Adaptive variation, including local adaptation, requires decades to become evident in common gardens

Population‐level adaptation to spatial variation in factors such as climate and soils is critical for climate‐vulnerability assessments, restoration seeding, and other ecological applications in species management, and the underlying information is typically based on common‐garden studies that are short duration. Here, we show >20 yr were required for adaptive differences to emerge among 13 populations of a widespread shrub (sagebrush, Artemisia tridentata ssp wyomingensis) collected from around the western United States and planted into common gardens. Additionally, >10 yr were required for greater survival of local populations, that is, local adaptation, to become evident. Variation in survival was best explained by the combination of populations’ home ecoregion combined with grouping of minimum temperature and aridity. Additional reductions in survival were explained by ungrouped (i.e., continuous) measures of garden‐to‐population‐origin separation in geographic distance (5% decrease in survival per 100 km increase in separation; R2 = 0.22) and especially in minimum temperature in younger plants (？4% per + °C difference, R2 = 0.56 vs. 0.29 in the 14th vs. 27th post‐planting years, respectively). Longer‐term common garden studies are needed. While we await them, uncertainty in adaptive variation resulting from short‐term observations could be quantitatively estimated and reported with seed‐transfer guidelines to reduce risks of introducing maladapted provenances in restoration. Knowledge of population‐level adaptation to spatial variation in climate and other factors is critical for predicting species’ responses to climate shifts (Valladares et al. 2014) and for ensuring that management actions such as restoration seedings are genetically and therefore ecologically appropriate. Phenotypic differences among populations that originate in different places but are grown together in common gardens provide the standard evidence used to identify and quantify adaptive variation among populations, including detection of local adaptation if both local and distant populations can be compared in multiple gardens (Blanquart et al. 2013). However, common gardens are rarely observed for more than a few years. For example, the mean duration of 308 common‐garden studies was 2 yr (Gibson et al. 2016), and many of these studies contribute to the mixed evidence for local adaptation, that is, only 70% of studies in a meta‐analysis detected it (Leimu and Fischer 2008). The short‐term focus is particularly prevalent among studies of perennial herbs and shrubs, though