A new approach to study local adaptation in long‐lived woody species: Virtual transplant experiments

Despite the importance of local adaptation and the extended literature that has addressed it, there are few methods available to explore local adaptation across large temporal scales. However, long‐term patterns are likely to be essential to understanding adaptation in long‐lived species, such as trees. Here, we propose a methodology named ‘virtual transplant experiment' (VTE), which uses long‐term climatic variability to explore local adaptation to climate in natural tree populations. VTEs evaluate the historical response of populations to their local climate and to climates representative of conditions in other populations. We tested our methodology using simulated data and applied it in two case studies on: (a) Pinus nigra populations at the edge of the species distribution, where previous research has suggested strong climate adaptation, and (b) Fagus sylvatica mesic populations, where parallel experiments showed no adaptation to macroclimate. VTE results from simulated and real‐world data matched our expectations, suggesting that the method accurately identified the patterns of local adaptation to climate in tree populations. VTEs consistently discriminated locally adapted populations in synthetic data with a known degree of local adaptation. As expected, P. nigra populations showed adaptation to local climate in the VTE, while F. sylvatica populations showed no overall local advantage. Our method provides a new way to test for local adaptation over time scales encompassing the complete lifespan of trees. VTEs can complement current methods to study local adaptation by adding the ability to explore the long‐term response to local climate in natural populations. The advantages and limitations of the different approaches to studying local adaptation stress the importance of combining multiple approaches to test for local adaptation in long‐lived organisms.