A synthesis of local adaptation to climate through reciprocal common gardens

Abstract

Contrasts of differences within plant species and ecotypes are often best examined in ecology, evolution and genetics through provenance and biogeographical comparisons. Climate adaptation studies in plants are no exception and benefit from experiments that use these sets of factors. Reciprocal common gardens are a tool used to test for local adaptation in species to different contexts including climate. A synthesis of common gardens and intraspecific tests for climate adaptation was used to compile over 200 studies that explored the relative efficacy of this tool and the ecology of change. Exclusion criteria were applied to review this literature and to compile specific tests that explicitly examined climate, plants and reciprocity in gardens for a total of 70 independent instances. A meta-analysis was used to test for consistency and significance of detecting ecotypes for the different categories of traits tested and by the transplanting of seeds or seedlings. This meta-analysis provides clear evidence for plant adaptation to climate change because all significant effect size estimates were positive, relatively large, and both seed and seedling transplants demonstrated consistent evidence for local adaptation. Emergence and germination responses from seed transplant experiments and relative growth and biomass differences from seedling transplants provided particularly strong support. Synthesis. Reciprocal common gardens were a highly effective experimental design to test for ecotypic differentiation and for climate adaptation. Nonetheless, we propose that future studies clearly define whether ecotypes are being explicitly tested in common garden experiments to enable evidence syntheses and discovery, and we highlight the need for reciprocal climatic gardens to clearly test for continued capacity for local adaptation in response to divergent climate selection processes in many plant species.