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13 Pág., The possibility of fine-scale intraspecific adaptive divergence under gene flow is established by theoretical models and has been confirmed empirically in tree populations distributed along steep altitudinal clines or across extreme edaphic discontinuities. However, the possibility of microgeographic adaptive divergence due to less severe but more frequent kinds of soil variation is unclear., This work was supported by a CGL2015-64164-R project grant to J.J.R.-A. and Delphine Grivet (ICIFOR-INIA, CSIC) and by a BES-2016-078969 Ph.D. dissertation grant to A.J.-R., both co-financed by the Spanish Ministerio de Economía y Competitividad (MINECO) and the European Regional Development Fund (ERDF). We thank Delphine Grivet, Ricardo Alía, Rodrigo Pulido, Fernando Del Caño, Sergio San Segundo, Eduardo Ballesteros, and Regina Chambel (ICIFOR-INIA, CSIC) for helping with field and greenhouse work. We express our gratitude to Salvador Sastre and Isabel González from the Forest Ecology Laboratory (ICIFOR-INIA, CSIC) for conducting the soil analysis. We also thank the staff at the Dirección Provincial de Agricultura, Medio Ambiente y Desarrollo Rural en Guadalajara (Dirección General de Política Forestal y Espacios Naturales) for field work permit number 1080766. We thank Ivan Scotti and an anonymous reviewer for constructive suggestions on the manuscript.

17 Pág. Instituto de Ciencias Forestales (ICIFOR), The estimation of recent gene flow rates among vast and often weakly genetically differentiated tree populations remains a great challenge. Yet, empirical information would help understanding the interaction between gene flow and local adaptation in present-day non-equilibrium forests. We investigate here recent gene flow rates between two large native Scots pine (Pinus sylvestris L.) populations in central Iberian Peninsula (Spain), which grow on contrasting edaphic conditions six kilometers apart from each other and show substantial quantitative trait divergence in common garden experiments. Using a sample of 1,200 adult and offspring chloroplast-microsatellite haplotypes and a Bayesian inference model, we estimated substantial male gametic gene flow rates (8 and 21%) between the two natural populations, and even greater estimated immigration rates (42 and 64%) from nearby plantations into the two natural populations. Our results suggest that local pollen shedding within large tree populations does not preclude long-distance pollen immigration from large external sources, supporting the role of gene flow as a homogenizing evolutionary force contributing to low molecular genetic differentiation among populations of widely distributed wind-pollinated species. Our results also indicate the high potential for reproductive connectivity in large fragmented populations of wind-pollinated trees, and draw attention to a potential scenario of adaptive genetic divergence in quantitative traits under high gene flow., This work was supported by CGL2015-64164-R project grant to JJRA and DG and by BES-2016-078969 PhD grant to AJR, both co-financed by MINECO (https://www.mineco.gob.es) and ERDF (https://ec.europa.eu/regional_policy/en/funding/erdf/). Research was also supported by AEG 17-048 project grant from MAPAMA (https://www.mapama.gob.es), established in the frame of the measure 15.2 “support to the conservation and use of forest genetic resources” and under Regulation (EU) No 1305/2013 of the European Parliament and of the Council of 17 December 2013 on support for rural development by the European Agricultural Fund for Rural Development (EAFRD; https://ec.europa.eu/info/food-farmingfisheries/key-policies/common-agricultural-policy/ rural-development_en) with 75% co-financing. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. No additional external funding was secured for this study.