Calibration models for diameter and height growth of Norway spruce growing in uneven-aged stands in Finland

The great interest in continuous cover forestry (CCF) in Finland has increased the need for tools to predict tree growth in uneven-aged stands. Growth predictions are commonly obtained with stand simulators like MOTTI, which is primarily targeted at even-aged stands for rotation forestry (RF) in Finland. The aims of this study were: 1) to test how the existing individual tree growth models of MOTTI worked for Norway spruce trees growing in uneven-aged stands in Finland; 2) to identify significant predictors if biases were detected; and 3) to compile calibration models in diameter or height growth. We used data based on 20 CCF Norway spruce permanent sample plots with a 20-25-year monitoring period measured in southern Finland. Simulation with MOTTI revealed obvious diameter and height growth biases in these stands. Growth was especially overpredicted for small trees. The current MOTTI predictions were unsuitable for CCF, and calibration models were therefore compiled using a linear mixed-effects regression approach. They were developed based on variables indicating tree size, tree- and stand-level competition, an uneven-aged stand structure, an asymmetrical competition variable, and dummy variables indicating the time since the last selection cutting in years. The resulting models predicted tree dbh and tree height more accurately than the existing MOTTI. The compiled calibration models will be incorporated into the MOTTI simulator to provide a more reliable prediction of tree diameter and height growth in uneven-aged stands. Moreover, the significant predictors reported in this study were considered as informative variables to develop calibration models for tree growth in CCF in a similar situation, where models are equipped only for RF.