PS 43-158 - Magnolias on the move: Evidence of an impending upslope range contraction in mountain magnolia associated with climate change

Abstract: Anthropogenic climate change is one of the top threats to biodiversity in the 21st century. In the past, mountainous landscapes with high topo-climatic variability have served as critical refugia for biodiversity, especially for small-ranged endemics; however, the limits of this buffering capacity will be tested as the pace and magnitude of modern climate change exceeds past fluctuations. Our research investigated population structure and tree growth rates of Magnolia fraseri, an endemic native to the central and southern Appalachian Mountains, along an elevational gradient spanning the species’ distribution at the core of its native range in Virginia. If the ecological niche of M. fraseri is being impacted by warming climate, we would expect to see a decoupling of adult and seedling life stages; specifically, an upward shift in recent seedling recruitment relative to the distribution of adult trees recruited in past decades. Additionally, we predicted that growth rates of mature individuals, as revealed by tree ring analysis, would show a trend toward increased growth at higher elevations. We detected a 278 m upslope shift in the mean elevation of seedlings relative to adult trees along a 682 m elevation gradient from near the species’ low elevation distribution limit (~700 m) to the highest elevation individual detected in the study area (~1387 m). Populations below ~1000 m elevation included many reproductive adult trees, but lacked substantial recent seedling establishment, and tree coring revealed that the last successful recruitment event into the forest canopy at these sites was 40+ years ago. In contrast, higher elevation populations (~1000 m+) showed abundant and ongoing seedling establishment and sapling recruitment, with seedlings per adult tree ratios peaking in the highest elevation plot. However, transects surveyed upslope of high elevation adult trees detected no seedling establishment beyond the species’ historical distribution limit, possibly due to dispersal and establishment limitations. Tree core analysis showed average radial growth per decade was ~50% greater at high elevation sites compared to low elevation sites. Overall, the patterns detected suggest that M. fraseri is being strongly affected by recent climate warming and faces an impending upslope range contraction toward the species’ historical upper distribution limit. These dynamics suggest the risk of widespread population extirpations at lower elevations, and the possibility of eventual extinction if the species is not able to disperse from shrinking high elevation refugia in the south to larger expanses of new potential habitat in the north.