Dynamics of diameter and height increment of Norway spruce and Scots pine in southern Finland

Key message Onset and cessation of radial and height increment of Norway spruce ( Picea abies (L.) Karst.) and Scots pine ( Pinus sylvestris L. ) in southern Finland were independent phenomena. They both contributed to the increment period duration, which was a more crucial factor defining the magnitude of annual radial and height increment. Context Phenology of diameter and height increment is a critical component of growth, also contributing to damage and survival of trees. Aims We quantified annual variation in intra-annual tracheid production and height increment of Norway spruce ( Picea abies (L.) Karst.) and Scots pine ( Pinus sylvestris L. ). Methods The number of tracheids and the day of the year for the onset and cessation of tracheid production were monitored from microcores collected repeatedly during growing seasons 2001–2012 in southern Finland. Weekly height increment was also measured in an adjacent sapling stand in 2008–2012. Results The first tracheids in pine were found around mid-May and in spruce a week later. The cessation of the tracheid production occurred during the last week of August for both tree species. Increment onset and cessation were independent phenomena, both contributing to the magnitude of tracheid production via increment period duration, which appeared to be a more crucial factor defining the number of tracheids. Duration of the height increment period was also related to shoot length but the connection was less tight than the link between the duration of tracheid production and the number of tracheids. A thermal threshold around 100 d.d. (degree days) was found for the onset of radial increment. No single environmental factor triggered the cessation of tracheid production, but in some years, soil water availability appeared to play a role. Conclusion The results indicate that extending growing seasons due to the climatic warming may increase growth in the Finnish forests.