Evaluating the Climatic Suitability of Gentiana Pneumonanthe L. in the Iberian Peninsula Through Bio-Ecological Indicators

Gentiana pneumonanthe L. (marsh gentian) is a wetland specialist and the exclusive host plant of the Alcon blue (Phengaris alcon). This species is considered a ‘flagship species' of wetland conservation, helping to raise awareness and support protection efforts. However, its long-term viability is increasingly threatened by climate change. In the Iberian Peninsula (IP), the species currently occurs within protected areas designated under Natura 2000 and RAMSAR. This study aims to identify the bio-ecological indicators that influence the viability and growth of marsh gentian, determine the suitable regions for its persistence, and assess the future suitability zones, particularly within protected areas. From an initial set of 14 bioclimatic variables (ISIMIP3b, processed using the CHELSA method at 1 km² resolution) and two topographic variables, four bio-ecological indicators were selected based on Pearson correlation and Variance Inflation Factor analyses (VIF): Thermicity Index (It), Ombrothermic Index (Io), Accumulated summer precipitation from June to August (RR_summer), and Maximum of the daily maximum temperature of the hottest month (August) (TXX_aug). Species distribution modelling was performed using the Biomod2 platform, applying six modelling algorithms (GLM, GAM, RF, GBM, CTA and MARS), which were evaluated using a combination of non-spatial metrics (TSS, AUCroc, BIAS, CSI) and a spatial metric (Boyce index). Projections were produced for a historical baseline (1995–2014) and future periods (2041–2060 and 2081–2100), under two anthropogenic radiative forcing scenarios (SSP3-7.0 and SSP5-8.5). The ensemble model demonstrated strong predictive performance (Boyce index = 0.98). Historically, 13.4% of the IP was climatically suitable for the species, primarily in mountainous regions. Under SSP3-7.0, suitable areas are projected to decline by 74.2% (2041–2060) and 99.3% (2081–2100), while under SSP5-8.5, the reductions are estimated at 75.5% and 99.9%, respectively. Although minor gains may occur in the Pyrenees (up to 3.5% under SSP3-7.0 in 2041–2060 and 0.05% under SSP5-8.5 in 2081–2100). Furthermore, most protected areas are expected to lose climatic suitability for species persistence. Such declines may disrupt ecological processes and directly threaten the survival of P. alcon. These findings underscore the urgent need for climate-informed land-use planning and effective habitat conservation strategies. Funding: This work is supported by National Funds by FCT – Portuguese Foundation for Science and Technology, under the projects UID/04033/2025: Centre for the Research and Technology of Agro-Environmental and Biological Sciences (https://doi.org/10.54499/UID/04033/2025) and LA/P/0126/2020 (https://doi.org/10.54499/LA/P/0126/2020). Teresa R. Freitas acknowledges the financial support of Fundação para a Ciência e a Tecnologia (FCT) through the Individual Scientific Employment Stimulus contract 2024.09620.CEECIND.