Spatial grain of adaptation is much finer than ecoregional-scale common gardens reveal.

Adaptive variation among plant populations must be known for effective conservation and restoration of imperiled species and predicting their responses to a changing climate. Common-garden experiments, in which plants sourced from geographically distant populations are grown together such that genetic differences may be expressed, have provided much insight on adaptive variation. Common-garden experiments also form the foundation for climate-based seed-transfer guidelines. However, the spatial scale at which population differentiation occurs is rarely addressed, leaving a critical information gap for parameterizing seed-transfer guidelines and assessing species' climate vulnerability. We asked whether adaptation was evident among populations of a foundational perennial within a single "empirical" seed-transfer zone (based on previous common-garden findings evaluating very distant populations) but different "provisional" seed zones (groupings of areas of similar climate and are not parameterized from common-garden data). Seedlings from three populations originating from similar conditions within an intermediate elevation were planted into gardens nearby at the same elevation, or 250-450 m higher or lower in elevation and 0.4-25 km away. Substantial variation was observed between gardens in survival (ranging 2%-99%), foliar crown volume (7.8-22.6 dm