Long-term fertilization drives genetic trait differentiation without changing intrinsic drought resilience in grassland populations

Long-term fertilization alters plant traits and is widely thought to reduce grassland drought resilience. While these effects are often attributed to species turnover, trait responses can also arise within species, through phenotypic plasticity and evolutionary divergence among populations. However, whether intraspecific evolutionary changes in response to long-term fertilization alter ecosystem responses to drought remains poorly understood. Here, using the >100-year Park Grass Experiment (UK), we tested whether long-term fertilization has led to heritable trait divergence and altered drought resilience in a dominant grass species. We collected 20 genotypes of Anthoxanthum odoratum from each of two fertilized plots and control plots and grew them under common garden conditions to isolate genetic differentiation from phenotypic plasticity in plant traits. Plants were then subjected to a two-week simulated drought followed by two weeks of recovery, during which CO₂ fluxes were measured. We found that genotypes originating from fertilized plots showed higher vegetative and reproductive height compared to those from control plots, indicating heritable divergence after a century of nutrient enrichment. Nevertheless, drought resilience of CO₂ fluxes, including drought resistance and recovery, did not differ among genotypes from different plots. Across all genotypes, drought recovery was positively associated with shoot biomass, root tissue density, and root diameter, but negatively associated with reproductive height. Our findings reveal that long-term fertilization can drive evolutionary shifts in plant height-related traits without affecting intrinsic drought resilience, highlighting a decoupling between evolutionary responses to nutrient enrichment and functional responses to climate extremes. This underscores the need to integrate eco-evolutionary processes into predictions of ecosystem responses to global change.