Flora of Chihuahuan Desertscrub on Limestone in Northeastern Sonora, Mexico Thomas R. Van Devender and Ana Lilia Reina-Guerrero Sky Island Alliance, Tucson, Arizona J. Jesús Sánchez-Escalante Departamento de Investigaciones Científicas y Tecnológicas, Universidad de Sonora, Hermosillo, Sonora, Mexico Abstract—Transects were done in desertscrub on limestone to characterize the flora of the westernmost Chihuahuan Desert. Most of the sites (15) were in the Municipios of Agua Prieta and Naco in northeastern Sonora, with single sites near Ascensión, northwestern Chihuahua and east of Douglas in southeastern Arizona. A total of 236 taxa were recorded on transects. Dicot perennial herbs (66 species) were the most numerous life forms, followed by annual herbs (36 species), subshrubs (36 species), and woody shrubs (33 species). The most diverse limestone floras were on Rancho La Morita in the Municipio of Naco (87 to 119 taxa/transect), Rancho La Calera (88 taxa), and Cerro El Caloso and Cerrito Los Janitos (84 taxa each) in the Municipio of Agua Prieta. Chihuahuan desertscrub at its western limits is a mixture of typical Chihuahuan species that reach Texas and Coahuila with western and tropical species in associations that are regionally distinct from other parts of the Chihuahuan Desert. Introduction During the spring of 2007, we attempted to relocate the endangered Cochise pincushion cactus (Coryphantha robbinsorum) reported The Chihuahuan Desert is the largest of the three creosotebush- from Sonora by Lopresti (1984). Although C. robbinsorum was not dominated warm deserts in North America (Shreve, 1942). It primar- found, local floras on calcareous substrates were surveyed in north- ily occurs from San Luis Potosí northwest through Chihuahua and eastern Sonora and adjacent Arizona and Chihuahua (Van Devender Coahuila to Texas and New Mexico (Lowe, 1964; Rzedowski, 1978; and Reina-G., 2007). Here we use the results of these inventories to Brown, 1982). Most (90%) is in Mexico on the Mexican Plateau characterize Chihuahuan desertscrub at its westernmost extension. bounded to the east by the Sierra Madre Oriental and to the west by the Sierra Madre Occidental. Most of this area is above 1000 m elevation and regular incursions of frigid Arctic air from the north Methods have shaped the evolution of the flora. More than 80% of Chihua- We visited a C. robbinsorum population on Permian limestone on huan desertscrub is found on limestone, and the gray gravel of this the Magoffin Ranch in Cochise County, Arizona, to refine our search substratum give the characteristic view of the landscape (Lowe, 1964; image for the plant and its habitat. Potential areas for C. robbinsorum Brown, 1982). Throughout the area, there is a mosaic of Chihuahuan were identified in Sonora based on the presence of Paleozoic or lower desertscrub on limestone bedrock and desert grassland in fine-soiled Cretaceous limestone substrates on Instituto Nacional de Estadística valley bottoms. In the Santa Catalina Mountains of Arizona, Shreve y Geografía (INEGI) geological maps, the proximity to the Arizona (1922) and Whittaker and Niering (1968) concluded that vegetation C. robbinsorum populations, and habitat descriptions in Lopresti is more xeric and open and that both vegetation and species reach (1984). Seven areas in Sonora in the Municipios de Agua Prieta and higher elevations on limestone than on other substrates. We have Naco and one in Chihuahua were selected for plant inventories (table 1, noticed that this effect is most prevalent in northern areas with drier fig. 1). climates, but hardly noticeable in foothills thornscrub on limestone in The climate of the area is arid, with biseasonal rainfall, hot sum- east-central Sonora and tropical deciduous forest in southern Sonora. mers, and cold winters. The weather stations nearest all transects are located in Agua Prieta and Naco, Sonora. The mean annual rainfall is 353.5 mm in the Municipio of Agua Prieta, with 40-47% during the summer monsoon season in July-September. The mean annual temperature is 17.3 °C, with a summer maximum of 43 °C in June 1994 and July 1989; a winter minimum of –14 °C and wind chill of In: Gottfried, Gerald J.; Ffolliott, Peter F.; Gebow, Brooke S.; Eskew, Lane –24 °C was registered in February 2011. Low winter temperatures G.; Collins, Loa C., comps. 2013. Merging science and management in limit biological activities. Another climatic factor is the prevailing a rapidly changing world: Biodiversity and management of the Madrean wind from the south for more than 60% of the year, with a strength Archipelago III; 2012 May 1-5; Tucson, AZ. Proceedings. RMRS-P-67. of 5.4 to 8.5 meters/second recorded in the international airport in Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Douglas, Arizona (Reza et al., 2011; CONAGUA, 2010; Universidad Mountain Research Station. Autónoma de Tamaulipas, 2003). USDA Forest Service Proceedings RMRS-P-67. 2013 229 Van Devender and others Flora of Chihuahuan Desertscrub on Limestone in Northeastern Sonora, Mexico 230 Table 1—Localities for limestone transects in Arizona, Chihuahua, and Sonora. Identification Location Elevation Description Arizona 31°22’13”N 109°08’18”W, 1284 (Coryphantha robbinsorum population). Open, grassy Chihuahuan Cochise County: limestone hills on MG1: m elevation to 31°22’00”N desertscrub-desert grassland mix on very gentle slope with gray limestone Magoffin Ranch 109°24”W, 1311 m bedrock on upper portions of the hill. Chihuahua: Municipio de Ascensión Cerro de Cal (La Biznaga on geology 31°07’04”N 108°05’23”W, 1380 Open Chihuahuan desertscrub on dark gray limestone with white and CH1 map), west of Ascensión; two areas m elevation and 31°06’35”N orange secondary veins. combined 108°05’04”W, 1322 m elevation Sonora: Municipio de Agua Prieta Ridge above El Álamo, ca. 21 km south of the Cerro La Minita transect on Rancho Puerta Blanca, north of the Sierra Pitáicachi, ca. 2 km west of Cerro Chihuahuan desertscrub on west-facing, gray, fossiliferous limestone; AL1 1239 to 1289 m elevation Agua Zarca, 7 km northwest of Rancho good grass, not heavily grazed. El Capadero, 9.7 km (by air) southeast of La Cieneguita (= Oquita Montenegro) in Cajón Bonito Chihuahuan desertscrub on gentle hill, south and northeast slopes, highly ca. 15 km east of Agua Prieta on north modified Cretaceous limestone with bands of whitish quartz; southern AP1 1318 m elevation side of MEX 2; 31°19’03”N 109°22’36”W slope very grassy, northeast slope dominated by Flourensia cernua and Acacia neovernicosa. Rancho La Calera, ca. 10 km (by air) 31°14’07”N 109°37’40”W, 1289 Chihuahuan desertscrub on whitish gray limestone upper northeast-facing USDA ForestService ProceedingsRMRS-P-67. 2013 USDA CA1 southwest of Agua Prieta, outlier ridge of m elevation slope, alluvium below. Sierra Anibácachi Rancho La Calera, ca. 10 km (by air) 31°13’59”N 109°37’37”W, 1297 Chihuahuan desertscrub on whitish gray limestone upper northwest-facing CA2 southwest of Agua Prieta, outlier ridge of m elevation slope, alluvium below Sierra Anibácachi lower end: 31°08’23”N Chihuahuan desertscrub on east-northeast facing limestone ridge with Cerro Caloso (= Calichoso), ca. 8 km (by 109°28’15”W, 1215 m elevation, CL1 most layers nearly vertical, but with lots of bedrock exposed, dark gray (to air) east of Cabullona upper end: 31°08’14”N purple) with lots of secondary white calcite veins. 109°27’59”W, 1272 m lower end: 31°15’57”N Cerrito Los Janitos, east of the north end 109°40’15”W, 1334 m elevation, Chihuahuan desertscrub on light gray limestone (similar to CA1 and CA2); JN1 of the Sierra Anibácachi, ca. 1.5 km (by upper end: 31°15’56”N mostly on east-facing slope. air) west-southwest of Agua Prieta 109°40’27W, 1429 m Flora of Chihuahuan Desertscrub on Limestone in Northeastern Sonora, Mexico Van Devender and others USDA ForestService ProceedingsRMRS-P-67. 2013 USDA Table 1—Continued Identification Location Elevation Description Ca. 40 km east of Agua Prieta on upper end: 31°18’56”N Rancho Puerta Blanca, north of Arroyo Chihuahuan desertscrub on south-southwest slope; lower part limey PB1 109°06’30”W, 1294 m Guadalupe and MEX 2, just south of quartzite, upper gray limestone elevation Arizona border (below transect PB2) Ca. 40 km east of Agua Prieta on upper end: 31°19’22”N Rancho Puerta Blanca, north of Arroyo PB2 109°06’37”W, 1386 m Chihuahuan desertscrub on south-southwest slope; gray limestone. Guadalupe and MEX 2, just south of elevation Arizona border (above transect PB1) Ca. 37 km east of Agua Prieta on 31°17’58”N 109°08’40”W, 1277 Chihuahuan desertscrub on top of gentle north-south limestone PB3 Rancho Puerta Blanca, south of MEX 2, m elevation conglomerate ridge east of Arroyo Guadalupe Ca. 37 km east of Agua Prieta on Chihuahuan desertscrub on gentle west-facing slope, mostly loose 31°18’04”N 109°07’36”W, 1355 PB4 Rancho Puerta Blanca, south of MEX 2, limestone weathering from limestone conglomerate ridge. m elevation east of Arroyo Guadalupe Sonora: Municipio de Naco lower end: 31°08’23”N 109°28’15”W, 1215 m Hill west of Arroyo La Bellota, Rancho La Excellent gray bedrock limestone on gentle slopes (all directions) and MO1 elevation, upper end: Morita, ca. 25 km west of Agua Prieta large flat area on top; grasses common. 31°08’14”N 109°27’59”W, 1272 m lower end: 31°15’14”N 109°49’05”W, 1438 m elevation; upper east end: Chihuahuan desertscrub. Transect began on west-facing slope with Hill west of Arroyo La Bellota, Rancho La MO2 31°15’21”N 109°48’50”W, scattered gray bedrock limestone on alluvium; then to north slope bedrock Morita, ca.