The Forest Vegetation at Higher Altitudes in the Chiricahua Mountains, Arizona

Abstract

Forty 250-m2 plots were distributed at various aspects at altitudes above 2000 m. Tree diam were measured in each plot, and the frequencies of herbs and shrubs were counted in smaller quadrats. The plots were arrayed in a three-dimensional ordina- tion according to the method of Bray and Curtis with modifications by the authors; other ordination techniques were used as well. An environmental index (essentially a moisture index), computed from aspect, altitude and slope, correlated significantly with the axes of the ordinations. The ordinations produced five well-separated groups of stands that exhibited characteristic vegetational and environmental properties. The major tree species, ordered from xeric to mesic, are: Quercus arizonica, Pinus leiophylla var. chihuahuana, P. ponderosa (principally var. arizonica), Q. gambelii, P. strobiformis, Pseudotsuga menziesii, Abies concolor, Populus tremuloides and Picea engelmannii. The positions of A. concolor and Q. gambelii are less certain than those of the other species. Size structure reveals that Pseudotsuga menziesii, the dominant tree species in the study area, will be increasing in dominance in the future, mainly at the expense of Pinus ponderosa, particularly in moderately moist and shady sites. Picea engelmanni and Pinus leiophylla are stable in the stands that they dominate at the mesic and xeric ends of the gradient, respectively. Populus tremuloides is declining in the stands it now dominates due to the better reproduction and establishment of Abies con- color and Picea engelmannii. Pinus strobiformis is the species of second highest con- stancy and density, but it fails to dominate a single one of our stands. Zonation is not particularly strong and may be changing. Pinus ponderosa dominates altitudes below 2600 m, Pseudotsuga menziesii dominates altitudes above 2800 m, and the two species share dominance between 2600 and 2800 m. Other species dominate stands throughout the region and are variously restricted by altitude and aspect. Our assessment of the rather incomplete early records, as well as the present data, leads us to conclude that Pseudotsuga menziesii has increased strongly since the fires that ravaged the region a century ago. If the present patchwork of stands dominated by a variety of species also reflects the great fires, then the region may become more strongly zoned and less diverse as time goes on.