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Home , Chiricahua Mountains, Huachuca Mountains, Mule Mountains, Rincon Mountains, Santa Teresa Mountains, Sierra Madre Occidental

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An Overview of the Flora of the Sky Islands, Southeastern : Diversity, Affinities, and Insularity

Steven P. McLaughlin 1

Abstract.-The "sky-island" of southeastern Arizona is roughly coincident with the Apachian District of the Madrean Floristic Province. The region extends to the Pinaleno and Santa Teresa 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 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 are comparately rich in species while the are comparatively depauperate. Based on their distributions within the , 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 ; 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 , and about 1/5 reach southern . In comparison to true insular floras, those of the sky-island region display high species diversity, comparatively low degree of local and regional , and low percentage of exotic species.

INTRODUCTION southwestern , northeastern Sonoran, and northwestern . 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 and, landscape. In other words, flora refers only to the on the higher ranges, -oak forest and ­ presence of species, not to their 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 in the north to the Baboquivari Mountains in the west; the of southwestern New Mexico are also STUDY AREA part of this region. The isolated mountain ranges of 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

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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 ; 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 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 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 . 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 . 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 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 , 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, , 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 ; AM, Animas correspondingly smaller. The Pinaleno Mountains Mountains; BA, Buenos Aires National Wildlife Refuge; CH, (McLaughlin, 1993) have the greatest elevation National Monument; CW, Chiricahua Wilderness; FB, 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. Shreve (1919) pointed Mountains; SR, Northern ; SC~ Sycamore out that in the Pinalenos there are few sharply cut Canyon; and ST, Sierra el Tigre. and well-watered . Bennett and Kun­ zmann (1992) found an association between of southeastern Arizona. The Sycamore Canyon topographic "roughness" and plant-species rich­ flora illustrates the high gamma diversity of major ness among 6 mountain ranges from the canyon environments with their varied array of southwest. In comparison to other sky-island microhabitats. Chiricahua National Monument ranges, the Pinalenos also seem to be somewhat (Reeves, 1976) also contains several major can­ drier (Martin and Fletcher, 1943; McLaughlin, yons. While Buenos Aires National Wildlife 1993). Refuge does not span a great elevational range, it Sycamore Canyon Natural Area (Toolin et al., does have a large areal extent with many scattered 1980) and the Buenos Aires National Wildlife Ref­ aquatic and wetland . uge (McLaughlin, 1992b) have relatively large The floras for Fort Bowie National Historic floras for their narrow elevation ranges. Both are Site (Warren et al., 1992), the northern Santa Rita situated in the relatively wet southwestern section Mountains (McLaughlin and Bowers, 1990), and

Table 2.-Local floras from the Apachian Floristic District. Number Elevations Flora of s~ecies High Low Range Rincon Mountains 959 2643 915 1728 Sierra el Tigre 953 2360 700 1660 Huachuca Mountains 907 2887 1525 1362 Pinaleno Mountains 786 3267 1150 2117 Animas Mountains 638 2597 1433 1164 Northern Santa Rita Mountains 628 1919 1006 913 Chiricahua National Monument 619 2230 1598 632 Sycamore Canyon Natural Area 593 1464 1037 427 Buenos Aires National Wildlife Refuge 568 1401 926 475 Fort Bowie National Historic Site 438 1600 1400 200 Aravaipa Canyon 398 1678 763 915 Mule Mountains 387 2248 1464 784 Chiricahua Wilderness 287 2988 1876 1112

63 ).

the Animas Mountains (Wagner, 1977) all lie ap­ proximately on the regression line of fig. 2. The 700 Mule Mountains (Wentworth, 1982) and Aravaipa NCord APACHIAN 600 Canyon (Warren and Anderson, 1980) appear to en . w be somewhat species poor; both floras are prob­ U w CL ably under-collected. The notable outlier in fig. 2 en 500 "- is the flora for the Chiricahua Wilderness (Leith­ 0 0:: liter, 1980), which covers the upper elevations of W ID ~ the . The high elevations of ;:) <400 • eGa z the sky islands are notably depauperate at the z "WGB w~ community level (Whittaker and Niering, 1975) :I and apparently at the landscape level, also. 300 The gamma diversity of sky-island floras is compared with those from other floristic districts 200 in the western United States in figs. 3-4. McLaugh­ 0 500 1 000 1500 2000 2500 3000 lin (1992a) used a data base of 101 floras from the MEAN ELEVATION RANGE (m) western United States to identify 20 floristic dis­ tricts (see FLORISTIC AFFINITIES, below). In fig. 3 the average number of native species is plotted Figure 4.-Relationship between average number of native species and average elevation range of local floras In 20 floristic as a function of average area for each district; in districts from the western United States. Abbreviations are as fig. 4 average species number is plotted against In fig. 3. average elevation range. Five floras from the sky­ island region (the' Apachian District) were The plot of average species number vs. aver­ included in this analysis. The plot of average spe­ age elevation range (fig. 4) shows a much stronger cies number vs. average area (fig. 3) for this relationship. The group of 5 Apachian floras sample shows a weak species-area relationship. stands out as being farthest above the regression The Apachian floras have the second highest line. This figure shows that Apachian local floras diversity in fig. 3 despite representing a compara­ have the highest average diversity, when cor­ tively small average area. rected for elevation range, of all floristic districts from the western United States. The 5 floras in­ 700 cluded in this analysis had an average of 602 species and an elevation range of 1056 m; the sam­ • NCord 600 "APACHIAN ple of 13 Apachian floras in Table 2 spans a en 8 CAZ • peA similar average elevation range (1038 m) but has ""U .NMD CL""en EGB • SCord somewhat higher average diversity (627 species). 500 • SNV • CHI "- • MOG 0 • CCord The high gamma diversity in the sky-island iii: • CMC • AZU region is a consequence of both the high alpha ID • CLB "":I ;::, 400 • CMD diversity of some of its plant communities (Table z • VAN • CGB z ·WGB 1) and high beta diversity. Temperature and pre­ ~ ~ NCP :I Cipitation change rapidly over short map 300 II SCP distances on mountain ranges in the sky-island re­ gion, leading to rapid species turnover and community change. Stated in another way, the re­ o 500 1000 1500 2000 2500 3000 3500 gion has very high ecosystem-level diversity. MEAN AREA (sq. km)

Figure 3.-Relatlonship between average number of native species Total Diversity (Epsilon) of the and average area of local floras in 20 floristic districts from the Sky-island Region western United States. Abbreviations are: AZU, Arizona Upland; CAZ, Central Arizonan; CCord, Central Cordilleran; CGB, Central ; CHI, Chihuahuan; ClB, Columbia The flora of the sky-island region within the Basin; CMC, Cismontane California; CMD, Colorado-Mojave United States is reasonably well known. I have Desert; EGB, Eastern Great Basin; MOG, Mogollon; NCord, Northern Cordilleran; NCP, Northern Colorado ; NMD, compiled a preliminary checklist of this flora, Northern Mojave Desert; PCA, Peninsular California; SCord, based both on written works (local floras, regional Southern Cordilleran; SCP, Southern ; SNV, treatments, monographs) and collections housed Sierra ; VAN, Vancouverlan; and WGB, Western Great Basin. at the University of Arizona herbarium. The total

64 flora of the sky-island region in southeastern Ari­ ranges. It is then possible to determine which 2 zona, an area of approximately 40000 km , floristic elements make up a significant propor­ consists of approximately 1940 native and 166 ex­ tion of any particular flora and how the spectrum otic species (Table 3). The largest families are the of floristic elements varies with latitude, longi­ Asteraceae (308 native spp.), Poaceae (204 native tude, elevation, substrate, and other spp.), Fabaceae sensu lato (161 native spp.), environmental factors. Such an analysis of floristic Cyperaceae (71 native spp.), Euphorbiaceae (57 affinities using floristic elements simplifies the native spp.), and Scrophulariaceae (52 native comparison of floras within and between . spp.). It may also be helpful in understanding the phyto­ Southeastern Arizona represents only about geographic history of a region. half of the sky-island region (fig. 1). The size of Shreve (1915) was the first to discuss the af­ the entire flora of the region is not known. There finities of the flora of southeastern Arizona in any are 225 species in the flora of the Bavispe region detail. He noted that the plants of the desert have (White, 1948) that do not occur in the sky-island strong affinities with the floras of the Sonoran and region of southeastern Arizona. The total number Chihuahuan deserts and almost no relationship to of sky-island species not found in Arizona is those of the Mojave and Great Basin deserts. The likely to be between 2 and 4 times this number, flora of the encinal (oak woodland) region of the giving an estimate for the regional flora of ca. Santa Catalinas appeared to Shreve to be related 2300-2800 species. to that of the Sierra Madre of northwestern Mex­ ico and the . The forest flora appeared to have affinities both with the Rocky Mountains FLORISTIC AFFINITIES to the north and the mountains of Mexico to the south. Shreve also noted that there was a distinct Affinity refers to the geographical relation­ endemic element in the forest flora, i.e. species ships of the flora. Species may be either endemic restricted to the isolated ranges of southern Ari­ (restricted to the area under consideration) or oc­ zona and New Mexico. cur both within and outside the area. In a local or The of the higher elevations regional flora endemics generally constitute a of the sky islands has received much attention. small proportion of the total number of species. Moir and Ludwig (1979) felt that the floristic af­ The nonendemics can be classified, either subjec­ finities of the mixed-conifer and spruce-fir forests tively or objectively, into floristic elements - of Arizona and New Mexico were primarily with groups of species with more or less coincident the northern Cordillera (Petran) region, i.e., the Table 3.-Summary of the flora of the Apachian Floristic Rocky, Cascade, and mountains. District In southeastern Arizona. Muldavin and DeVelice (1987) classified all plant Taxon Native S~~. Exotic S~~. Total S~~. species found in their montane habitat-type plots Ferns and Fern Allies 67 0 67 as Petran, Madrean, Southwest Endemics, or Gymnosperms 17 0 17 other; they found that spruce-fir and white fir for­ Flowering Plants ests are strongly Petran in floristic composition, Dicotyledons 1477 108 1585 Douglas fir and ponderosa pine forests contain a Monocotyledons 3n 58 435 mix of Petran and Madrean species, and TOTAL 1938 166 2104 pine and Chihuahua pine forests (i.e., the pine­ oak woodlands), are strongly Madrean. Largest Families: Robinson (1968) noted that Madrean species Asteraceae 308 20 328 (taxa with southern affinities) form an important Poaceae 204 56 260 element in the high-elevation flora of the Chirica­ Fabaceae 161 10 171 hua Mountains, particularly on south-facing Cyperaceae 71 1 72 slopes. In the lower-lying Rincon Mountains, spe­ Euphorbiaceae 57 2 59 cies with Madrean affinities predominate at all Scrophulariaceae 52 8 60 elevations (Bowers and McLaughlin, 1987); even Cactaceae 42 0 42 above 2300 m species with northern affinites ac­ Malvaceae 39 40 count for less than half of the flora. Lamiaceae 35 5 40 Whittaker and Niering (1964) were the first to Boraginaceae 35 0 35 attenlpt to define a comprehensive· set of floristic Convolvulaceae 34 1 35 categories for southeastern Arizona, Using re­ Brassicaceae 33 10 43 gional manuals and herbarium records; they " 65 grouped species into "areal types" as a means of Wentworth concluded that the Mules represented analyzing the floristic affinities of the plants of the the northwesternmost extent of Chihuahuan de­ Santa Catalina Mountains. These areal types sert vegetation and flora. Wentworth (1985), group species by pattern of geographic distribu­ however, found relatively small proportions of tion, i.e., they are essentially geographic elements. Chihuahuan species on higher-elevation lime­ This subjective classification has been used in stone sites in the Huachuca Mountains. other studies (Bowers, 1980; Wentworth, 1982, McLaughlin (1992a) developed a hierarchical 1985). system of floristic elements and floristic areas us­ Whittaker and Niering (1965) found, in agree­ ing factor analysis, an ordination technique, with ment with Shreve (1915), that their Rocky presence-absence data from 101 local floras from Mountain, Temperate, Northern, and Holarctic the western United States. This data base was di­ areal types-those with northern affinities-were vided into three subsets: narrow species, those best represented at high altitudes. Their Madrean occurring in 9 or fewer local floras; regional spe­ areal type was most abundant at mid-latitudes in cies, those occurring in 10-19 local floras; and the oak and pine-oak woodlands, and their Sono­ widespread species, those occurring in 20 or more ran and Latin American areal types predominated local floras. Five floristic areas were defined for at low elevations. Endemics appeared to be uni­ the widespread species, nine for regional species, formly distributed at all elevations above 915 m and 20 for narrow species. The floristic areas de­ (Table 4). fined at these different scales were nested in a Whittaker and Niering (1965) stated that the hierarchical manner, and corresponded to tradi­ affinities of the desert grassland were with the tional phytogeographic categories-provinces, short-grass plains east of the Rocky Mountains. subprovinces, and districts, respectively, for wide­ They placed relatively few species in their Plains spread, regional, and narrow species. Floristic areal type, however, and although these were elements corresponding to each floristic area can most abundant in the desert grassland, the overall be determined indirectly from the ordination of affinities of the desert grasslands were similar to floras (McLaughlin, 1994). those of the open oak woodlands. The 5 floristic provinces in the western United The Chihuahuan areal type was not well­ States are the Cordilleran Province, including the represented in the flora on the south side of the Rocky Mountains, Cascade Mountains, the higher Santa Catalinas. Whittaker and Niering (1968) ranges of the central Great Basin, and the north­ found more species with Chihuahuan affinites on western part of California; the Intermountain on the northern side of the range be­ Province, including most of the Great Basin, the tween 1830-2135 m. Wentworth (1981) compared Columbia Plateau, the Colorado Plateau, and the the floras of and limestone substrates in Wyoming Basin; the Sonoran Province, including the Mule Mountains and found an even stronger both the Sonoran and Mojave desert areas; the pattern. Granite slopes had a high percentage of Californian Province, including the cismontane Madrean species while limestone slopes had a portions of California exclusive of the northwest­ high proportion of Chihuahuan species. ern coastal area, and the Madrean Province. The

Table 4.-Florlstlc affinities of vegetation zones within the Santa Catalina Mountains (Whittaker and Nlerlng, 1965). Data In the table are the total numbers of species found In all study plots of each vegetation type over the elevation al gradient. The spruc.flr forests are actuall' from the Plnaleno, not the Catalina Mountains. Abbreviations for areal types are: RM, Rocky Mountain; W, Western; H, Holarctlc; N, Northern: T, Temperate: MA, Madrean; SW, Southwestern; P, Plains; CH, Chlhuahuan; LA, Latin American; SO, Sonoran; and End, Endemic. Vegetation Elev. (m) RM W H N T MA SW P CH LA SO End Total Spruce-fir 2950-3260 13 5 2 3 3 2 4 2 35 Mixed-conifer 2740-2920 11 13 4 4 2 2 7 3 47 Pine forest 2440-2740 11 9 5 11 11 5 55 Pine-oak forest 2130-2440 9 6 3 17 18 1 2 59 Pine-oak woodland 1830-2130 6 3 19 19 2 3 56 Pygmy conifer-oak 1520-1830 2 3 23 28 1 4 1 3 66 Open oak woodland 1400-1700 4 22 38 4 2 9 6 4 92 Desert Grassland 1220-1700 2 17 39 6 5 13 '15 5 105 Spinose Desert 915-1220 4 33 2 4 12 21 3 82 Upper Bajada Desert 850 20 6 5 11 44 Lower Bajada Desert 760 12 4 4 22

66 Table S.-Floristic affinities of the flora of the sky-islands region of southeastern Arizona. Numbers are percentages of the total flora of each area. Abbreviations for floristic elements are: MAD, Madrean; CORD, Cordilleran; SON, Sonoran; INT, Intermo untain; CAL, Californian; APA! A~achian; CHI, Chihuahuan; AUp, Arizona U~land; and MOG, Mogollon. Flora All Species: Narrow Species: MAD CORD SON CAL INT APA CHI AUP MOG Total Flora of Sky-island Region in SE Arizona 57.2 17.4 15.1 5.6 4.7 30.9 9.2 6.4 5.5

Pinalenos, above 2745 m 20.8 74.3 0.0 1.2 3.7 9.0 0.4 0.0 14.7 Chiricahua Wilderness 57.0 36.6 0.7 3.9 1.8 30.5 2.1 0.0 10.8 Pinalerio Mountains (alQ 52.7 27.2 10.7 4.8 4.6 20.9 2.4 2.0 5.6 Huachuca Mountains 69.9 18.0 5.0 3.6 3.5 38.8 6.4 0.7 6.1 Animas Mountains 72.4 12.9 6.6 3.4 4.7 28.1 8.9 0.9 5.0 Rincon Mountains 61.5 10.6 19.4 5.6 2.9 31.6 4.3 6.9 2.1 No. Santa Rita Mtns 71.9 4.0 16.8 4.1 3.2 31.8 6.6 4.8 1.1 Fort Bowie NHS 72.6 4.6 15.9 2.3 4.6 19.8 9.7 2.8 1.6 Buenos Aires NWR 61.0 3.0 23.0 9.4 3.6 27.7 5.3 8.9 0.4 Mule Mountains 85.8 2.6 9.3 0.5 1.8 37.6 13.4 1.0 0.5

Madrean Province in the United States includes a Historic Site, and the Animas Mountains. The per­ single subprovince with 3 floristic districts: the centage of Apachian species is particularly high in Apachian District in southeastern Arizona and the Huachuca and the Mule mountains. southwestern New Mexico; the Chihuahuan Dis­ The Madrean elements include widespread trict in southern New Mexico and west ; and and regional species and narrowly distributed a Central Arizona District occurring across the species in the Apachian, Chihuahuan, and Central center of the state below the and Arizonan elements. "Narrowly distributed" refers above the lower deserts (McLaughlin, 1992a). to the distribution of species in the western In this classification, the sky-island region is United States. Many of the Madrean species that coextensive with the Apachian District of the Ma­ are narrowly distributed in the western US are drean Floristic Province. The Arizona Upland much more widespread south of the international District of the Sonoran Floristic Province lies to border. In order to better characterize these Ma­ the west, the Central Arizona District to the north­ drean elements, I tabulated the number of species west, the Mogollon District of the Cordilleran classified as Madrean that also occur in the floras Province to the northeast, and the Chiltuahuan of Durango (Gonzalez et al., 1991), Valle de Tehua­ District to the east and southeast. can (Davila et al., 1993), and Chiapas (Breedlove, Although this system consists of 34 floristic 1986) (Table 6). A higher percentage of species elements, only a few elements are important in with widespread and regional distributions ex­ any particular region. In southeastern Arizona, tend to Durango than species with narrow Madrean elements account for over half (57%) of distributions. In other words, species that are the species in the flora (Table 5). Cordilleran and more widely distributed north of the international Sonoran elements are also important, accounting border are also more widely distributed south of for 170/0 and 15% of the total flora. the border. However, a similar percentage of The affinities of selected local floras are also widespread and regional Madrean and Chihua­ given in Table 5. The flora of the Pinalefto Moun­ huan species extend to the Tehuacan Valley, and a tains above 2745 m, i.e., within the mixed-conifer higher percentage of Apachian species extend to and spruce-fir forests, has a flora with mostly Cor­ Chiapas. The high proportions of all elements (ex­ dilleran affinities. The flora of the Chiricahua cept for Central Arizonan) reaching Durango Wilderness area, which lies mostly at high eleva­ indicate that the majority of the taxa from south- tions, is primarily Madrean, however, with a large Cordilleran component. The Mogollon element is Table 6.-Percentages of Madrean elements represented In the well represented in the high-elevation flora of the flora of south eastern Arizona also found in the floras of Durango (Gonzalez et al., 1991), Valle de Tehuacan (Davila et Pinalefto Mountains and in the Chiricahua al., 1993), and Chia~as (Breedlove, 1986). Wilderness. The Sonoran elements constitute a Element Durango Tehuacan Chiapas high percentage of the floras of Buenos Aires Widespread and Regional 63 23 14 Wildlife Refuge and the Rincon Mountains. The Apachian 49 18 23 Chihuahuan element is best represented in the flo­ Chihuahuan 59 24 9 ras of the Mule Mountains, Fort Bowie National Central Arizonan 17 5 2

67 eastern Arizona that are classified as Madrean are The endemics account for just 2% of the estimated distributed throughout the northern Sierra Madre regional flora. Occidental. Approximately 17% of all the species classified as Madrean reach Chiapas; this group can best be characterized as a widespread INSULARITY Neotropical element. Almost 600 species, nearly a third of the flora The names "sky-island region" and "Madrean of southeastern Arizona, is classified as Apachian. Archipelago" imply that the biota of the region This means that their distribution in the western has characteristics similar to that found on true United States in concentrated in the Apachian islands. Floristic District. Only a small proportion of these­ Certain floristic attributes of the sky islands -54 species, or less than 100/0--appear to be can be compared with those from two oceanic ar­ endemic to the Apachian District (Table 7). The chipelagos, the Hawaii Islands (Wagner et al., number of Apachian endemics is likely to 1990) and Galapagos Islands (Wiggins and Porter,. shrink-not grow-with further study. Although 1971) (Table 8). There are similarities in the inter­ additional rare taxa are likely to be discovered island distributions of species. The larger ranges from within the area, others will prove to be more in the sky-island region each contain about 35- widespread, and some (Plummera ambigens?, Poly­ 45% of the total regional flora, similar to the gala piliophora?) may not prove to be good species. percentages for the larger islands in Hawaii (39- 47%). The larger islands in the Galapagos have a Table 7.-Sky-island endemics in the flora of southeastern Arizona. higher percentage (46-71 %) of the flora of the en­ Family Species tire archipelago. The inter-island similarity is Aspleniaceae Woodsia cochisensis Windham approximately the same in the sky islands (62%) Apocynaceae Amsonia grandif/ora Alexander, A. kearneyana as in the Galapagos (65%), both of which are Woodson Asteraceae Brickellia floribunda Gray, B. lemmon; Gray, higher than in the Hawaiian Islands (54%). How­ Cirsium rothrockii (Gray) Petrak, Erigeron ever, the regional flora of the Apachian Districts he/iographis Nesom, E. kuschei Eastw., E. has fewer endemics (2-3%) than either the Hawai­ lemmoni Gray, E. scopulinus Nesom & Roth, Hieracium carneum Greene, H. lemmon; Gray, ian (89%) or Galapogos (30%) floras; the Hymenoxys quinquesquamata Rydb., Plummera sky-island region also has a much lower percent­ % ambigens Blake, P. flori bunda Gray, Tagetes age of exotic species (ca, 8 ) than either the lemmoni Gray, Viguiera triloba (Gray) Olsen Hawaiian (47%) or Galapogos (15%) floras. Total Brassicaceae Arabis tricornuta Rollins, Draba petrophila Greene Cactaceae Coryphantha recurvata (Engelrrr.) Britt. & Rose, floras are known from too few of the sky-island Cochiseia robbinsorum W. H. Earle, Echinocereus region mountains to compare their species-area ledingii Peebles relationships with those of true archipelagos. The Caryophyllaceae Silene thurberi S. Wats. larger ranges in the sky-island region are much Chenopodiaceae Atriplex griffithsii StandI. smaller in area than the larger Hawaiian and Crassulaceae Graptopetalum bartramii Rose Galapagos islands, but the floras of the mountains Euphorbiaceae Tragia laciniata (Torr.) Muell.-Arg. in the sky-island region are much larger (780-950 Fabaceae Acacia millefolia S. Wats., Astragalus hypoxy/us S. native species) than those of the larger islands of Wats., A. thurberi Gray, Dalea tentaculoides Gentry, Lupinus lemmon; C. P. Smith, Phaseolus Table B.-Floristic comparison of Hawaiian, Galapagos, and "Sky" supinus Wiggins & Rollins Islands, Hydrophyllaceae Phacelia arizonica Gray Hawaii Galapagos Sky Islands Islands Included Hawai'i Isabela Huachucas Lamiaceae Agastache brevif/ora (Gray) Epling, Hedeoma dentatum Torr., Salvia lemmoni Gray Kaua'i San Cristobal Pinalenos Polygalaceae Polyga/a orthotricha Blake, P. pili ophora Blake Maui San Salvador Rincons Ranunculaceae Delphinium andesicola Ewan O'ahu Santa Cruz Rosaceae Potentilla albiflora L. Williams O. 2 Mean Island Size (km ) 3886 1675 488 Rubiaceae Hedyotis greenei (Gray) W. H. Lewis Mean island Altitude (m) 2521 1048 1736 Rutaceae Choisya arizonica Stand!., C. mo/lis StandI. Mean Number of Native 421 278 884 Scrophulariaceae Limosella pubiflora Pennell, Penstemon disc%r Species Keck Percentage Endemic 88.8 29.9 2.9 Agavaceae Agave palmeri Engelm., A parviflora Torr., Yucca Species (of total native schottii Engelm., Y. thornberi McKelvey species in archipelago) Percentage Exotic Species 47.31 Liliaceae Allium pfummerae S. Wats. 4.8 8.2 (of total species in Poaceae Boute/oua eludens Griffiths, Muhlenbergia archipelago dubioides C. D. Goodding, M. xerophila C. O. Mean Inter-Island Similarity .538 ,654 ,620 Goodding, Setaria arizonica Rominger (Otsuka's index)

68 either the Hawaiian (370-450 native species) or McLaughlin, S. P. 1993. Additions to the flora of the Pi­ Galapagos (220-390) archipelagos. Thus, in terms A naleno Mountains, Arizona. Journal of the of endemism, percentage exotics, and overall di­ Arizona-Nevada Academy of Sciences 27: 1-27. versity, the floras of the mountains of the McLaughlin, S. P. 1994. Floristic plant geography: the clas­ sification of floristic areas and floristic elements. sky-island region appear to differ substantially Progress in 18: 185-208. from those of true islands. McLaughlin, S. P., andJ .E. Bowers. 1990. A floristic analysis and checklist for the northern Santa Rita Mountains, ~ Pima Co., Arizona. The Southwestern Naturalist 35: LITERATURE CITED 61-75. Moir, W. H., and J. A. Ludwig. 1979. A classification of spruce-fir and mixed conifer habitat types of Arizona Bennett, P. S., and M. R. Kunzmann.1992. Factors affecting and New Mexico. USDA Forest Service Res. Pap. 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