Defining Boundaries Across Borders: A Case Study Extending a Major Land Resource Area Into Mexico Rebecca MacEwen School of Natural Resources, University of Arizona, Tucson, AZ Roy S. Mann NRCS, Retired Philip Heilman and Jeffry Stone Southwest Watershed Research Center, USDA-ARS, Tucson, AZ Alicia Melgoza Castillo La Campana Experimental Ranch, INIFAP, Chihuahua City, Chihuahua D. Phillip Guertin School of Natural Resources, University of Arizona, Tucson, AZ Abstract—Geographic information science (GIS) and field work were applied to extend Major Land Resource Area (MLRA) 41, Southeastern Arizona Basin and Range, from Arizona and New Mexico into Sonora and Chihuahua, Mexico. The result of this analysis is a tentative boundary line that delineates MLRA 41 for both the United States and Mexico based on elevation, soils, temperature, precipitation, and vegetation. This delineation will allow for a more effective linkage of research and increased availability of information across the international border. The focus of this study, MLRA 41, Southeastern Basin and Introduction Range, encompasses southeastern Arizona and a small portion The United States and Mexico not only share a border, of southwestern New Mexico. MLRA 41 is a very diverse but also many natural, economic, and cultural resources. The ecological area in the transition zone between the Sonoran and Chihuahuan and Sonoran Deserts occupy significant areas Chihuahuan Deserts, with a relatively homogeneous pattern on both sides of the border and demonstrate similar climate, of topography, soil, climate, water resources, and land use, geology, soil and water resources, and vegetation. Comprised consisting of a series of isolated mountain chains and arid river of arid and semi-arid regions, the area includes California, basins. There are three Land Resource Units (LRUs) defined by Arizona, New Mexico, and Texas and is divided by botanists the NRCS (1978) within MLRA 41. LRU 41-1: Mexican oak- into three deserts (EPA 2002) and by the USDA Natural pine woodland and oak savannah is found in the higher range Resources Conservation Service (NRCS 1978) into three cor- of elevations, above 1,300m, the precipitation ranges from responding Major Land Resources Areas (MLRAs) for land 400-800 mm, and the vegetation is dominated by oak savannahs management purposes (figure 1). and perennial grasses, with conifer woodlands in the higher The NRCS defines MLRA 40, Central Arizona Basin and elevations, above 1,700m where there is greater than 500 mm Range, as primarily Sonoran Desert, with no perennial water of precipitation. The relatively small area of land, consisting and vegetation dominated by desert shrub and cacti, especially primarily of river valleys, with elevation ranging from 880- the Giant Saguaro cactus. Winters are warm, and precipitation 1,440 m and annual precipitation ranging from 200-300 mm is and elevation are relatively low, with a few mountain peaks classified as 41-2: Chihuahua-Sonora Desert shrub mix due to reaching 1,400 meters. its sparse cover of perennial grasses and shrubs and only a few MLRA 42, Southern Desertic Basins, Plains, and Mountains, trees. The third LRU, 41-3: Arizona semi-desert grassland, is dominates southern New Mexico and part of western Texas. A by far the largest, and covers the mid-range of elevation from large portion of this area is the Chihuahuan Desert, and thus is 975-1,500 m, and of precipitation from 300-400 mm. The home to only a few perennial watercourses, the Rio Grande, vegetation here is characteristically warm season perennial Conchos, and Pecos Rivers. Winters are cold, with elevation grasses, shrubs, and cacti, with a few summer annuals. ranges from 800-1,500m, and precipitation falls primarily in The need for a common land classification scheme between the summer. This area is also dominated by desert shrub and the United States and Mexico is discussed in Heilman et al. grass, but has several species of deciduous and coniferous trees (2000), which proposes a method to link NRCS ecological at higher elevations. sites with the Comisión Técnico Consultiva de Coeficientes USDA Forest Service Proceedings RMRS-P-36. 2005. 525 MOJAVE AND SONORA N DESERTS � 40 41 42 CHIHUAHUA DESERT 0 1 00 200 300 400 500 Kilomete rs Figure 1—Major Land Resource Areas and deserts in the Southwest. de Agostadero (1978) (COTECOCA-Technical Commission managed by conservation and those naturally bio-diversely for Stocking Rates) Sitios de Productividad Forrajera (Forage rich, was completed for the State of Arizona, but not New Production Sites). COTECOCA was chosen because of its Mexico; therefore, vegetation and land use data at a scale correspondence with ecological sites, and more detailed level of 1:100,000 were only available for Arizona. Soil data was of classification. This linkage will allow land managers and obtained from the USDA-NRCS (1994) State Soil Geographic investigators in Mexico to take advantage of the existing data (STATSGO) Database, which has a scale of 1:250,000 for the for MLRA 41, and thus future resource data could be more conterminous United States. easily shared across the border, reducing time and cost to both sides. Sonora and Chihuahua Data Precipitation and temperature Precipitation data were obtained from Comisión Nacional Arizona and New Mexico para el Conocimiento y uso de la Bioversidad (CONABIO) and Instituto Mexicano de Tecnología del Agua (IMTA). Data Precipitation and temperature from these sites that were available included both monthly and annual minimums, maximums, and averages in predefined The precipitation and temperature data used for the United ranges. States were obtained from the Oregon Climate Service’s Spatial Climate Analysis Service, which applies PRISM Vegetation and soils (Parameter-elevation Regressions on Independent Slopes Model) to adapt orographic effects into the interpolation of The initial vegetation information was created by digitiz- temperature and precipitation (Daly 1996). ing a Brown and Lowe (1994) vegetation map of both Sonora and Chihuahua. Site maps developed by COTECOCA (1978) were then used to compare vegetation found through field Vegetation and soils observations with the digitized Brown and Lowe information The National Gap Analysis Program (2003), a geographic for verification and adjustment of vegetation communities. planning tool that identifies gaps between land areas that are Numerous sites were sampled comparing soil descriptions in 526 USDA Forest Service Proceedings RMRS-P-36. 2005. Arizona with soils in Chihuahua and Sonora. Vegetation and within the elevation and precipitation limits of MLRA 41. This soils maps from INEGI (2004) were also used to reference the large area was further divided by use of the field notes that field observations and other sources. Their more general classi- stated an area was possibly characteristic, or not indicative of, fications are consistent with those of the specific COTECOCA any of the three LRUs of MLRA 41and then compared to the in that MLRA 41 includes pastizal (grassland) and bosques vegetation data for Mexico. Vegetation was one of the biggest de encino (oak forests), and has a southern boundary north of factors in determining the extent of MLRA 41. Elevation and, the selva caducifolia (thorn forest) and bosques de coniferas to a degree, precipitation were then used to separate the area (conifer forests). into the three LRUs based on the NRCS defined and statisti- cally determined ranges. Elevation The Digital Elevation Model (DEM) was retrieved from Results the National Elevation Dataset center, and then converted The elevation range was found to be 716 to 3,261m. The from decimal degrees to meters, having a resolution of ap- published elevation range is 800-1,800 m in the mountains, proximately 900 m. The DEM covers the western coast of the with the highest peaks around 3,300 m. This indicates the United States, and extends down into Mexico to include the ranges determined from the statistics are within the param- States of Sonora and Chihuahua. eters set by the NRCS, and could be applied to the Mexico side. We were also able to determine that MLRA 41 had Field Data annual precipitation ranging from 229-1,143mm, minimum temperatures ranging from 1.47-12.91 °C, and maximum Field data were collected on several occasions. This included temperatures ranging from 15.93-28.86 °C. These values being collecting GPS points that indicated changes in vegetation or similar to the USDA-NRCS published data, which includes landform, vegetation field notes, and soil/site correlation data. having an average annual precipitation range of 275-375mm, Pictures were also taken to document differences or similari- but as much as 900 mm in higher elevations, an average ties in vegetation and landform within MLRA 41 in the United annual temperature of 13-17 °C with lows below freezing and States (figure 2). highs above the 37.8 °C, allowed us to apply similar techniques to the Mexico data sets. Through field observation it was discovered that the western Methods part of Sonora, like that of the Arizona/California border, is Zonal statistics in ArcMap were applied using the elevation more typical of the Sonoran Desert, like that of MLRA 40, data to calculate the range for all three LRUs. These ranges and is accordingly the western limit. The vegetation of the were used to determine areas in Northern Mexico that were Chihuahuan Desert becomes more evident toward Texas, as Figure 2—Example of vegetat- ion indicative of MLRA 41-1 in Sonora, Mexico. USDA Forest Service Proceedings RMRS-P-36. 2005. 527 ARIZONA NEW MEXICO Figure 3—New MLRA 41 area and previous N boundary. � MLRA 41- 2000 CHIHUAHUA MLRA 41- 2004 41-1 41-2 41-3 SONORA 0 50 100 200 300 Kilometers in the vegetative composition of MLRA 42; and thus becomes the eastern boundary of MLRA 41. The southern boundary References was drawn where the thorn forests begin.