Vegetation of the Santa Catalina Mountains, Arizona: A Gradient Analysis of the South Slope

Abstract

Vegetation of the southwest slope of the Santa Catalina Mountains of southeastern Arizona was sampled and transects prepared for 1,000—ft (305 m) elevation belts on granite and gneiss soils from the summit forests (2,440—2,750 m) to the base of the mountains (900 m). Transects also represented subalpine forests above 2,750 m in the Pinaleno Mts. and vegetation of the valley plain or bajada below the mountains, and samples were taken from volcanic soils below 900 m in the Tucson Mts. Principal community—types from high elevations to low are: subalpine forest (Picca engelmanni in the Pinaleno Mts. and Abies lasiocarpa), montane fir forest (Abies concolor, Pseudotsuga menziesii), pine forests (Pinus ponderosa, P. strobiformis), pine—oak forests (P. ponderosa, Quercus hypoleucoides), pine—oak woodlands (P. ponderosa, P. chihuahuana, Q. hypoleucoides, Q. arizonica), pygmy conifer—oak scrub (Pinus cembroides, Juniperus deppeana, Q. arizonica, Q. emoryi, Arctostaphylos pringlei, A. pungens, monocot shrubs), open oak woodland (Q. emoryi, Q. oblongifolia, Vauquelinia californica, monocot shrubs, and grasses), desert—grassland (Agave schottii, Haplopappus laricifolius, and grasses), Sonora desert of mountain slopes (north—slope shrub phase, and south—slope spinose—suffrutescent phase), upper bajada desert (Cercidium microphyllum, Franseria deltoidea), and lower bajada desert (Larrea tridentata). Forests of canyons and arroyos are also described. Relations of communities to elevation and topograhic moisture gradients are represented in a mosaic chart. Physiognomic relations of communities are represented in charts of growth—form coverage in relation to elevation and topographic moisture gradients. Growth—form diversity increases from high—elevation forests strongly dominated by evergreen—needleleaf trees to desert of lower mountain slopes in which pinnate leguminous trees, spinose shrubs, suffrutescent semi—shrubs, and stem—succulents share dominance. Among Raunkiaer life—forms hemicryptophyte species are most numerous at middle and higher elevations, phanerophyte species at lower elevations. In open oak woodlands and desert grasslands phanerophytes, hemicryptophytes, and suffrutescent chamaephytes each make up about one—third of the perennial flora. Desert floras of mountain slopes are characterized by predominance of suffrutescent chamaephytes over both phanerophytes and hemicryptophytes, and large numbers of therophyte species. Analysis in terms of geographic areas of species shows decreasing numbers of Rocky Mountain, Western, and Northern species from high—elevation forests downward, increasing numbers of Southwestern and Latin American species at lower elevations. Madrean species of the Mexican Plateau and Southwestern species predomonate in pine—oak forests and woodlands and pygmy conifer—oak scrub. Sonoran, Chichuahuan, and Latin American species predominate in the desert of lower mountain slopes, and widely distributed Southwestern species in the Larrea desert. Flora of the Catalina Mountains is rich and community species diversities are high. Species diversities increase toward lower elevations desert—grasslands and deserts of lower mountain slopes are among the riches communities in the United States. Floristic diversity is higher in continental than maritime climates, as indicated by comparison of species diversities and community differentiation along topographic moisture gradients in the Catalinas and in California mountains.