Vegetation on Limestone in the Huachuca Mountains, Arizona

Abstract

-Major plant communities on open limestone slopes (1600-2100 m) in the Huachuca Mountains of southeastern Arizona are pine-oak woodland, mesic phase Cercocarpus breviflorus scrub, and xeric phase Cercocarpus scrub. Other communities of more limited areal extent include pine forest, deciduous woodland, and desert grassland. Vegetation patterns as represented by the first axis of reciprocal averaging ordination are strongly correlated (Spearman's r = 0.86, p 2 mm diameter). Levels of organic matter and exchangeable calcium were high in these soils, whereas extractable iron and aluminum were generally undetectable. Data from Fort Huachuca, located at 1422 m near the northeastern base of the range, illustrate general climatic trends in the study area. The mean annual precipitation is 372 mm, of which 114 mm (31%) falls in the month of July, the start of the summer rainy season (Sellers and Hill, 1974). Wallmo (1955) estimated that precipitation at the highest elevations in the Huachucas exceeds 750 mm annually, and snow accumulations in winter may be a meter or more (Swarth, 1904). Mean annual temperature at Fort Huachuca is 16.8°C (7.9°C and 25.3°C in January and July, respectively). Regional lapse rates are -0.0073°C'm-' for mean July temperature and 0.347 mm.m-' for mean annual precipitation (Wentworth, 1976). Vegetation at the lowest elevations in the Huachucas (below 1370 m) is desert scrub, with a zone of desert grassland (1370-1520 m) above (Wallmo, 1955). Between 1520 m and 2130 m many sites support dense scrub dominated by evergreen oaks (encinal). Pine-oak woodlands are found above 1680 m and extend to 2290 m, while mixed coniferous forests occur as low as 1980 m and extend to the highest peaks. Occasional stands of quaking aspen (Populus tremuloides) are found on the highest, coolest sites. Superimposed on the elevational zonation of communities are shifts caused by topography and parent material. Exposed ridges and open south-facing slopes are xeric while north-facing slopes, draws, and canyons of similar elevation are relatively mesic. Vegetation on limestone usually differs in species composition, structure, and physiognomy from that found on soils derived from the more widespread acidic rocks, such as granite and quartzite (cf. Whittaker and Niering, 1968b; Wentworth, 1981). METHODS.-Topographic and geologic maps were employed to locate sample sites representing the variation in elevation, slope, aspect, and topographic position on limestone. During the summer and fall of 1973, vegetation at each of 77 sites was measured in a 0.1 ha (20 X 50 m) quadrat extending 10 m to either side of a 50 m tape (Whittaker and Niering, 1965). Site information included location, bedrock type, elevation, slope, aspect, topographic position, and exposure. Visual cover estimates for all vascular plant species (and miscellaneous classes such as litter and bedrock) were made in columns projected vertically upward from the soil surface in 25 m2 subquadrats arranged at 1 m intervals on alternate sides of the 50 m tape. Cover estimates were then averaged over the 25 subquadrats for each species or miscellaneous class. Each 0.1 ha quadrat was also searched for species not occurring in the subquadrats. The vegetation of each quadrat was classified by physiognomy and dominance into one of several community types. The classification was refined through use of reciprocal averaging ordination (Wentworth, 1976) and also confirmed by subsequent numerical classification of the quantitative data (Wentworth, unpublished). The various types and the numbers of quadrats allocated to these were: pine-oak woodland of open slopes (15), mesic phase Cercocarpus scrub of open slopes (23), xeric phase Cercocarpus scrub of open slopes (22), canyon deciduous woodland (2), canyon pine forest (3), scrub and woodland of draws (6), high elevation ( > 2100 m) scrub and woodland (2), low elevation ( < 1600 m) desert grassland (2), and vegetation of heterogeneous parent material (2). Nearly all of the 77 sites were judged to be free of disturbances. However, the 2 low elevation sites in desert grassland had burned recently (alternative sites were not available), and fire scars on large shrubs or trees in 6 higher elevation sites provided evidence of past fires. Only 1 site was judged to have been grazed by cattle in the recent past, while 2 others showed evidence of minor disturbance by native fauna. No evidence of logging or woodcutting was noted in any of the sites sampled. 386 vol.30, no.3 This content downloaded from 207.46.13.180 on Thu, 08 Sep 2016 06:03:11 UTC All use subject to http://about.jstor.org/terms Wentworth-Vegetation on Limestone 6700 I PINEOAK WOODLAND / w Pinus cembroides / 6500 o00 Quercus arizonica */ zu Cercocarpus ./ < u breviflorus