This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. An Overview of the Flora of the Sky Islands, Southeastern Arizona: Diversity, Affinities, and Insularity Steven P. McLaughlin 1 Abstract.-The "sky-island" region of southeastern Arizona is roughly coincident with the Apachian District of the Madrean Floristic Province. The region extends to the Pinaleno and Santa Teresa Mountains in the north and to the Baboquivari Mountains in the west. The total flora of this region includes approximately 2100 species of which 166 are exotics. The largest mountain ranges within the sky-island region of southeastern Arizona have local floras possessing about 1/3 to Y2 of the regional flora. For their size and elevational range, the Rincon and Huachuca Mountains are comparately rich in species while the Pinaleno Mountains are comparatively depauperate. Based on their distributions within the western United States, the native species of southeastern Arizona can be classified as Madrean (57%), Cordilleran (17%), Sonoran (15%), Californian (6%), and Intermountain (5%). The Madrean element is a heterogenous group of species found mostly south of the international border and reaching their northern limits in southern Arizona; included are species with Chihuahuan, Sierra Madrean, and Neotropical affinities as well as Apachian (sky-island) endemics. Over half of the Madrean species from southeastern Arizona extend to Durango, and about 1/5 reach southern Mexico. In comparison to true insular floras, those of the sky-island region display high species diversity, comparatively low degree of local and regional endemism, and low percentage of exotic species. INTRODUCTION southwestern New Mexico, northeastern Sonoran, and northwestern Chihuahua. This region is char­ Flora refers to the plant species present in a acterized by its many small, isolated mountain region, irrespective of their importance in the ranges (fig. 1), covered with oak woodland and, landscape. In other words, flora refers only to the on the higher ranges, pine-oak forest and conifer­ presence of species, not to their dominance or ous forest. These ranges are separated from one abundance. The study of flora is referred to as another other by plains and valleys covered with floristic plant geography. The purpose of this pa­ desert and desert grassland. per is to discuss the flora of the sky-island region, Included in the sky-island region are all the particularly that of southeastern Arizona, in terms mountain ranges in southeastern Arizona from of its diversity, affinities, and insularity. the Santa Teresa Mountains in the north to the Baboquivari Mountains in the west; the Animas Mountains of southwestern New Mexico are also STUDY AREA part of this region. The isolated mountain ranges of northern Mexico display clear physiographic The sky-island region refers to an area consist­ and floristic continuity with those of the south­ ing of portions of southeastern Arizona, western United States. Mountains of northern Mexico that are part of the sky-island system in­ clude the Sierra de San Luis, Sierra el Tigre, Sierra 10ffice of Arid Lands Studies, University of Arizona, Tucson, AZ. de los Ajos, Sierra San Jose, Sierra Cananea, Sierra 60 111" 110' lOS" 34" ~----~--~--~~~--~.r----------------__ ~ ,.----+34" 33" AR.IZONA MEXJ:CO 32." 31" SONORA 30" . lCILOKE1'EllS o 60 I i! 'i I o 40 KILZS H1!lU«)S., lUO 29"~--------------------'-------------------~"-------~~------~--r---~~~----L---~~29" 112" 111' 110" 109" lOS" Figure 1.-Map of the sky-island region, southwestern United States and northwestern Mexico. The boundaries for the mountain ranges correspond to the lower elevationailimits of the oak woodlands; shaded areas are coniferous forests (after Brown and Lowe, 1980). The approximate boundaries of the Apachian Floristic District are shown; the dashed line In Mexico indicates greater uncertainty in the exact placement of this boundary. 61 Azul, and Sierra de Pinitos (fig. 1). It is not clear, Table 1.-Plant-community diversity (0.1 ha plots) in the Santa however, exactly where the southern boundary of Catalina Mountains (from Whittaker and Niering, 1975). Community-type Elev. (m) No. of Species the sky-island region should be placed on floristic Corkbark fir forest 2720 15 grounds. I separate it at the point where wood­ Douglas fir-white fir forest 2640 16 land and forest vegetation form a continuous Douglas fir forest 2650 10 cover in the northern Sierra Madre Occidental Ponderosa pine-white pine forest 2740 13 (fig. 1). Ponderosa pine forest 2470 8 The sky-island region has a semi-arid climate Ponderosa pine-silverleaf oak forest 2180 12 with biseasonal precipitation. Percent summer Pine-oak woodland 2040 18 rainfall increases from northwest to southeast Pygmy conifer-oak scrub 2040 20 across the region. Total annual precipitation gen­ Open oak woodland 1310 58 erally decreases from west to east in southeastern Desert grassland 1220 46 Arizona (McLaughlin, 1992b, 1993). Spinose-suffrutescent desert 1021 41 Upper Bajada desert 870 33 Lower Bajada desert 760 6 DIVERSITY southwestern mountain ranges have been inter­ Diversity refers to the number of species in a preted as an example of high diversity at particular area. Diversity can be analyzed on sev­ intermediate productivity. It is more likely that eral scales (Whittaker, 1977): (1) alpha diversity, gradients of decreasing diversity with increasing the number of species in a plot or community, aridity (downslope) and decreasing area generally 10-100 species; (2) gamma diversity, the (upslope) interact to produce high diversities at number of species in a landscape, usually 100- midslope. 1000 species; and (3) epsilon diversity, the number Plant ecologists often measure alpha diversity of species in a region, generally more than 1000 in 0..1 ha plots. Temperate North American sites species. I apply Whittaker's (1977) concept of ep­ typically average 20-30 species/0.1 ha (Whittaker, silon diversity to that of areas the size of floristic 1977; Gentry, 1988). Oak woodlands and grass­ districts and larger, and gamma diversity to that lands in the sky-island region display of local floras. In the case of the sky-island region, comparatively high alpha diversity (40-60 spe­ epsilon diversity refers to the diversity of the en­ cies/0.1 ha plot) while forest communities have tire region, while gamma diversity refers to the comparatively low alpha diversity (10-20 spe­ diversity of the individual mountain ranges (or cies/0.1 ha). Plant communities from neotropical other comparable areas) making up the sky-island sites typically have alpha diversities of 100-250 region. In addition to these inventory diversities, species/D.l ha plot (Gentry, 1988). beta diversity and delta diversity refer to change in species composition between plots or commu­ nities within a landscape, and between landscapes DiverSity (Gamma) of Local Floras within a region, respectively (Whittaker, 1977). Patterns of gamma diversity can be investi­ gated by examining the numbers of species in Diversity (Alpha) of Plant Communities local floras. Many biogeographers have noted that the number of species increases in a regular man­ Whittaker and Niering (1975) examined spe­ ner as a function of the amount of area sampled. A cies diversity over the elevational gradient of the log-log plot of the number of species vs. area is Santa Catalina Mountains. They found that the generally a straight line. Bowers and McLaughlin highest diversities were not in the most mesic, (1982) compared species diversity in 20 local flo­ highest-productivity forests, but in the open com­ ras from throughout Arizona. They found that munities at mid elevations-in woodlands, elevation range, rather than area, provided the grasslands, and the spinose-suffrutescent desert best predictor of species diversity. Species diver­ (Table 1). Lowest alpha diversity was found in sity increases with increasing habitat diversity, ponderosa pine forest and in the lower bajada which, in mountainous areas, is more closely re­ (creosote bush) desert. Wentworth (1982), work­ lated to elevation range than to areal extent" ing in the Mule Mountains, also recorded A number of local floras have been compiled maximum species diversity in oak woodlands and (or are in progress) for different sites in southeast­ grasslands. The patterns of species diversity on ern Arizona (Table 2), including four of the larger 62 ranges-the Pinaleno Mountains, Huachuca Mountains, Rincon Mountains, and Sierra el Tigre. 1200 Number of species in this discussion refers to the 1100 number of native species only-naturalized exotic 1000 ST species are excluded. Patterns in gamma diversity - • RM en I&J 900 within the sky islands region are shown in fig. 2, (3 I&J BOO which displays species number vs. elevation en0.. .... range for the 13 local floras listed in Table 2. There 0 700 ac: I&J is a stronger relationship between species number CD 600 ::I and elevational range (r = 0.653, P = .016) than :) z 500 between log species and log area (r = 0.514, p. = 400 .072), as previously observed by Bowers and MM- 300 McLaughlin (1982). - cw Of the four large ranges, the Rincon Moun­ 200 tains (Bowers and McLaughlin, 1987) and Sierra el 0 400 800 1200 1600 2000 2400 Tigre (White, 1948) have the largest floras. The ELEVATION RANGE (m) compilation of the Huachuca Mountains flora is still in progress (Bowers and McLaughlin, this symposium). The Huachucas have a lower total Figure 2.-Relatlonshlp between number of native species and elevation range for 13 local floras from the sky-Island region. elevation range than the Rincons and their flora is Abbreviations are: AC, Aravaipa Canyon; AM, Animas correspondingly smaller. The Pinaleno Mountains Mountains; BA, Buenos Aires National Wildlife Refuge; CH, (McLaughlin, 1993) have the greatest elevation Chiricahua National Monument; CW, Chiricahua Wilderness; FB, Fort Bowie National Historic Site; HM, Huachuca Mountains; range, yet their flora is smaller than those of the MM, Mule Mountains; PM, Pinaleno Mountains; RM, Rincon three other large ranges.