lected during the month of October, 1995. A voucher for all of the species of plants encountered at each site were collected (trees, shrubs, herbaceous 102 USDA Forest Service Proceedings RMRS-P-5. 1998 annuals and perennials, grasses and vines). The study was originally intended to be resurveyed for winter annuals, but the absence of winter rains in the year of the study made it impossible, as none of those plants appeared. Table 1. The geographic location of the sampling sites. Site Site Alt. number name Latitude Longitude (m) 1 La Faja 31° 19' N 110° 35' w 1250 2 Camino a Cananea 31°17'N 110° 36' w 1f20 3 Poblado Santa Cruz 31° 12' N 110° 35' w 1200 4 ElAlamo 31° 10' N 110° 36' w 1180 5 Poblado Miguel Hidalgo 31° 09' N 110° 39' w 1140 6 Paredes 31° 10' N 110° 43' w 1120 7 Ejido Cadenas Valdez 31° 12' N 110° 44' w 1110 8 Ejido Lopez Mateos 31° 15' N 110° 47' w 1100 9 Estaci6n Mascarefta 31° 18' N 110° 49' w 1060 10 Substaci6n 31° 18' N 110° 50' w 1040 The identification of the plants was conducted in coordination with the University of Arizona herbarium and the University of Sonora, and the vouchers were deposited in the herbarium of DICTUS (Departamento de Investigaciones Cientificas y Tecnol6gicas de la Universidad de Sonora) in Hermosillo, Sonora. The nomenclature and identifications were accom­ plished through the use of Kearney and Peebles (1961), Lehr (1978), Gould (1951), and from the most recent and/or most widely accepted botanical taxonomic literature. The author names and abbreviations follow the standards of Brummet and Powell (1992). Rather than simply reporting the presence of a species, a second set of data was made to measure the abundance of the species on the sites as a whole. To determine the rate of abundance of species occurrence a scale of 5 classes was used to describe each plants continuity and abundance over the range of the study, described in Table 2 (Oosting, 1956). Table 2. Abundance classes. Class Abundance 1 Rare (1 to 20% of the sites) 2 Present infrequently (21 to 40% of the sites) 3 Present frequently (41 to 60% of the sites) 4 Present consistently (61 to 80% of the sites) 5 Present very consistently (81-100% of the sites) USDA Forest Service Proceedings RMRS-P-5. 1998 103 RESULTS The basin of the Santa Cruz River has an area of 783 square kilometers in Sonora. The River's total length is approximately 360 kilometers (Swallen and Shaw 1957), 56 kilometers of which is in Sonora.