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One of the most important processes in maintaining genetic diversity is gene flow. Trees are efficient at moving their genes across the landscape as they produce large quantities of pollen and seed and these are well adapted to disperse over long distances. Other flora and fauna that occupy our woodlands are often less efficient at achieving this as they may produce fewer offspring and/or are less capable of dispersing these over long distances. This may impact on their ability to survive the challenge of climate change if their capacity to genetically adapt to novel conditions is restricted and their ability to relocate themselves or their offspring to more favourable locations is limited. The structure of the landscape in which they exist and the distribution of suitable habitat at a range of spatial scales may also impact on gene flow in poorly dispersing species.
Tree populations tend to maintain high levels of genetic diversity and this is partly due to their ability to practice long distance gene flow. Using microsatellite markers, we have examined pollen flow in species such as oak and cherry and shown that a considerable proportion of the seed is the product of pollen originating from outside the stand (Cottrell et al. 2009; Gerber et al. 2014). This regular influx of pollen from distant sources may provide genetic material that is preadapted to the warmer, drier conditions that are predicted to result from climate change and this may contribute to the population’s ability to genetically adapt to the novel conditions.
Timing of flowering is also important when considering gene flow because effective pollen‐mediated gene flow among plant populations requires synchrony of flowering. A recent study explored the annual and spatial differences in timing of male flowering in Scots pine populations spanning a steep environmental gradient across the Scottish Highlands. Observed differences in reproductive synchrony were sufficient to limit gene flow via pollen between populations of Scots pine at opposite ends of the environmental gradient across Scotland (Whittet et al. 2017).
In comparison to the trees themselves, gene flow can be much more limited in species that occupy the woodlands. In twinflower, a rare plant which grows in the Caledonian pinewoods, gene flow is highly restricted. This is because twinflower depends on small insects for pollination and these do not move large distances between flowers. This species has experienced severe habitat loss which has resulted in many of the remaining patches consisting of a single clone. As this species is self-incompatible these single clone patches are largely genetically isolated because the pollinators cannot travel the long distances between single clone patches. Consequently, the opportunity to reproduce sexually is very limited (Wiberg et al. 2016). This greatly limits the ability of the Scottish population of twinflower to adapt to climate change.
The distribution of trees in the landscape can also influence gene flow in our woodland fauna. Our studies demonstrate that populations of wood ants are highly differentiated in a landscape in which suitable habitat is fragmented compared to a landscape in which suitable areas of woodland habitat are more connected. (Vanhala et al. 2014).
Cottrell, J.E., Vaughan, S.P., Connolly, T., Sing L., Moodley, D.J. & Russell, K. 2009. Contemporary pollen flow, characterization of the maternal ecological neighbourhood and mating patterns in wild cherry (Prunus avium L.). Heredity 103 (2) 118-128.
Gerber, S. Chadoeuf, J. Gugerli, F. Lascoux, M. Buiteveld, J. Cottrell J. et al. 2014 High rates of gene flow by pollen and seed in oak populations across Europe. PLOS One. January 2014 | Volume 9 | Issue 1 | e85130 DOI: 10.1371/journal.pone.0085130
Vanhala, V., Watts, K., A’Hara S. & Cottrell J. 2014 Population genetics of Formica aquilonia wood ants in Scotland: the effects of long-term forest fragmentation and recent reforestation. Conservation Genetics DOI 10.1007/s10592-014-0584-1
Whittet, R., Cavers, S., Cottrell, J., Rosique-Esplugas, C. & Ennos, R. 2017 Substantial variation in the timing of pollen production reduces reproductive synchrony between distant populations of Pinus sylvestris L. in Scotland. Ecology & Evolution, DOI: 10.1002/ece3.3154
Wiberg, R. A. W., Scobie, A. R., A’Hara, S. W., Ennos, R. A. Cottrell J. E. 2016 The genetic consequences of long term habitat fragmentation on a self-incompatible clonal plant, Linnaea borealis L. Biological Conservation 201: 405-413.
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