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Question: Is there a difference in plant species and life form composition between two major patch types at a biome transition zone? Are subordinate species associated with different patch types at the shortgrass steppe - Chihuahuan desert grassland transition zone? Is this association related to differences in soil texture between patch types and the geographic range of associated species?

Location: central New Mexico, USA.

Methods: 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 for the occurrence of subordinate species and soil texture within a 1500-ha transitional mosaic of patches.

Results: Of the 52 subordinate species analysed, 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. 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 composition. The association of species to patch types was not related to their geographic range and occurrence in the adjacent biomes.

Conclusions: 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.