This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. Trees of the Northern Sierra Madre Occidental and Sky Islands of Southwestern North America Richard S. Felger1 and Matthew B. Johnson2 Abstract.-This report covers the naturally occurring montane tree flora of the northern Sierra Madre Occidental of eastern Sonora and western Chihuahua and the sky islands extending into southeastern Arizona and southwestern New Mexico. This flora is comprised of 233 species in 130 genera and 61 families, which represents approximately 5 percent of the total flora of the region. The region is a meeting place of the temperate North American and Neotropical tree floras with intrusions of Sonoran and Chihuahuan desert at lower elevations. The legume (Fabaceae), oak (Fagaceae), and pine (Pinaceae) families are the most diverse, and the oaks (Quercus) and pines (Pinus) are the largest genera. This is the first comprehensive listing of the trees of the northern Sierra Madre Occidental. Extensive areas of tropical deciduous forest (TDF) cover the lower elevations of the southern part of the region in Sonora and Chihuahua and harbor 60 percent of the regional tree flora. Oak woodland and pine-oak woodland occur at higher elevations and mixed conifer forest at the highest elevations, and support 43, 33, and 11 percent respectively of the regional tree flora, The Madrean forest once stretched unbroken into the American tropics but accelerating deforestation is leading to fragmentation of the keystone species populations, INTRODUCTION We are including all of the tree species known to us in the montane regions at elevations above This publication covers the trees of the sky is­ the deserts and grassland in the north, and above land mountains of southeastern~rizona, tropical thornscrub in the south. The northern sky southwestern New Mexico, northeastern Sonora, islands, especially at higher elevations, have a and northwestern Chihuahua, and the contiguous continental and temperate makeup. Many north northern Sierra Madre Occidental in western Chi­ temperate tree species penetrate far southward in huahua and eastern Sonora. The northern limit of the interior of Mexico at intermediate or higher this bioregion is marked by the Pinalefto Moun­ elevations in areas such as the Central Plateau. tains (Mt. Graham) in Arizona. Out of practicality Fourteen percent of the tree flora, or 33 tree taxa we have set the southern limit of this study at the (species and a few subspecies or varieties) reach Sonora-Sinaloa border and adjacent mountains of their southern limits in the region (Table 1, p. 78- southwestern Chihuahua. The mountains just east 83). of the Cascada de Basaseachic in southern Chi­ Overall, the region is arid to semi-arid except huahua form the southeastern point, and the at the highest elevations. The lower elevations in Sierra de Alamos in southern Sonora marks the the northern part of the region are bounded by the southwestern point. The sky island ranges in Ari­ Sonoran Desert on the western flanks and the Chi­ zona include the Chiricahua, Galiuro, Huachuca, huahuan Desert and grassland on the eastern Pinalefto, Rincon, Santa Catalina, Santa Rita flanks. The southern mountains, especially at mountains, and in New Mexico the Animas and lower and intermediate elevations, support tropi­ southern Peloncillo mountains. A land of ex­ cal and subtropical biota. This southern flora, in tremes, it is topographically and geologically southeastern Sonora and southwestern Chihua­ complex. hua, consists largely of a flora that is continuous with the American tropics. As one moves north­ ward through our region, there is a tendency 1Dry/ands Institute, Tucson, AZ. towards a reduction in stature and an attrition of 2Desert Legume Program, University' of Arizona, Tucson, AZ. tropical species and genera. Seventy-two percent 71 of the tree flora, or 164 species of southern or would not be surprising to find the flex and the tropical origin reach their northern limits in the two Prunus species in northern Sinaloa. A number region (Table 1; also see Felger et al., this volume). of others occur only in the southern part of our Nine plant families represented by trees here do region and in northern Sinaloa, e.g., Albizia si­ not extend farther north in western North Amer­ naloensis, Brongniartia alamosana, Diospyros ica, i.e., Bombacaceae, Clethraceae, sonorae, Opuntia thurberi var. alamosenses, O. Cochlospermaceae, Erythroxylaceae, Magno­ wilcoxii, Quercus albocincta, Q. tarahumara, and liaceae, Myrsinaceae, Myrtaceae, Olacaceae, and Sabal uresana. In fact, the flora of northern Si­ Opiliaceae. naloa flora is not separable from that of southern The total annual precipitation generally de­ Sonora and southwestern Chihuahua (Gentry creases from south to north, but increases with 1946a, 1946b, 1982). elevation. Precipitation is largely bi-seasonal. The The characters, including size, that constitute monsoon-like, summer rainy season is most pro­ a "tree" are often highly subjective but useful for nounced and dependable towards the south, indicating trends, and important for considering while winter precipitation increases in importance keystone or habitat-modifying organisms. The de­ northward. Soil moisture is the principal limiting marcation between the larger shrubs and smaller factor in this dry region. The northward expan­ trees is especially subjective. We have chosen a sion of many of the more tropical or subtropical height of 5 m as the artificial limit between shrubs species is blocked by an invisible "frost-line" of and trees, and when in doubt have favored in­ freezing weather coupled with increasing aridity. cluding woody plants and excluding the more Frosts are infrequent or virtually absent towards herbaceous species with seasonal die-back. We the southern part of the region at lower to inter­ have also decided that if a plant is classified as a mediate elevations. Increasing aridity northward tree one should be able to climb up into it, or tends to raise the lower elevational limits of the theoretically climb it if the spines are removed, tropical/subtropical trees while the frost-line de­ without causing it to collapse. Many have a single scends in elevation. This results in narrowing trunk at least 10 cm in diameter at about 1 m elevational distributions or bands of tropical/ sub­ above ground level. Some species included in this tropical species northward and fragmentation of listing have multiple trunks arising at or near the frost-sensitive and drought-intolerant popUlations ground and could be classified as shrubs rather into specific microhabitats. There is often some­ than trees. In some cases a particular species may what of a paradox, because microhabitats with the be a shrub across most of its distribution in the most favorable moisture conditions (riparian bot­ region, but in favorable habitats such as moist tomlands and north-facing slopes) tend to tropical canyons it may develop into a sizeable experience the most severe freezing temperatures Table 2.-Summary of tree species of the northern Sierra (see Burquez et al., in press). Madre Occidental and the Sky Islands including the seven The total flora for the region is estimated to largest families and five largest genera. include at least 4,000 species of vascular plants No. of No. of No. of (Felger et al., this volume). Within this rich flora families genera species we have documented 233 species of trees (Table Gymnosperms 2 7 26 Dicotyledons 56 119 198 1), which represent about 5 percent of the total Monocotyledons 3 4 9 flora. These tree species are distributed in 130 Total 61 130 233 genera and 61 families (Table 2). In our opinion Families: this tree flora is approximately 95 percent com­ Fabaceae 23 36 plete, with additional records likely to be found in Fagaceae 1 21 the remote mountains and canyons in southwest­ Pinaceae 4 18 Moraceae 4 9 ern Chihuahua and adjacent Sonora. The largest Euphorbiaceae 7 8 families of trees are the legume (Fabaceae), oak Cupressaceae 3 8 (Fagaceae), and pine (Pinaceae) families, and the Burseraceae 1 8 Salicaceae 2 7 most diverse genera are the oaks (Quercus; fig. 1) Cactaceae 4 6 and pines (Pinus) (Table 2). There are about seven tree species and one va­ Largest genera: QuerclJs 21 riety endemic to the region: Fraxinus gooddingii, Pinus 12 flex rubra, Nolina matapensis, Opuntia th urberi Bursera 8 var. thurberi, Prunus gentryi, P. zinggii, Yucca Acacia 6 Ficus 6 schottii, and Y. grandiflora (Table 1). However, it Juniperus 5 72 Figure 1.-leaves of nineteen species of Madrean and sky island oaks (Quercus): (a) Q. albocincta, cusi, encino roble, encino prieto, a Red (Black) Oak. (b) Q. arizonica, Arizona white oak, encino blanco. (c) Q. chihuahuensis, Chihuahua oak, encino blanco, encino chino, a White Oak. (d) Q. chryso/epis, canyon live oak, an intermediate Oak. (e> Q. coccolobifolia, encino negro, a Red Oak. (1) Q. durifolia, a Red Oak. (g) O. emoryi, Emory oak, blackjack oak, bel/ota, a Red Oak. (h) O. gambelll, Gambel oak, Rocky Mountain white oak. (I) Q. gr;sea, / gray oak, a White Oak. (j) 0, hypo/eucoides, silverleaf oak, encino blanco, encino c%ra do, cusi, a Red Oak. v;. -I: (k) O. hypo/eucoides, the southern form, sO,metimes ~~~...J..,~. ~.. ~ J),~........... -:.' .. ~ known as Q. scytophylla Liebm. (I) O. mcvaughii, encino roble, a Red Oak. (m) O. oblongifol/a, Mexican blue oak, encino azul, a White Oak. en) O. rugosa, net-leaf oak, a Red Oak. (0) O. sideroxy/a, encino prieto, a Red Oak. (p) Q. subspathulata, a White Oak. (q)Q. tarahumara, hand basin oak, encino cajete, a Red Oak. (r) Q. toumeyl, a ,\-".' . " ~ " White Oak. (s) Q. tubercu/afa, encino amarillo, a White I Oak. (t) Q. vlminea, willow leaf oak, saudllo, a Red Oak. I /' Drawings by MBJ. 73 tree (e.g., Aralia humilis and Stemmadenia ture used here results from our interpretation of tomentosa).