Re‐Interpreting Genetic Offset: Quantifying the Least Required Evolutionary Rate Under Climate Change at the Mediterranean Range Margin of European Beech

Abstract

European beech (Fagus sylvatica L.) spans a wide range of European climates and exhibits evidence of local adaptation, which supports its persistence under diverse conditions. We analysed 18 populations—distributed across an altitude gradient on the southwestern geographic range edge of the species—using landscape genomics to assess their adaptive variation and vulnerability to future climatic conditions. We uncovered weak but structured genetic differentiation, revealing three main climate-tied genetic groups. Combining multiple Genotype–Environment Association (GEA) approaches—linear, such as Latent Factor Mixed Models (LFMM) or Redundancy Analyses (RDA), and non-linear (Gradient Forest)—we identified 373 Single Nucleotide Polymorphisms (SNPs) detected by all GEA methods as being putatively associated with climate gradients. Using the Gradient Forest model, we mapped genetic offset across all 21st century periods under a key climate scenario: Shared Socioeconomic Pathway (SSP) 5, forcing 8.5 W/m2 (SSP585) and across all SSPs for 2061–2080, identifying Pyrenean and pre-Pyrenean regions as maladaptation hotspots. To capture temporal dynamics, we introduce a novel approach to interpret genetic offset. The Required Evolutionary Rate (REvoRate) quantifies the minimum genetic change per year needed to keep pace with projected climates. Joint interpretation of offset and REvoRate revealed that some stands with moderate offsets face high short-term adaptive demands, while others with larger offsets are required to evolve more gradually. The drought-temperature gradient emerged as the main driver of allele frequency turnover, with geography contributing through isolation by distance. Together, genetic offset and REvoRate provide a dynamic framework to assess temporal maladaptation risk. Our results highlight the need to integrate standing genetic variation and evolutionary potential into forest management and conservation planning to ensure the persistence of F. sylvatica in one of its most climate-vulnerable range margins.