Vegetation Classification and Map: El Malpais National Monument

Muldavin, Esteban; Chauvin, Yvonne; Neville, Teri; Kennedy, Amanda; Hulse, Hugh; Arbetan, Paul; Neville, Paul

Natural Heritage New Mexico, in cooperation with the Earth Data Center at the University of New Mexico and Nature- Serve, developed a vegetation classification and high-resolution vegetation map for El Malpais National Monument, New Mexico. The project was part of the U.S. Geological Survey (USGS) Vegetation Characterization Program, a cooperative effort by the USGS and the National Park Service Inventory & Monitoring - Vegetation Mapping Program to classify, describe, and map vegetation communities in more than 280 national park units across the United States. The classification and map follow the guidelines and requirements of the national program, and are based on data collected from 476 field plots between 2006 and 2009. Based on field sampling, we identified 50 plant associations following the guidelines of the National Vegetation Classification Standard (NVCS). Using the classification as a foundation, we constructed a vegetation map from a combination of image analysis and photo interpretation of highresolution (0.6 m) color aerial ortho-photography combined with satellite imagery (Landsat Thematic Mapper). The map legend is hierarchically structured, with an upper Level 1 of nine vegetation map units corresponding more or less to the Group Level of the NVCS, and a Level 2 composed of 42 nested map units defined by the plant associations from the vegetation classification. The vegetation pattern of El Malpais National Monument (ELMA) is intricately interwoven with its geology. In particular, the Zuni-Bandera volcanic lava field with its five lava flows and associated cones and craters dominates the landscape. The flows range in age from 3,200 to 54,000 years old and have differing degrees of soil development and commensurate vegetation expression. The youngest flows, McCartys (≈ 3,200 years old), Bandera (≈ 10,000 years old), and Twin Craters (≈ 18,750 years old) are characterized at higher elevations by stunted forests of scattered Douglas-fir and ponderosa pine, with occasional aspen growing among the cracks and fissures of the lava. Large areas of exposed lava are still common (both rough “`a`a” lava and smooth “pahoehoe” forms). Lower down the elevation gradient, these open forests give way to equally open and stunted woodlands dominated by pinyon pine and scrub-like oneseed junipers, and at the lowest elevations, transition to shrublands dominated by Apache plume, skunkbush sumac, and wax currant. On these sparsely vegetated young flows, fire is infrequent and limited in extent, creating an environment where trees and shrubs can live to extended ages and sites develop unique assemblages of species. On the older flows (Hoya de Cebola, 50,000 years old, and El Calderon, 54,000 years old), a similar elevation gradient of vegetation is apparent, but because these flows have developed a deep and more continuous soil mantle over time, the understory vegetation is more continuous and typically dominated by grasses. Trees grow taller on these more productive sites, but the more continuous grass cover can foster more frequent surface fires resulting in woodlands with a more open, savanna-like character. The cinder cones provide a different environment of steep, unstable slopes, yet vegetation has managed to establish in patches, which we have mapped as distinct forest, woodland and scrub classes. Vegetation adjacent to young lava flows can be quite different in character. Old basalt plains (700,000 years old) and older cinder cones, as well as hills of sedimentary limestone and sandstone are prevalent around the perimeter of, or as islands (kipukas) within, the flows. With relatively deep soils, these flows support ponderosa pine and pinyon-juniper woodlands and grasslands that are representative of typical non-volcanic landscapes of the southern Colorado Plateau. Overall, the vegetation map was designed to facilitate ecologically-based natural resource management at a 1:24,000 scale with 0.25 ha minimum map unit size.