Tree density and population size affect pollen flow and mating patterns in small fragmented forest stands of pedunculate oak (Quercus robur L.)

In many parts of the world, once large and continuous forests have been replaced by a mosaic of isolated forest fragments embedded in an agricultural or urban landscape matrix. At the same time, stand characteristics such as tree population size and tree density have been strongly modified as a result of anthropogenic activities. Because increased geographical isolation can impede gene flow among forest fragments, and because decreased population size and tree density may reduce the number of local mating partners, this can be expected to lower the opportunity to trees for outcrossing, and to result in increased inbreeding, negatively impacting the viability of fragmented tree populations. In this study, we examined eight isolated stands of the wind-pollinated tree species Q. robur that strongly differed in population size, tree density and spatial isolation. In the centre of each Q. robur stand, adult leaves and seeds were collected in a circular plot, in which we evaluated the diversity and differentiation of the local pollen pool, and examined mating patterns. Most forest stands showed high proportions of out-of-plot pollen flow (range: 0.24–0.77), which were positively correlated with the number and density of adult trees within the forest stands. Despite high outcrossing rates (>0.998), seeds within seed families were stronger related than what could be expected under panmixia, which could be attributed to small but significant levels of correlated paternity (0.018–0.107) and biparental inbreeding (0.025–0.118) within the study plots. Next to increased coancestry coefficients, deviations from random mating also resulted in significant pollen pool differentiation (0.008–0.059) among seed parents. We also found that stand population size and tree density were significantly correlated to the relatedness of the seedlings and the degree of pollen differentiation within the study plots. These results suggest that, in small and isolated low density forest stands, reduced mate availability may decrease local pollen pool diversity, increasing the likelihood of consanguineous mating and pollen pool differentiation in the next generations. We conclude that preserving high levels of pollen flow within and between forest fragments may be more important in wind-pollinated tree species than what was previously thought.