Tracking the desert’s edge with a Pleistocene relict
Plants, far from their core area of distribution, are often found isolated on various mountain peaks. In addition to the Sky Islands of the southwestern U.S. and northwestern Mexico, a series of 900–1200 m desert peaks surrounded by arid lowlands also have temperate affiliated species at their summits.
Photo credit: BTW
When we went to the summit of Isla Tiburón, we found a surprising population of a species previously known far to the north. But there was the crucifixion thorn, Canotia holacantha, over 200 kilometers to the south of the next nearest populations.
The presence of this and several other disjunct long-lived plant taxa on Isla Tiburón suggested a more southerly extent of Ice Age woodlands than previously understood. We investigated the phylogeography of this desert edge crucifixion thorn to see if indeed its presence on the largest island in Mexico represents remnants of Pleistocene woodlands rather than recent dispersal events.
Results suggest that a Canotia common ancestor occurred on the landscape, which underwent a population contraction ca. 15 kya. The Isla Tiburón population and the Chihuahuan Desert microendemic C. wendtii have the greatest genetic differentiation, are sister to one another, and basal to all other Canotia populations. Three haplotypes within C. holacantha were recovered, which correspond to regional geography and thus identified as the Arizona, Sonora, and Tiburón haplotypes. These results indicate a once broad distribution of Canotia, now present in relict populations on the fringes of the southern desert, in the Chihuahuan Desert, with scattered populations on desert peaks, and a common or abundant distribution at the northern boundary of the Sonoran Desert. These results suggest Canotia has tracked the shift of the desert’s edge both in latitude and elevation since the end of the last Ice Age.
Related Studies
Wilder, B.T, A.T. Becker, A. Munguia-Vega, M. Culver. 2022. Tracking the desert’s edge with a Pleistocene relict. Journal of Arid Environments 196:104653. https://doi.org/10.1016/j.jaridenv.2021.104653
