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.

Nomenclature for most species follows Kearney & Peebles (1969). Gould (1951) and Benson(1969) were used for identification and nomenclature of Gramineae and Cactaceae, respectively. Gould (1968) was used for taxonomy of Gramineae at the tribe and subfamily levels. Voucher specimens are on deposit in the L. H. Bailey Hortorium of Cornell University.

I wish to express my appreciation to J. A. Teeri for suggesting that I investigate distributions of C₄ plants in southeastern Arizona, and to J. D. Elson for valuable assistance with preliminary data analyses. The data discussed in this paper were collected as part of a Ph.D. dissertation completed at Cornell University under the direction of R. H. Whittaker. U. Blum, S. C. Huber, R. K. Peet and J. F. Reynolds provided helpful reviews of the manuscript. I gratefully acknowledge financial support provided by the North Carolina Agricultural Research Service, the National Science Foundation, Cornell University, and E. I. DuPont de Nemours and Company.

Paper No. 8258 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27650 USA.