Abstract
The ability of a population to genetically adapt to a changing environment is contingent not only on the level of existing genetic variation within that population, but also on the gene flow received from differently adapted populations. Effective pollen‐mediated gene flow among plant populations requires synchrony of flowering. Therefore differences in timing of flowering among genetically divergent populations may reduce their ability to adapt to environmental change. To determine whether gene flow among differently adapted populations of native Scots pine (Pinus sylvestris) in Scotland was restricted by differences in their flowering phenology, we measured timing of pollen release among populations spanning a steep environmental gradient over three consecutive seasons (2014–2016). Results showed that, over a distance of 137 km, there were as many as 15.8 days’ difference among populations for the predicted timing of peak pollen shedding, with the earliest development in the warmer west of the country. There was much variation between years, with the earliest development and least synchrony in the warmest year (2014) and latest development and greatest synchrony in the coolest year (2015). Timing was negatively correlated with results from a common‐garden experiment, indicative of a pattern of countergradient variation. We conclude that the observed differences in reproductive synchrony were sufficient to limit gene flow via pollen between populations of P. sylvestris at opposite ends of the environmental gradient across Scotland. We also hypothesize that continually warming, or asymmetrically warming spring temperatures will decrease reproductive synchrony among pine populations.
Highlights
A characteristic of many boreal and northern temperate tree species is the capacity for long-distance pollen dispersal by wind, and high levels of gene flow between populations are thought to be widespread (Kremer et al, 2012; Savolainen, Pyhäjärvi, & Knürr, 2007)
When thermal time (GDD) was considered in place of calendar time, we found that the pattern was reversed whereby a lower heat sum has been accumulated at Allt Cul by the time trees are predicted to be shedding pollen than at Beinn Eighe (Figure 6b)
There were large differences in the predicted timing of peak pollen production between the sites sampled in each year and between years
Summary
A characteristic of many boreal and northern temperate tree species is the capacity for long-distance pollen dispersal by wind, and high levels of gene flow between populations are thought to be widespread (Kremer et al, 2012; Savolainen, Pyhäjärvi, & Knürr, 2007). In Scotland, P. sylvestris persists in 84 fragmented semi-natural populations, known as the “Caledonian pinewoods,” thought to represent only 1% of its former maximum distribution (McVean & Ratcliffe, 1962) Despite this severe fragmentation, levels of neutral genetic variation remain similar to those observed in more continuous parts of the species range in Eurasia, with the majority of the genetic variation held within rather than between populations (Forrest, 1980; Forrest, 1982; Kinloch, Westfall, & Forrest, 1986; Provan et al, 1998; Wachowiak, Iason, & Cavers, 2013; Wachowiak, Salmela, Ennos, Iason, & Cavers, 2011). We consider whether the degree of synchrony in reproductive phenology observed between populations in different environments could limit long-distance gene flow and compare our observations with the extent of genetic connectivity suggested by previous marker-based studies (Forrest, 1980; Kinloch et al, 1986; Provan et al, 1998; Wachowiak et al, 2011, 2013)
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