Local adaptation is highly dependent on common garden conditions where seeds were propagated: evidence from a 7-year study on a dominant alpine meadow species

1. Combining common garden experiment with reciprocal transplant or sowing experiment is widely used to assess local adaptation in plants. This approach effectively minimizes the potential influence of maternal environments derived from seed origin. However, the impact of divergent common garden environments on local adaptation assessment has received limited attention in previous studies.2. To investigate the effects of diverse common garden conditions on assessment of local adaptation, we conducted a 2-year common garden experiment followed by a 5-year reciprocal sowing experiment, both carried out at two different elevations. Seeds from low and high elevation populations of Elymus nutans were directly sown in the common garden experiment to propagate seeds for the subsequent reciprocal sowing experiment. Multiple traits, including seedling emergence, survival, plant height, aboveground biomass, number of reproductive branches, and number of seeds per individual, were measured in both experiments. This comprehensive method allowed us to examine variations in local adaptation across different growth times and life cycle stages.3. The original low-elevation population consistently outperformed the foreign population at the low-elevation site, regardless of whether seeds were propagated in low or high-elevation common gardens, indicating clear local adaptation. In contrast, the original high-elevation population showed local adaptation only when seeds were propagated in the low-elevation common garden. Long-term experiments revealed a gradual decline in the strength of local adaptation in E. nutans over time. This adaptation was most evident in number of seeds, with smaller advantages observed in seedling survival and number of reproductive branches. No evidence of local adaptation was detected in other traits.4. Synthesis. The local adaptation of E. nutans was primarily observed during early growth time and was highly dependent on the common garden conditions in which the seeds were propagated. Therefore, it is crucial to fully consider common garden conditions when evaluating plant local adaptation through common garden and reciprocal transplant experiments.