Effects of Chronic Habitat Fragmentation on Population Genetic Processes in Temperate Tree Species. The example of rowan and ash in a deforested landscape and implications for native woodland restoration in southern Scotland

Concerns have been expressed regarding the viability of forest remnants due to detrimental genetic consequences of habitat fragmentation. However, empirical studies conducted so far suggest that population genetic processes respond in more varied ways than expected, thus highlighting the need for evidence from a wider range of species and in situations where fragmentation is long-standing. In southern Scotland, human- mediated deforestation for pasture since the Neolithic has dramatically altered the landscape. A single catchment (Moffat Dale) was intensively surveyed for severely fragmented populations of Sorbus aucuparia L., an insect pollinated bird dispersed species, and Fraxinus excelsior L., which is wind pollinated and wind dispersed. These remnants are being considered for seed collection in a native woodland restoration programme currently being implemented. Quantifying genetic variation at isozyme and chloroplast DNA markers in S. aucuparia remnants revealed that high levels of genetic diversity are maintained. However, genetic differentiation among remnants was detected for both types of marker and the estimated ratio of pollen flow to seed flow between fragments is close to one (r=1.36) suggesting reduced historical pollen-mediated gene flow but efficient seed dispersal. Similarly, F. excelsior remnants maintain high levels of genetic diversity at nuclear microsatellite markers and low interpopulation differentiation (Q=0.080). Using the neighbourhood model, it was estimated from open-pollinated progeny arrays that contemporary pollen flow is extensive and that effective pollen dispersal distance within the catchment averages 328 m. A detailed paternity analysis conducted on progeny arrays confirmed these results. Although pollen flow is an important component of realised gene flow, a parentage analysis showed that it is not predominant as 56.6% of the seedlings that recently established in Moffat Dale immigrated into the catchment. S. aucuparia and F. excelsior remnants in a severely deforested landscape are part of a wide reproductive network. Genetic diversity within remnants and gene exchange among them have been maintained by efficient long distance seed and pollenmediated dispersal, making remnants an appropriate seed source for planting stock.