Exploring Past and Future Distributions of the Rare Appalachian Oak Fern Using MaxEnt Modeling

Abstract. Anthropogenic climate change is projected to have an especially negative impact on the survival of plants that are dependent on limited microclimatic refugia or that already reside at their climatic extreme. Gymnocarpium appalachianum is a narrowly endemic fern restricted to cold mountaintops and algific vents in the central and southern Appalachian region of eastern North America. It is the much rarer of the two documented diploid parents of the circumboreal allotetraploid G. dryopteris––one of the most widespread fern species on the planet. Gymnocarpium appalachianum is a good case study for forecasting how evolutionarily significant, but rare, species might survive on a warming planet. We utilize an ecological niche modeling approach (MaxEnt) to explore the projected distribution of G. appalachianum under past (Last Glacial Maximum) and future climate models. All known verified herbarium records of G. appalachianum were georeferenced, for a total of 70 occurrence points. Nineteen standard bioclimatic variables extracted from WorldClim were used to model near-current climate projections; representative concentration pathways (RCPs 2.6 and 8.5) were used for future climate projections (2070). The temperature annual range, mean temperature of warmest quarter, precipitation of driest month, precipitation of coldest quarter, and mean diurnal range were identified as the key variables for shaping the distribution of G. appalachianum. An unanticipated result from our analyses is that G. appalachianum has past and current projected habitat suitability in Alaska. Because this overlaps with the current range of G. disjunctum, the other diploid parent of G. dryopteris, it suggests a possible region of origin for this circumboreal tetraploid descendent of G. appalachianum––a research avenue to be pursued in the future. Our study envisions a dire fate for G. appalachianum; its survival will likely require an urgent contingency plan that includes human-mediated population relocation to cooler, northern locations. Understanding the long-term sustainability of narrowly endemic plants such as G. appalachianum is critical in decisions about their management and conservation.