The distribution and abiotic drivers of subtropical plant taxa in the southwestern U.S. sky island region: identifying hotspots of conservation significance with an aggregation of peripheral species

A contributing element of the exceptional plant biodiversity of the Arizona-New Mexico sky islands is the numerous southern taxa at their northern extent, yet the local distribution and drivers of subtropical plant richness have not been delineated. We assess the proportion of subtropical richness (PSR) in sky islands as the ratio of subtropical taxa to total species richness. We (1) identified 284 subtropical vascular plant species at or near their northern range in 24 sky islands, (2) calculated PSR for each sky island, (3) quantified spatial patterns of PSR and subtropical beta diversity, (4) determined regional hotspots of PSR, and (5) analyzed independent eight variables as potential drivers of PSR and subtropical species turnover. Sky islands with the highest PSR occur in the southwestern portion of the region close to the international border. Four predictor variables (ordered by significance) strongly correlate with PSR: minimum winter temperature, monsoon season precipitation, area, and latitude. The primary subtropical hotspot occurs mostly within Santa Cruz County (southeastern Arizona), representing the largest regional concentration of rare subtropical taxa. We determined three primary clusters of species cooccurrence with a strong longitudinal trend driving subtropical beta diversity. Nearly half of subtropical taxa are regionally rare or vulnerable, illustrating the potential value of assessing regional peripheral plant taxa to discern hotspots of ecological significance. The sky islands of Santa Cruz County represent a unique botanical hotspot with exceptional subtropical richness and rarity. Understanding regional drivers of subtropical influences helps forecast potential responses to global change, while strengthening conservation strategies that minimize future losses of biodiversity.