Botanizing in the Madrean Sky Islands of Arizona
Spend a few days collecting in Southeastern Arizona exploring the northwestern face of the Santa Rita Mountains – Madera Canyon. The Santa Rita Mountains is one of the Madrean Sky Islands in Arizona. These sky islands are one of the most diverse inland archipelagos formed by the confluence of two mountainous spines of North America: 1) the Colorado Plateau and Rocky Mountains characterized by thick forests of Ponderosa pine and deep canyons that form the Mogollon Rim, which signifies the abrupt edge found on the Plateau’s southern flank and 2) the Sierra Madre Oriental and its subtropical forests of pines before it reaches the Arizona-New Mexico border.
40 distinct mountain ranges form the Sky Island region of North America. In addition to this globally unique convergence—the north-south overlap of two major cordilleras spanning the temperate and subtropical latitudes— an additional biogeographical phenomenon occurs at the Sky Island intersection, as well. The Sonoran desert and its iconic towering saguaro cacti creep eastward into higher elevations. Tucson, which sits at the eastern edge of the Sonoran desert, marks the western gateway into the Sky Islands. East from there, the Sky Island landscape increasingly represents the cold-adapted constituents of the Chihuahuan desert, which spill westward over the lowest point in the continental divide from southcentral New Mexico and Chihuahua, Mexico. This intermingling of bioregional edges brings together different life forms evolved from vastly different places on the continent, finding themselves tucked together in unusual associations within the Sky Islands.
Madera Canyon is fueled by Madera Creek. This stream system and the abundant plants along its banks form a riparian corridor. The highest peak in the Santa Rita Mountains towers high above the desert floor – Mount Wrightson (9483 ft). Approaching Madera Canyon we drove through the Lower Sonoran Zone from the Santa Cruz Valley which is characterized by Sonoran desert scrub with desert trees, barrel cactus, and chollas. Here we encountered a nice stand of fruiting Ephedra trifurca. With its wing-bracted strobili, Ephedra trifurca is well adapted to the open sandy habitats and a strong breeze put wind dispersal in motion for us. The scrubland transitions to desert grassland with velvet mesquite, before reaching cooler temperatures in the Upper Sonoran Zone, which is characterized by several species of evergreen oaks (Quercus oblongifolia, Quercus emoryi, Quercus hypoleucoides), alligator juniper (Juniperus deppeana), Mexican piñon pine (Pinus edulis), shrubs (Garrya wrightii, manzanita), bunch grasses, and cottonwood (Platanus wrightii) along the banks of Madera Creek. In this zone we found nice exemplars of Arizona grape Vitis arizonica often trailing on Garrya wrightii (gray-leaf dogwood).
On to the Santa Catalina Mountains north of Tuscon, where Mt. Lemmon towers high above the Santa Catalinas with a summit elevation of 2792 m. Mount Lemmon was named for botanist Sara Plummer Lemmon who sumitted the peak with her botanist husband J.G. Lemmon and local rancher E. O. Stratton. Along the Mt. Lemmon Highway we encountered rolling hillsides and canyons of Oak-Grassland that dominate the landscape. The most abundant plants here are Emory oak (Quercus emoryi), Arizona white oak (Quercus arizona), Mexican Blue Oak (uercus oblongifolia) and many dominant grasses—side-oats grama, cane beardgrass, Arizona panicgrass, invasive Lehmann lovegrass, and the elevationally wide-ranging bullgrass, as well as a dozen other less-common species. Other abundant plants of the Catalinas’ Oak-Grassland biome include pointleaf manzanita, shindagger agave, sotol, beargrass, and mountain yucca. Along the Catalina Highway our first encounter of Arizona grape (Vitis arizonica) was at Molina Basin at the lower end of Molino Canyon in a dry creek bed lined with willow and cottonwood. The grapes were rather abundant and fruiting profusely at this transition between Sonoran Desertscrub and Grassland or Oak-Grassland at 4300 ft. Further up General Hitchcock highway the grapes were still in bloom at 5761 ft. and I observed the functionally unisexual flowers of Vitis arizonica very well, what a treat!
Near the Arizona Mexican border we also explored another sky island – the Patagonia Mountains, which represents one of the southernmost archipelagos in Arizona and rises to 2,201 meters (7,221 feet) at the summit of Mount Washington, the range’s highest peak. First we stopped at Patagonia Lake, one of the premier birding sites in AZ, where we saw several unique birds. Driving towards Tuscon, through the Sonoita Valley, about 7 km SE of Patagonia at Harshaw Creek we collected another Vitis arizonica populations.