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| A Conceptual Model for Individual Tree Development Based on Physiological Processes |
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Grote
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A conceptual tree growth model is presented, which calculates the three-dimensional development of a tree in dependence on its individual carbon, water, and nitrogen balance. The basis to model such an individual development is the estimation of available radiation, soil-water, and nutrients at the scale of crown and root system fractions. The resources available within these fractions are calculated from environmental conditions, which are modified according to the height, crown size, foliage density, and root extension of the surrounding trees. The concept does not include a sophisticated radiation model, but determines radiation interception by means of a modified competition-index in order to decrease computational demands.
Temperature and the availability of resources determine exchange rates of carbon and nutrients for each fraction of the crown- and root system. Senescence is represented by compartment-specific turnover rates, which can be enhanced by environmental stress factors. Allocation of carbon and nitrogen into foliage, fine roots, branches, coarse roots, and bole are calculated according to functional-balance and pipe-model principles. The challenge, however, had been the representation of growth within the spatial fractions. Therefore, efficiency-indices are defined from the relation between assimilation and respiration in these fractions, and carbon increase of each compartment besides stem wood is distributed according to the relative size of these indices.
The new concept combines advantages of two types of models. Firstly, it represents the dependency of tree development on individual environmental conditions and vice versa, as is the basic feature of stand growth models currently used for representing long-term forest developments. The individual representation of trees enables the application of these models to any kind of species mixture or stand structure. Secondly, the physiological-based approach ensures that the model could be used for investigations of complex environmental changes, e.g. CO2-concentration, precipitation, temperature, nitrogen deposition, or air pollution, which have formerly been investigated with one-dimensional models only. The model is particularly meant to support the interpretation of measurement results at intensively investigated forest sites, where tree parameter could be determined and modeling results could be evaluated. It could also be used, however, for scenario analysis, including combined changes of environment and management in order to develop adapted management strategies.
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