Modeling the geographic distribution of pawpaw (Asimina triloba [L.] Dunal) in a portion of its northern range limits, western New York State, USA

Abstract

Pawpaw (Asimina triloba [L.] Dunal) is a small deciduous tree in eastern North America notable for its edible fruit. Its northern range extends into New York State (NYS) where it is listed as threatened, with only approximately 20 sites known to contain pawpaw. This research models the geographic distribution of pawpaw in western NYS (land area = 27,263 km²) for four reasons: to understand factors shaping its distribution, to assess the degree to which it is protected, to locate potential introduction or reintroduction sites, and to locate potential sites with undiscovered pawpaw. This study uses an ensemble of small models (ESM) approach devised for modeling the distribution of rare species, which averages predictions from simple (i.e., bivariate) models weighted by model predictive performance. The ESM in this study, constructed from boosted regression tree (BRT) models, demonstrated a high ability to discern pawpaw presences from pseudo-absences. The area under the receiver operating characteristic curve (AUC) value was 0.986 when calculated using training data, and BRT models comprising the ESM exhibited AUC values calculated using cross-validation data as high as 0.957. The ESM suggests that pawpaw is found in the warmest (mean annual temperature > 9.0 °C,) and driest (May–September precipitation < 44 cm) areas of western NYS, and on mildly acidic (pH = 5.5–7.0) and deep (> 110 cm) soils. Locations predicted to be suitable for pawpaw are overwhelmingly found on unprotected lands; forested areas in PAs with “high” or “highest” suitability for pawpaw only comprise 0.1–0.2% of the study area.