Remaking a stand: Links between genetic diversity and tree growth in expanding Mountain pine populations

In long-living trees, the links between individual genetic diversity and fitness are not fully understood. Yet, the lack of information on the relationships between genetic diversity and radial growth in tree populations is alarming, particularly at species distribution limits given that these marginal populations are expected to be vulnerable against climate warming and to colonize new areas. We aged and genotyped 160 individuals in two mountain forests forming distribution limits of Mountain pine (Pinus uncinata) in northern Spain: a treeline population (n = 60 trees) and a relict population (n = 100 trees). We reconstructed their radial growth using dendrochronology to evaluate their long-term fitness. We aimed: (i) to quantify the levels of genetic diversity and its contribution from contemporary gene flow and, (ii) to evaluate the associations between genetic diversity and tree growth. Parentage analysis was used to determine if individuals were the product of local reproductive events or long-distance pollen vs. seed dispersal during the second half of the past century. Furthermore, we modeled growth patterns as a function of site and tree size together with genetic diversity. We found that genetic variation in the two distribution limits of Mountain pine remains relatively high, and it was greater in juvenile trees. The positive association between individual genetic diversity and growth in juvenile trees suggests that favourable climate conditions in the last decades appear to have facilitated growth in juvenile individuals with greater genetic diversity. We also found that long distance gene flow have occurred in both populations likely contributing to population expansion and encroachment, but it only gradually increased in the treeline population during the last 50 years. Our results suggest that the levels of genetic diversity and its positive relationship with growth might have implications in the local tree response under warmer climatic conditions and in the way how trees colonize new areas. Further studies are necessary to determine the extent and implications of the relationship between individual genetic diversity and growth at the tree level.