Individual-tree growth models for Sitka spruce (Picea sitchensis) in Iceland

Sitka spruce (Picea sitchensis) is one of the main tree species used in forestry in Iceland, growing well in all parts of the country. In view of the importance of Sitka spruce in Iceland, there is need for a reliable growth model that will support multifunctional forest management and planning. This study developed growth models for Sitka spruce using data from permanent sample plots established by the Icelandic Forest Service between 1970 and 2020. The measurement interval on the plots varied from 3 to 16 years. To deal with irregular measurement intervals, an optimization approach was used to recover models for annual diameter increment and survival rate. The aim was to find parameters for diameter increment and survival models that would minimize the difference between the measured and simulated diameter distributions of the plots at the end of the measurement interval (when the simulation begins from the diameter distribution at the beginning of the interval). To enable the simulation of stand development, models for dominant height development and tree height were also developed.