For plant species important in ecological restoration, seed transfer zones have been developed to maximize the probability that sown seed will germinate, establish, persist, and reproduce without negatively impacting the genetic composition of remnant plant populations. However, empirically based seed transfer zones have not been developed for most species. In their absence, maps based on ecological or climatic variables have been suggested as proxies. In the United States, these maps typically include the Environmental Protection Agency's Levels III and IV Ecoregion maps and the US Forest Service's Provisional Seed Zones. Maps of different spatial scales represent a compromise between economic and ecological considerations; those that delineate larger seed transfer zones are less costly to implement but impose more risk of poor adaptation to local conditions. To test the relative suitability of each map in delineating seed transfer zones, we conducted common garden experiments using five forb species found throughout the Great Basin and measured variation in traits thought to influence plant performance. We distinguished between environmentally and genetically controlled variation in measured traits and assessed how well this variation was explained by different candidate seed transfer zones. We found significant, population-level variation in all species for most measured traits. All tested seed transfer zones significantly explained some of this variation, but the proportion explained generally decreased with increasing zone size. Results suggest the intersection of Provisional Seed Zones and Level III Ecoregions was the best proxy for formal seed transfer zones developed based on common garden studies. This spatial scale captured 80% of the variation among source populations on average, and represents a viable compromise between ecological and economic considerations.
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