Bioclimatic Determinants of the Niche of the Vulnerable Malagasy Tree Delonix decaryi (R.Vig.) Capuron: Insights from Maximum Entropy Modelling for Spiny Thickets Conservation

Abstract

Delonix decaryi (R.Vig.) Capuron, locally designated as "Fengoky," is an iconic pachycaul tree endemic to the sub-arid spiny thickets of southwestern Madagascar. Threatened by severe habitat fragmentation, localized charcoal exploitation, and accelerating climate shifts, understanding its environmental boundaries is critical for regional conservation. This study employs Maximum Entropy (MaxEnt) predictive modelling to delineate the ecological niche of Delonix decaryi and identify the primary macroclimatic drivers governing its distribution. Using 94 spatially filtered occurrence records, 19 bioclimatic variables, and a persistent vegetation fraction layer, we generated a highly robust predictive model (Training AUC = 0.9839). Our analysis reveals that seasonal precipitation patterns exert overriding control over the species’ spatial limits. The precipitation of the wettest month (bio13) emerged as the single most critical predictor, accounting for 68.9% of total model contribution and displaying a dominant permutation importance of 96.7%. Annual precipitation (bio12) contributed an additional 17.1%. Marginal response curves show that Delonix decaryi occupies a highly specialized, narrow hydrologic window, with habitat suitability peaking sharply where wettest-month rainfall remains below 150 mm and annual precipitation ranges strictly between 330 mm and 600 mm. Temperature seasonality (bio4) represented the most significant source of unique, non-redundant environmental information. These findings establish a clear quantitative baseline to guide habitat restoration, define spatial boundaries for protected area expansion, and inform ex-situ germplasm collection in the face of shifting arid dynamics in southern Madagascar.