The biodiversity of our Sky Islands is truly astounding. Here at the meeting point of 4 major bioregions - The Sonoran Desert, the Chihuahuan Desert, The Rocky Mountains, and the Sierra Madre Occidental - we have towering and isolated mountain ranges that loom as mesic, forested beacons above the drier interceding habitat of grasslands and deserts. At our relatively low latitude - for a temperate region - of about 32 degrees we already are candidates for high levels of diversity. Add to this the Galapagos-like element of our Sky Island ranges - isolation that contributes to evolution - and the seemingly competing influence of connectivity that the stepping-stone arrangement of our mountains facilitate and the resultant mix of flora and fauna is truly astounding!
As a seasoned and professional Naturalist, Ethnobotanist, & Wildlife Biologist I am still discovering Sky Island plant species new to me with each passing year. More importantly, I continue to forge lasting and meaningful connections with the species whose acquaintance I’ve already made. Among my favorite plants are those whose ranges barely extend into our region. Often these are more tropical species generally less tolerant of freezing temperatures than their Sky Island compatriots. Thus, Madrean plants rub elbows, so to speak, with their temperate and desert brethren. Here I hope to introduce you to a few plants living along the edge - our edge.
Given my past explorations in the wilds of Florida and the Caribbean, I was initially floored to encounter an engaging little plant seemingly more suited to those climes. Many years ago I found myself exploring the incomparable beauty of Sycamore Canyon along the border with Mexico. Having heard tales of its beauty and profusion of life I was not disappointed with its otherworldly character. Two plants in particular caught my keen eye as I meandered along Sycamore Creek during the verdancy of Monsoon season.
Casting a glance towards the familiar One-seed Junipers lining the canyon bottom, I noticed intriguing little clumps festooned upon certain branches. Not quite believing what my brain was already registering I indeed confirmed the presence of a member of the Pineapple Family - a bromeliad! Later research proved this to be Ball Moss, an epiphytic member of the Bromeliaceae - Tillandsia recurvata. The range of this species extends from south central Arizona - the location of Sycamore Canyon - all the way across the southern tier of states and into the Caribbean. Indeed, I realized that I had encountered this species on numerous occasions in Florida, and likely in Jamaica as well! For perspective, such epiphytes - represented by a number of plant families - become at once common and spectacular in the lower clutches of Mexico’s rightly famous Barranca del Cobre, Copper Canyon.
Here, they lurk on the fringes, inhabiting almost exclusively Juniper Trees - now referred to as Coahhuila Junipers (Juniperus coahuilensis). Only once have I witnessed a Ball Moss growing in a rock instead of a Juniper. They render an otherwise Great Basin
Desert-esque scene into a subtropical one. They are commensals upon the Juniper, meaning that they have no measurable adverse or positive effect upon their hosts, while they themselves indeed benefit by living upon the shady, moister branches. Ever the curious Ethnobotanist, I nibbled a few leaves from a hardy plant the other day at a site a mere handful of miles from Raven’s Nest, our 42-acre Nature Sanctuary near Patagonia Lake. The taste was not unpleasant - a bit astringent - and I’m still here to tell the tale!
During the same Sycamore Canyon saunter I met with an even more rare Sky Islands botanical wonder. Growing in a canyon wall cleft was a primitive plant whose next nearest populations reside in 1200 miles away in Texas and about 300 miles south in Mexico, near Yecora in Sonora! Whisk Fern (Psilotum nudum) indeed seems an anomaly akin to Ball Moss. One expects such a rare, primitive fern to show up in the Gondwanaland relict forests of Queensland more than in our borderlands. My initial impression of this strange species was that of a horsetail crossed with a fern. The family Psilotaceae contains only 2 species, including the one in Sycamore Canyon.
Much easier to locate and admire is the Mexican Blue Oak, whose range, never-the-less is very constricted in the U.S. Despite the well-known proclivity of members of the genus Quercus (i.e. Oaks) to hybridize, Quercus oblongifolia is one of our easiest species to identify. Its lowland habitats on the fringes of grasslands and canyon mouths, combined with the bluish cast of its notably blunt-tipped leaves readily distinguishes it. Mexican Blue Oak only enters the U.S. in southeast Arizona and extreme southwest New Mexico.
Recently my wife Claudia and I stumbled upon a truly dazzling one while exploring the more hidden canyons near our home. This venerable tree must be hundreds of years old given the likely rather slow growth rate of Oaks in our region. Its DBH - diameter at breast height - is well over 3 feet across! Nor does it lack in the character, given its nearly horizontal bole and the fact that its own trunk has a firm hold on that of a young Netleaf Hackberry (Celtis reticulata). We were thrilled to encounter this magnificent tree, especially given the double gauntlet that this and other plant species ran two years ago of drought and extreme freezing temperatures. It renders our usual haunts rather unusual in fact.
Rounding out our brief exploration of the plants that, barely, call the Sky Islands home is a more familiar face. Anyone who’s spent significant time hiking in the canyons and rocky foothills of the Sky Islands has likely come across a distinctive member of the
Bean Family - Southwestern Coralbean (Erythrina flabelliformis). It too is frost sensitive and often gets killed back to the ground during severe Winter weather. Thus, we merely see it as a tall shrub for the most part in Arizona, which it barely breeches from the south. In those warmer climes lurking near us in Mexico it regularly reaches respectable tree proportions.
Once, while wandering through the Tropical Deciduous Forests just outside the colonial town of Alamos, Sonora I stumbled upon the distinctive and brilliantly crimson seeds of Southwestern Coralbean. No, I thought, it can’t be, as the tree from which the seeds emanated towered a good 40 feet above me and sported a DBH of about 2 feet! Subsequent reading, however, confirmed that this species regularly attains tree size where its not limited by freezing temperatures. Local cultures even value it for lightweight furniture. Thus, even when a plant species from warmer climes invades our Sky Island region, it may express itself quite differently than in the heart of its range.
Had I more time and space I would love to wax poetic/scientific over a much longer list of fringe plants. Far too many people either seem unaware of the botanical treasures at their doorsteps and/or simply take them for granted. In a day and age of increasingly imperiled biodiversity, we feel it’s imperative to not only take notice, but to sing the praises of both the common and the rare wild things that enrich our lives. Consider, then, finding and familiarizing yourself not only with the few species mentioned above, but also with the Yellow Trumpets, Mala Mujers, Madrean Yuccas, and Sayas that collectively constitute our unique subtropical flare.
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