Modelling the long-term dynamics of populations and communities of trees in boreal forests based on competition for light and nitrogen

This paper describes an individual-based and spatially explicit model for computing the long-term succession of a population or community of trees and the turnover of carbon and nitrogen in a forested ecosystem. In the model ecosystem trees are located within a simulated plot in a grid of cells that are sufficiently small to contain not more than one tree. Each tree consists of five mass compartments (stem, branches, leaves/needles, coarse roots and fine roots) and has its own area, varying in time, for the acquisition of nitrogen. Each tree competes with its nearest neighbours for light and nitrogen; i.e. growth depends on the limitations on light or nitrogen. The calculation of biomass production is based on the potential biomass increment, obtained by means of an integrating parameter for tree net primary production (NPP) in the form of the maximum possible biological productivity of the leaves/needles. Growth under the limited light and soil nitrogen are calculated, and the smaller of the two is used as the realised growth. The total growth of each tree is allocated to different mass compartments using species-specific proportions related to the age of the tree. The litter cohorts are assumed to decompose to form a pool of soil organic matter (SOM) in a manner that is dependent on climatic conditions and the quality of the litter. The simulated plot has an explicit nitrogen–carbon balance based on the turnover of these in the ecosystem linked to the dynamics of organic matter in the soil. The model, which allows standard forest inventory data to be used as input, has been constructed using an object-oriented approach. Comparison of the output of the model with growth and yield tables shows that the current model provides quite similar time courses for the main tree parameters (height, diameter, basal area, etc.) in the case of Scots pine ( Pinus sylvestris), Norway spruce ( Picea abies) and birch ( Betula pendula) throughout Finland (60–70°N).