Restructuring tree provenance test data to conform to reciprocal transplant experiments for detecting local adaptation.
Local adaptation is a fundamental assumption in delineating seed zones and developing seed transfer guidelines to safeguard climatic adaptation of tree and plant species during forest regeneration and ecological restoration. It is also broadly assumed for forest tree species that show genetic differentiation among geographic populations, especially for those with widespread natural distributions that occur in distinct environments. However, due to a scarcity of suitable data, the inference of local adaptation has rarely been validated for forest tree species through reciprocal transplant experiments (RTEs). In this study, we illustrate a novel approach to restructure tree provenance test data to conform to RTEs and use recently proposed statistical models to detect local adaptation, using white spruce Picea glauca (Moench) Voss as an example. Our research demonstrates how similar studies can be conducted to validate local adaptation in other forest species and populations, for which RTEs are lacking, but where large, high-quality provenance test data sets are available. Contrary to common belief, our results show that local adaptation is absent in survival and height for white spruce populations from Ontario, Canada, although they have evolved in considerably different climatic habitats, and exhibit substantial and clinal genetic differentiation and significant genotype-by-environment interactions in these two adaptive traits. Synthesis and applications. Our results show that the common assumption of local adaptation in forest tree species may not necessarily be correct within significant portions of their natural range. In forest genetic studies, population differentiation in adaptive traits has often been attributed to local adaptation without rigorous validation. Our results caution against such interpretation of experimental results. In the absence of local adaptation as shown by reciprocal transplant experiments, assisted migration may be considered as an option for enhancing forest adaptation to climate change.