Distributions of C4 plants along environmental and compositional gradients in southeastern Arizona

Abstract

Distributional patterns of C4 plants were investigated in 4 study areas located in se Arizona: granite slopes in the M ule M ountains, limestone slopes in the M ule M ountains, calcareous bajada (alluvial plain) below the M ule M ountains, and limestone slopes in the H uachuca M ountains. Cover data for all vascular species were obtained from 238 0.1 ha (20 × 50 m) sample quadrats located over ranges of elevation and topographic position within the study areas. Overall, 69 C4 species representing 6 angiosperm families were encountered. C4 species accounted for 13.5% to 22.3% of vascular species within the study areas. C4 species frequency in quadrats (on the basis of all species or of grasses only) increased from mesic to xeric community types in all study areas except the calcareous bajada. Similar, but less consistent, trends were evident in the relative cover contributed by C4 species. I n two of the study areas (granite slopes in the Mule Mountains, limestone slopes in the Huachuca Mountains) regression analyses revealed statistically significant trends of C4 species frequency and relative cover along environmental (elevation/solar-irradiation scalar) and compositional (reciprocal averaging ordination) gradients. A lack of consistent trends on limestone slopes in the Mule Mountains may be the result of grazing and/or recent invasion of low-elevation limestone areas by a Chihuahuan Desert flora dominated by C3 dicot shrubs. The calcareous bajada below the Mule Mountains was studied less intensively, but its flora was found to contain the highest frequency of C4 species of the 4 study areas. In contrast, C4 cover on the bajada was low, presumably as a consequence of heavy grazing pressure on the grasses. The results of the present investigation support the prediction that C4 species should be proportionally more successful in habitats characterized by high temperatures, high irradiance and low moisture.