Population-level ecological niche models to assess the impact of climate change on endangered and relict tree species: a case study of Parrotia subaequalis in China

• We identified two climatically distinct populations separated by the Yangtze River. • MaxEnt models at species/population levels show combined local habitats > whole. • Significant niche overlap difference: local vs local > local vs entire population. • Local adaptation is confirmed within isolated populations of Parrotia subaequalis. • Intraspecific variation in ENMs enhances model accuracy of endangered relict trees. Ecological niche models (ENMs) are pivotal for predicting species’ environmental suitability under global warming. For an endangered and relict tree, projection of ENMs often ignores intraspecific variation, thus leading to over- or underestimation of future climate tolerance. In this study, we focused on a Tertiary relict species Parrotia subaequalis , a critically endangered tree endemic to China, whose populations exhibit distinct geographic isolation exerted by the Yangtze River. Firstly, we conducted multivariate statistical analysis to delineate different environmental populations. We then constructed MaxEnt models at the levels of species (i.e., whole population) and population (i.e., local population) to forecast the current and future distributions of P. subaequalis , and measured niche overlap between each population pair over time. Our analysis showed that all of its occurrence records were clustered into two distinct categories, which were consistent with both local populations. Species-level suitable area (2.78 × 10⁴ km²) was 16.3% smaller than the summed population-level areas (3.32 × 10⁴ km²). Future projections indicate more than 10% habitat gain for local populations but a nearly 6% decline for the whole species. Moreover, niche overlap between the two local populations was significantly lower than that between each local population and the species-level niche ( p < 0.01). This study demonstrates that niche divergence manifests itself within the two isolated populations of P. subaequalis. Our findings underscore the necessity of incorporating intraspecific variation into ecological niche modeling for endangered and relict tree species to develop appropriate regional conservation strategies.