Madrean Sky Islands of Arizona

MtWrightson_MaderaCanyon_sm_0026The Madrean sky islands of Arizona 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 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.

Santa Rita Mountains  & Madera CanyonEphedra_trifurca_sm_0043Approaching 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 Ephedra_trifurca_sm_0049 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 (us 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).

Santa Catalina Mountains & Mt. Lemmon  The drive on Catalina Highway (also known as General Hitchcook Highway) from Tuscon to Mt. Lemon in the Santa Catalina Mountains, Pima County, Arizona was spectacular.  After driving through climax Sonoran Desert hillsides covered in Saguaros (Carnegiea gigantea) and creosote bush (Larrea tridentata) we approached Molino Canyon, which was an impressive gateway to outlandish rocks capes adorning the scenic highway towards the summit of Mt. Lemmon. Here at Molino Basin we came across a nice stand of Arizona grape (Vitis arizonica) in a dry creek bed fruiting profusely.  Further up the mountain bright green box elder trees (Acer negundo) were spotted along the highway and soon the trees communities changed to oak woodlands and mixed coniferous forest with large pines, douglas fir, alligator juniper, cottowoods and sycamore along riparian areas.  We found two additional maples near Bear Wallow Rd., Rocky Mountain maple (Acer glabrum) and Large-leaf maple (Acer grandidentatum).

Vitis arizonica

Patagonia Mountains – The last stop in this trifecta took us to the southernmost sky island archipelago in Arizona, the Patagonia Mountains. State Route 82 from Nogales to Patagonia winds through Chihuahuan Desert hillsides with pure stands of ocotillo (Fouquieria splendens). About midway we arrived at Patagonia Lake State Park, which was created by the damming of Sonoita Creek.  The park is taming with wildlife and is a birders paradise. We saw lots of unique birds.
PatagoniaLakeDSC_0657Passing through Patagonia, we took Harshaw Road (State Route 49) towards the abandoned mining town of Harshaw on the hunt for an additional Vitis arizonica population, which we found along Harshaw Creek climbing over some shrubs. Large cottowood trees (Populous fremontii) lined the creek bed.