Modelling the historic distribution and habitat of American chestnut (Castanea dentata) in Georgia, USA using edaphic and landform predictors

The loss of American chestnut ( Castanea dentata Marsh. Bork.) caused ecological change in many community types of the eastern United States. Restoration is challenged by climate change and two naturalised invasive non-native diseases, chestnut blight (caused by Cryphonectria parasitica [Murrill] M.E. Barr) and Phytophthora root-rot (caused by Phytophthora cinnamomi Rands). Learning how to overcome these challenges in the southern portion of the former chestnut range may make range-wide restoration efforts more successful because these pressures are likely strongest in southern portions of the range. To establish a baseline of chestnut distribution and environmental correlates, we used ca. 1830 land lottery maps to document the historic abundance and distribution of chestnut in Georgia. Land lottery surveyors documented 717,901 trees within our study area, identifying 15,710 as chestnut. We used their data to create a species distribution model with soil and landform predictors, factors possibly related to disease risk and predicted the relative habitat suitability for chestnut throughout much of the State. Our results revealed that long-held assumptions about chestnut range and abundance in Georgia are incorrect. Chestnut was less common in northern Georgia and more common in the Piedmont than previously postulated, and chestnut ranged well into the Coastal Plain. Soil and landform variables adequately predicted chestnut distribution and habitat suitability models predicted that chestnut occupied a wide diversity of habitats; habitat preferences were complex and differed by physiographic province. All the models agreed that chestnut preferred higher elevation, more slope and landform curvature, but the relationship of chestnut to other variables was difficult to generalise and may have been context dependent. These results can be used to guide re-introduction and maximise success in the face of changing climate and non-native disease risk. Anecdotal accounts limit the historic range of American chestnut in Georgia to the Blue Ridge, Appalachian Plateau and Ridge and Valley physiographic provinces; however, historic land lottery maps dating from the early 1800s documented chestnut throughout the Georgia Piedmont and much of the Coastal Plain; Chestnut was found in a wide range of Georgia habitats in every physiographic province before the introduction of non-native pathogens, sometimes in high abundance and sometimes as less dense components of habitats with other dominant or co-dominant species; Although patterns of chestnut distribution were largely driven by elevation and slope, other environmental factors including aspect, landform curvature, number of frost-free days, percentage clay in soil and soil pH significantly influenced chestnut distribution in Georgia. These patterns varied by physiographic province, suggesting chestnut habitat was not monotypic across Georgia; Restoration efforts that recognise regional differences and the full suite of diverse habitats that once included chestnut may be more likely to succeed. Our models and regional habitat suitability maps can be used to guide planting site selection, while also considering the large variability inherent in ecological restoration of chestnut across multiple physiographic provinces; As they are relatively fine scale, edaphic and landform factors are useful in predicting suitable habitat for plants and may be used in conjunction with climate change predictions to guide site selection for species reintroductions that will be sustainable into the foreseeable future.