Range-Wide Habitat Suitability, Climate Change Exposure and Field Stand Structure in Two Andean Polylepis Species

Polylepis forests are the highest-elevation woody ecosystems on Earth and are severely threatened by climate change and land use, yet habitat models that go beyond binary presence/absence remain scarce. The current and future habitat suitability (2021–2100; CMIP6, four to five GCMs × four SSPs) of two co-occurring Andean species, Polylepis rugulosa and Polylepis tarapacana, was modelled with MaxEnt and Random Forest calibrated across their full range (Peru, Bolivia, Chile and Argentina) and clipped to the Tacna region of southern Peru. Internal spatial cross-validation was complemented by two external validations: 228 field-mapped stand polygons and 35 inventory plots (883 trees), the latter used to test whether suitability also tracks measured stand structure. Both algorithms performed well (AUC =0.78–0.94 across internal and external validation). Once within-plot non-independence and residual spatial autocorrelation were accounted for, no structural attribute was significantly associated with suitability: a positive suitability–multi-stemming trend persisted in Polylepis rugulosa (p=0.06–0.09) but not in Polylepis tarapacana, so this coupling is reported as a preliminary, hypothesis-generating result rather than as a validated predictive relationship. Within Tacna, suitable habitat above the maxSSS threshold is projected to contract by 17.4% for Polylepis rugulosa and by 100% for Polylepis tarapacana under SSP585 by 2081–2100 (MaxEnt ensemble; the near-total loss is unanimous across the contributing GCMs), revealing divergent climatic vulnerabilities. Reading suitability as a map of habitat quality rather than of habitat presence therefore remains a working hypothesis that larger plot networks and demographic data are needed to test.