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

Boreal plants growing along 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 to be 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 uppermost elevation spruce-fir forests of the Chiricahua Mountains, together with a theoretical modeling of the species9 tolerances to three climate change cues: warming, drought, and forest fire, in order to explore the persistence of frontier boreal plant species in the frame of climate changes. We hypothesized 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 functional traits and principal component analysis to model tolerances of the plants to combined actions of warming, drought, and forest fire. Our model predicted selective sensitivity to warming for two species: Vaccinium myrtillus and Rubus parviflorus, while possible decline of the other species could be explained by drought and/or fire. We surveyed the study area in 2019 and found eight of the ten 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 highly affected by fires. Both warming-sensitive species did not show signs of decline: population 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 over one degree Celsius between years 1975-1993 and 1994-2019, our study did not find evidence of the direct warming effect on the observed species. We conclude that severe wildfires and multi-decadal decrease in precipitation rather than warming are the main limiting factors of the remnant boreal species remarkable but limited persistence in the Chiricahua Mountains. Our study demonstrates how field observations can be combined with modeling to evaluate species selective responses to different environmental stresses for better environmental management decisions, particularly in light of climate change.