Intra-annual tracheid production of Norway spruce and Scots pine across a latitudinal gradient in Finland

Considerable changes in tree growth are projected due to the expected climate change. The expected changes of climate call for a better insight into the growth responses of trees to varying environmental conditions over large geographical regions. The aim of this study was to analyse the intra-annual tracheid production of Norway spruce (Picea abies (L) Karst.) and Scots pine (Pinus sylvestris L.) across a latitudinal gradient in Finland (60-68N). The number of tracheids and the day of the year for the onset, fastest rate, and cessation of tracheid production were determined from microcores repeatedly collected in nine stands during growing seasons of 2001-2009. The onset of tracheid production varied from late May in southern Finland to mid-June in northern Finland. On all stands, tracheid production initiated earlier and ceased later for Scots pine than for Norway spruce. On average, the fastest tracheid production rate occurred slightly after the summer solstice, but variation between sites and years was high. In the northernmost Scots pine stand, the length of the growing season was less than two months and the onset of tracheid production required clearly lower TS than elsewhere. The results imply that within Finland, year-to-year weather variation has a marked impact on the timing of tracheid production. However, the results indicate that the Norway spruce and Scots pine have adapted and are able to adjust their tracheid production according to the local conditions. 2014 Elsevier B.V. All rights reserved.