Plant Species Composition of Grassland Patches Dominated by Black and Blue Grama

Transition zones between biomes consist of a mosaic of patch types dominated or co-dominated by species from the adjacent biomes. Most studies of biome transition zones have focused on the dominant life forms or characteristic species, and patterns in subordinate species composition are not well documented and understood, yet it is these species that contribute to biodiversity. The objective of this study was to compare the plant species and life form composition of two patch types at a shortgrass steppe – Chihuahuan desert grassland transition zone in central New Mexico, USA. We sought to determine if species were associated with different patch types, and if that association was related to differences in soil texture between patch types and the geographic range of subordinate species. Patches dominated by either Bouteloua gracilis, the dominant species in the shortgrass steppe, or Bouteloua eriopoda, dominant species in the Chihuahuan desert grasslands, were sampled at multiple scales for the occurrence of subordinate species and soil texture within a 1500 ha transitional mosaic of patches. Of the 52 subordinate species analyzed, 16 species were associated with B. gracilis-dominated patches and 12 species with B. eriopoda-dominated patches. Patches dominated by B. gracilis were richer in annual grasses and forbs, whereas patches dominated by B. eriopoda contained more perennials forbs and shrubs and subshrubs. Soils of B. gracilis-dominated patches had higher clay and lower rock contents compared with soils of B. eriopoda-dominated patches. Differences in species characteristics of the dominant species as well as differences in soil texture between patch types contribute to patch-scale variation in species and life form composition. The effect of soil texture on life form composition is supported by the correlation between texture and life form composition within the patch types. The association of species to patch types was not related to their geographic range and occurrence in the adjacent
biomes. Our results show that patch types at this biome transition zone have characteristic life form and species composition, but species are associated to patch types due to local constraints, independently from their affinity to the adjacent biomes.