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Master of Science
Magnolia fraseri, climate change, range shift, range contraction, recruitment, escalator effect, biodiversity, Southern Appalachian Mountains
Anthropogenic climate change is emerging as one of the top threats to biodiversity in the 21st century as climate conditions shift faster than many species are capable of adjusting their distributions. Mountainous terrain is expected to provide some buffering against rapid climate change in the short term, as species in these areas can potentially migrate short distances upslope in elevation to track cooler habitat rather than moving long distances poleward; however, in the longer term, this dynamic could also lead to increasing contraction of viable habitat into smaller areas at higher elevation and even risk extinction via the “escalator effect”. A range of methodologies are being used to assess and predict climate change threats to individual species and to explore conservation options for the most at-risk species, particularly small-ranged endemic species which are thought to be most vulnerable to near-term extinction. Unfortunately, the most common method for gauging species risk, Species Distribution Modeling (SDM), which typically uses pre-existing data on species occurrences compiled from historical records in herbaria or other sources, is known to be less accurate and often unreliable for endemic species with small geographic distributions. As such, de novo on-the-ground assessments that generate detailed and up-to-date data on population structure and dynamics will be crucial for providing accurate assessments of status and risks to inform conservation actions for many endemic species. In this study, I investigated Magnolia fraseri, a small-ranged forest tree native to cool, higher elevation forests in the Southern Appalachian Mountains of the eastern U.S., and surveyed the elevational distribution of its adult versus seedling life stages in the Mount Rogers National Recreation Area in southwestern Virginia, near the center of the species’ small native range. If warming climate is impacting this species’ elevational distribution, I expected to detect a decoupling of life stages; specifically, a shift in seedling recruitment toward higher elevation population sites compared to where existing adults recruited in the past. Additionally, I predicted that tree recruitment histories and the growth rates of mature individuals, as revealed by tree coring, would show an upward elevational shift if the species has been affected by recent climate change. My results showed a 277.6 m upslope shift in the elevational mean of seedlings relative to reproductive canopy 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 at 1387 m elevation. Population sites below ~1000 m elevation lacked substantial recent seedling establishment, and tree coring revealed that the last successful recruitment event from the seedling stage into the forest canopy at these sites was often 40+ years ago. In contrast, most higher elevation populations (~1000 m and above) showed abundant and ongoing seedling establishment and sapling recruitment in recent years, with seedlings per adult tree peaking in the highest elevation plot. A generalized linear model confirmed a strong positive relationship between elevation and seedling abundance, and also detected a significant interaction between elevation and canopy disturbance on seedling recruitment. Specifically, M. fraseri seedling recruitment responded positively to plot canopy gap area only above ~1000 m, but failed to recruit under canopy gaps below this elevation, despite their presence in plots. In addition, analysis of tree cores showed showed that average radial growth per decade was ~50% greater at high elevation sites than at low ones. Overall, the patterns detected suggest M. fraseri is being strongly affected by recent climate warming and appear to show the species exhibiting some population dynamics typical of the “escalator effect”; however, the survey data collected to-date suggest that rather than shifting upslope into new habitat areas, the species might be experiencing an even more dire scenario of distribution contraction, where the expected increases in new seedling recruitment at higher elevations are only occurring where reproductive adults were already present near the species historical upper distribution limit. These dynamics suggest the risk of widespread population extirpations at lower elevation, genetic bottlenecks, and, potentially, even extinction. In the absence of a rapid and natural route to northward migration for M. fraseri, this research points to a perilous future for the species in a shrinking area of viable habitat at higher elevations within its historical native range. Avoiding eventual extinction may require active management strategies such as ex situ conservation in botanical gardens or, potentially, assisted migration.
©2022 John Berryhill. Access limited to the Smith College community and other researchers while on campus. Smith College community members also may access from off-campus using a Smith College log-in. Other off-campus researchers may request a copy through Interlibrary Loan for personal use.
130 pages. Includes bibliographical references (pages 111-124).
Berryhill, John M., "Investigating the Potential Upslope Migration of Magnolia fraseri in Response to Climate Change" (2022). Masters Thesis, Smith College, Northampton, MA.
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