Estimating annual GPP, NPP and stem growth in Finland using summary models

In this study we introduce and test a new simple approach for estimating annual stand-level gross primary production (GPP), net primary production (NPP) and stem biomass growth based on carbon acquisition and allocation, by combining existing summary models. The focus is on the variation of GPP and NPP across different parts of Finland caused by climate. A summary model for estimating light-use efficiency is applied to calculate the annual GPP of a fully closed canopy (P0) for different sites as a function of climatic conditions. The site-specific P0s are first determined for five sites using long-term series of daily weather data from the Finnish Meteorological Institute (FMI), and these results are further generalised to all sites using effective temperature sum (ETS) as a predictor. Actual canopy GPP is calculated as a fraction of the estimated site-specific P0 using a summary model of the effect of shading on photosynthesis in non-homogeneous forests. Leaf area is estimated from measured tree data using the pipe model. NPP:GPP ratio is estimated on the stratum level from mean height. The allocation to stem growth is estimated from stand age and site type, using a summary equation based on results from earlier studies. The method was tested against data from permanent sample plots of the Finnish National Forest Inventory (NFI) from years 1985 and 1995. The results indicate that the approach produces realistic short-term estimations for stem growth. At the stand level the model was nearly unbiased (2.1% underestimation), with RMSE of 34% and R2 of 0.52, and it provided a clearly better fit than a simple linear prediction of stem growth from the estimated GPP. More importantly, we showed in a model comparison that in the present data set our model provided results of similar accuracy as a well-established empirical tree-level growth model.