Substantial variation in the timing of pollen production reduces reproductive synchrony between distant populations ofPinus sylvestrisL. in Scotland

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.

Hysteretic response of sap flow in Scots pine (Pinus sylvestris) to meteorological forcing in a humid low‐energy headwater catchment

“…Driven by the meteorological conditions, sap flux rates were higher in May–August than in April and September. The sap flux in April and September would be consistent with these 2 months being in the early and late period of annual growth of Scots pine (Martínez‐Sancho, Dorado‐Liñán, Hacke, Seidel, & Menzel, ; Whittet, Cavers, Cottrell, Rosique‐Esplugas, & Ennos, ). May in this year had relatively low precipitation input (~37 mm), which potentially limited the replenishment of root‐zone soil moisture.…”

Cryptic genetic variation and adaptation to waterlogging in Caledonian Scots pine, Pinus sylvestris L.

“…The extent of local adaptation is dependent upon the strength of differential selection relative to the homogenizing effects of gene flow (Kawecki & Ebert, ). Scots pine is wind‐pollinated, enabling reproduction across large distances; however, in Scotland, it has been shown that asynchrony in flowering phenology may serve to inhibit gene flow among populations (Whittet, Cavers, Cottrell, Rosique‐Esplugas, & Ennos, ). Related work on needle anatomical traits has shown differentiation for stomatal band and resin canal densities, consistent with adaptation to the same longitudinal rainfall gradient that generates adaptive differences in flood tolerance in P. sylvestris in Scotland, is also modest (Donnelly et al., ).…”

“…Scots pine is wind-pollinated, enabling reproduction across large distances; however, in Scotland, it has been shown that asynchrony in flowering phenology may serve to inhibit gene flow among populations (Whittet, Cavers, Cottrell, Rosique-Esplugas, & Ennos, 2017).…”

Local climate and habitat continuity interact to alter contemporary dispersal potential

“…Furthermore, it is a biologically meaningful climate variable as it reflects the required heat sum associated with the onset of growth (Beaubien and Hamann 2011). It is predicted to be particularly sensitive to climate change (McGinn and Shepherd 2003) as shifts in DD5 for spring perennials, such as Geum triflorum, may have substantial influence on the timing and duration of the growing season (Beaubien and Hamann 2011;Whittet et al 2017). Note: we also ran our analyses with the precipitation variables that loaded highly on PC2, however the results were rarely significant.…”