Unveiling future maladaptation and preadaptation in locally adapted Mediterranean beech populations under climate change

European beech (Fagus sylvatica) exhibits a broad distribution across Europe and has undergone local adaptation in many populations, enabling its persistence across diverse climatic conditions. We analysed eighteen Catalan populations using landscape genomics. We uncovered weak but structured genetic differentiation, revealing three main genetic provenances.  Leveraging Genotype-Environment Association (GEA) methods, we also examined adaptive potential, identifying linear (LFMM, BayPass, RDA) and non-linear (Gradient Forests) relationships between allele frequencies and environmental variables. We identified numerous Single Nucleotide Polymorphisms (SNPs) correlated with environmental gradients, with an over-representation of gene classes potentially—and in some instances proven to be—associated with drought and heat stress responses. This suggests that genes regulating key physiological processes are involved in climate in adaptation. By applying Gradient Forest, we further mapped allelic turnover across the region and delineated three distinct genetic zones. Furthermore, we projected future genetic offset (2061–2080) under all the climate scenarios available for this period, pinpointing regions at heightened risk of maladaptation and reduced fitness. Due to the greater climate shift predicted in the Pyrenees, offset values were also higher there than in lower latitude forests, and specifically we observed the highest offsets in transition areas between valleys and ridges. We define a new index, Expected Genomic Sensitivity (EGS), that describes the required change of genomic composition by unit of climate (EGS = Genetic offset / Environmental offset), obtaining a normalised offset that let us compare populations identifying the ones that are more preadapted to their future climate. This index also allowed us to recognize which populations are the more idoneous for a translocation experiment in a Pyrenean population, being those the ones more preadapted and with a lower offset to the future climate of the receiving population. Our results highlight the drought-temperature gradient as the primary driver of allele frequency shifts, alongside an isolation-by-distance pattern shaped by geographic distribution. These results underscore the importance of considering standing genetic variation in conservation and forest management strategies to mitigate the adverse effects of climate change, which threatens global biodiversity and may lead to local extinctions near the species’ drought tolerance limit.