Persistence of remnant boreal plants in the Chiricahua Mountains, southern Arizona

Boreal plants growing along the southern edge of their range on isolated mountains in a hot desert matrix live near the extreme of their physiological tolerance. Such plants are considered sensitive to small changes in climate. We coupled field observations (1974, 1993, 2019) about the abundance and vigor of small populations of ten remnant boreal plant species persisting in the uppermost elevations of spruce-fir forests of the Chiricahua Mountains, together with modeling of the species sensitivities to three stress factors associated with climatic change: warming, drought, and forest fire, in order to explore the persistence of frontier boreal plant species during climate change. We hypothesize that populations of these cryophilic plants have declined or become locally extinct during an adverse warming period since 1993, enforced by two large forest fires (1994, 2011). We used plant traits and principal component analysis to evaluate sensitivities of the studied plants to the combined actions of warming, drought, and forest fires. Our model predicted selective sensitivity to warming for two species: Vaccinium myrtillus and Rubus parviflorus. Other cryophilic species could be more sensitive to drought and fire. We surveyed the study area in 2019 and found eight of the ten previously investigated species still occur in the area. Five species occurred in wet canyons at lower elevations, but three species persisted in low vigor at the uppermost elevation, which was highly affected by fires. Neither warming-sensitive species showed signs of decline: populations of R. parviflorus increased in abundance and vigor, while V. myrtillus persists without significant changes since 1993. Despite the recorded increase in temperature in the study area >1 °C between years 1975–1993 and 1994–2019, our study did not find direct evidence of warming effects on the observed species. We conclude that severe wildfires and the multi-decadal decrease in precipitation rather than warming are the main limiting factors of remarkable but limited persistence of the remnant boreal species in the Chiricahua Mountains. Our study demonstrates how field observations can be combined with modeling to evaluate species selective responses to different environmental stress factors to make better environmental management decisions, particularly in light of climate change.