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Simulating the effect of nitrogen dynamics to the growth of boreal forest under changing climate, using a combination of interacting process-based models
Tapio Linkosalo, Annikki Mäkelä, Heljä-Sisko Helmisaari and Raisa Mäkipää

The growth of most boreal forests is currently limited by the amount of available nitrogen. Plants take up N from the soil, then return it to the soil systems in form of litter that is decomposed in the soil and released again for the plants to use. Increased temperature and atmospheric CO2 is expected to increase the growth potential of boreal forests, but this potential cannot be realized if there is lack of required nutrients. Increased temperatures will hasten the rate of decomposition of plant litter, but they may also reduce the soil moisture, which would then slow down the decomposition.
There are already process-based models that describe and predict both the stand growth and soil decomposition, but to grasp the interaction and dynamics of the whole system, both models need to be combined to be enumerated simultaneously. In this paper, we have combined a process based stand growth model PipeQual with a soil decomposition model Romul and a bucket model of soil moisture. The model combination describes the dynamics of soil water, organic matter/carbon and nitrogen between external environment, the stand and the soil.

The combination model is currently being validated using empirical measurements of stand, soil organic matter and nitrogen content. Under current climatic conditions, the nitrogen released from the soil processes and needed for the stand growth are balanced, suggesting that the individual sub-models are in good agreement, and thus promising realistic predictions of the nitrogen dynamics under changing climatic conditions. The model combination has already been used to predict the development of forest ecosystem carbon balance under current and changed climatic conditions, with various management options. We will further utilise the model to predict the effect of new energy-related management options, like whole-tree harvest, to the nitrogen balance and hence growth potential of the boreal forests.