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Abstract
Anthropogenic climate change is perhaps the greatest threat to the Orchidaceae due to their incredible sensitivity to environmental conditions and highly specialized symbiotic interactions. The long generation times of orchids make an adaptive response unlikely considering the current rate of climate change. Instead, range modification to track favorable climatic conditions will be necessary to survive a rapidly warming climate. While previous works have investigated range modification as a response to climate change, rarely are obligatory symbionts considered. Herein we use ecological niche modeling and population genetic approaches to assess the potential of Cypripedium acaule to modify its range through projection of shifts in locality of C. acaule habitat, quantification of overlap in habitat of C. acaule and its mycorrhizal symbionts under a warmer climate, and assessment of colonization potential through quantification of historic gene dispersal. We further consider the role of intraspecific variation in mycorrhizal associations on migratory patterns through application of an isolation by environment framework. Our findings suggest southern extirpation at low elevations will be accompanied by northern expansion of suitable habitat. Simultaneously, we expect habitat capable of supporting both C. acaule and its fungal symbionts to continue to exist under a rapidly warming climate. We find that C. acaule harbors a moderate level of genetic diversity within populations (mean nuclear H_e=0.378; mean plastid π=0.092) and at the species level. Weak population structure is also observed (nuclear Φ_ST=0.095; plastid Φ_ST=0.200) with seed dispersal contributing more than interpopulation pollen movement to gene dispersal (m_p/m_s=0.39). Gene flow in C. acaule is frequent enough to swamp the genetic signature of historic northward colonization following glacial retreat, which ecological niche models suggest occurred. We also observe a significant correlation between genetic differentiation and mycorrhizal community distance among populations in the nuclear genome, suggesting successful recruitment and persistence of migratory individuals relies on the presence of fungal communities similar to that of the natal populations. Our results suggest a modest optimism for the persistence of C. acaule in the near future under a rapidly warming climate, though the rate of range modification will be influenced by fungal colonization rates.