Vegetation on Limestone and Granite in the Mule Mountains, Arizona

Abstract

Gradient analysis was employed to determine vegetation pattern in relation to environment on limestone and granite in the Mule Mountains, Cochise County, Arizona, USA, using data obtained from 175 0.1—ha (20 x 50 m) quadrats located over ranges of elevation and topographic position. Sequences of major community types from mesic to xeric positions on the mountain slopes were pygmy conifer—oak scrub, open oak woodland, and desert grassland on granite and Cercocarpus breviflorus scrub, mesic phase Chihuahuan desertscrub, and xeric phase Chihuahuan desertscrub on limestone. Calcareous bajadas (alluvial plains) southwest of the Mule Mountains support a disturbed Chihuahuan desertscrub type. An environmental scalar was developed to quantify site position along complex—gradients of elevation and topographic position. The scalar was correlated with indirect ordination axes from reciprocal averaging, principal components analysis, and Wisconsin polar ordination. Over similar ranges of elevation and topographic position, granitic sites have higher species richness, greater importance of herbaceous and arborescent species, and lower importance of shrubby species than limestone sites. Both rock types support vegetation rich in widely distributed southwestern species but the vegetation on limestone has a distinctly Chihuahuan character, while that on granite is Madrean. Investigation of species distributions indicated a modal shift toward more mesic environments on limestone as compared to granite. This shift complemented other observations which suggested more xeric conditions on limestone. The low—elevation plant communities on limestone represent the northwesternmost occurrence of the Chihuahuan Desert formation. This may be due to three factors: (1) limestone habitats are arid (as judged from observations of vegetation); (2) increased aridity favors desert species, but the local climate (particularly cool winter temperatures) selects for Chihuahuan, rather than Sonoran, species; and (3) among desert species the Chihuahuan are better adapted to calcerous soils, which occur widely in the Chihuahuan Desert.