Seasonal patterns of photochemical capacity and spring phenology reveal genetic differentiation among native Scots pine ( Pinus sylvestris L.) populations in Scotland

► Scots pine found over a spatially heterogeneous environment in Scotland. ► Little is known about patterns of adaptive genetic variation within the region. ► Variation in photochemical capacity and phenology studied in eight populations. ► Population and family differences found suggest adaption to home environments. ► A revision of current seed transfer guidelines is required. Environment-driven genetic differentiation among populations is a common feature among forest trees, and an understanding of how populations have adapted to their home site conditions is essential for management and conservation practices. In Scotland, 84 native Scots pine ( Pinus sylvestris L.) woodlands are recognised by the Forestry Commission and they occupy highly diverse environments from the maritime west coast to continental sites in eastern Scotland. However, it is not known whether adaptations to local environments along sharp temperature and rainfall gradients have occurred in different populations and as a result, the seed transfer guidelines of the species are based only on data from isozymes and monoterpenes. In this study of an outdoor common-garden trial, we used chlorophyll fluorescence to examine whether seedlings from 32 open-pollinated families and eight populations from sites experiencing contrasting annual temperature regimes differed in their response to variation in natural outdoor temperatures between September 2009 and May 2010. In addition, growth initiation in spring was recorded. Photochemical capacity at photosystem II F v/F m showed a distinct seasonal trend and remained at relatively high levels (∼0.7) until November. Following a period of over 2 weeks with temperatures below or close to 0 °C, F v/F m started decreasing towards its minimum values recorded in early March when population means varied between 0.35 and 0.45. By early May and along with rising temperatures, photochemical capacity had recovered to the same level as observed in early November. Populations were found to respond differently to the cold period starting in December. The largest drop in photochemical capacity was observed in seedlings from a low-altitude population located in the maritime western Scotland, while in seedlings from higher-altitude locations in the cooler eastern Scotland, the response was smaller. In March, the recovery of photochemical capacity was slowest in seedlings from the mildest and coolest sites. Evidence of adaptive genetic differentiation was also found