Incorporating phylogeography for modelling the distribution of the carob tree (Ceratonia siliqua, Leguminosae) in future climate change

Distribution estimates based on niche modeling are commonly used to forecast the impact of future climate change on biodiversity. The longer generation time of trees implies that adaptive responses to stress rely on the existing genetic diversity. However, the effects that climate change may have on the genetic diversity of fruit trees have rarely been addressed despite its agronomic importance. The carob tree has been used since ancient times to feed humans and livestock across Mediterranean rural landscapes. The long-term historical decline estimated for carob populations (Viruel et al. 2020 in J. Biogeogr.) have affected negatively to its genetic diversity. Here we study the potential future distribution of the carob tree and the likely consequences on its current genetic diversity. Niche models were constructed in MaxEnt with a large set of present occurrences (n=758) and six selected bioclimatic variables selected after pilot analyses and evaluated using TSS and AUC metrics. Distribution models were estimated for 2070 by projecting the niche-based models to two future climate scenarios (RCP 4.5 and 8.5) and five general circulation models. Our results suggest that the currently most suitable areas for carob trees could decrease by 44 % and 58 % under the RCP scenarios 4.5 and 8.5 respectively. Some genetic clusters might be more severely impacted. South Morocco which is a long term refugia for the carob tree according to Viruel et al. (2020 in J. Biogeogr.), could be also impacted. This prediction is particularly worrying since these populations contain the highest genetic diversity. However, when distribution modelling was conducted with respect to phylogeography, Maxent results supports south Morocco (SM) and the east Mediterranean (EM) as the most persistent areas whereas the most affected areas are in the south of Spain (SS) and more generally in the rest of the western basin (CM). Therefore, our preliminary results indicate that incorporating phylogeography may improve carob distribution modelling and forecasting. In several countries, the abandonment of traditional orchards and the forest fragmentation in favor of more intensive agricultural systems and urban expansion will worsen this trend. Immediate ex situ and in situ conservation actions are recommended.