Root biomass fraction of forest-planted sugar maple seedlings varies with climate of population origin across a latitudinal gradient

Abstract Planting seedlings beyond a species’ or population’s current distribution is a potential management strategy to help plants keep pace with changing climate. The extent to which both climate of origin and spatial climate differences affect seedling traits such as biomass allocation, however, remains poorly understood. Sugar maple ( Acer saccharum ) seedlings from three USA source populations with contrasting climates (from south to north; Missouri, Iowa, Minnesota) were grown for three years at nine forested sites, from Arkansas, USA, to Ontario, Canada, to test hypotheses about ecotypic and biogeographic variation in root biomass fraction (RMF, assessed herein with root biomass/[root biomass + stem biomass]). Sites spanned the north-south transect of the species’ range and extended beyond both northern and southern range margins. Our two main predictions were (i) seedling RMF would increase from southern to northern sites as a reflection of increasing resource constraints, and (ii) for similar reasons, seedlings with origins from further north would on average have both greater RMF and less plasticity in RMF than seedlings from further south. In contrast to our predictions, southern-origin seedlings overall had higher RMF than northern-origin seedlings, with central-origin seedlings intermediate. Southern-origin seedlings had greater RMF at northern than at southern sites, as predicted, but northern and central-origin seedlings did not. Determining how belowground allocation and its plasticity vary across populations and naturally existing biogeographic gradients offers a window into how species may respond to ongoing climate change.