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Summary Report No. 32

Evaluation of the physiologically-based forest growth model FORSANA

R. Grote, M. Erhard, F. Suckow (November 1997)

This report includes three separated papers, which together perform the evaluation of the forest growth model FORSANA. The papers emphasis on the models new features and include the description of the process formulation as well as a sensitivity analysis. Furthermore, some comparisons of model results with measurements from three Scots pine (Pinus sylvestris L.) plantations in eastern Germany are presented, to show the suitability of the approach.

Two of the papers are concerned about the physiological processes, which are calculated with a daily time step from initial stand and soil conditions, weather data, air pollution and deposition. The first one concentrates on the water balance and highlights the importance of fine root distribution, sapwood water storage and competition by ground vegetation for transpiration and drought stress of the trees. These features are simulated dynamically in FORSANA, using mechanistic dependencies on environmental conditions.

The second paper describes the carbon and nitrogen allocation as well as the representation of compartment mortality, which all are calculated daily in direct or indirect dependence on environmental conditions. The approach is based on the seasonal changes of carbon (and nitrogen) demand in different compartments, which is induced by the phenological development of the foliage and different mortality rates of foliage, sapwood and fine roots.

In the third paper, calculations are described, with which the annual change of the average tree dimensions and the new stem number are represented. The stand responses to the changing environmental conditions depend exclusively on the cumulative growth and mortality of the tree compartments during the previous year. An additional stem number reduction due to management can be taken into account, which considers an increased harvesting intensity with increasing stand height growth. This routine is tested against field data from several hundred stands, which were exposed to very different air pollution and deposition conditions over 27 years.

The papers demonstrate that FORSANA is suitable to assess short-term and long-term responses of Scots pine plantations up to several decades of changing conditions of (SO2) air pollution and (nitrogen) deposition. It can be initialised with mere forest inventory data and coarse information about the soil and can thus be applied also on a regional scale, as far as exposition and lateral water flows can be neglected. Additionally, the model has been developed to be fully sensitive to climatic variables (including CO2), because it is assumed that natural and anthropogenic impacts interact with each other. Thus, it can be used not only to investigate impacts of changes in air pollution but also for climate change assessments.

Grote, R., Suckow, F.: Integrating dynamic morphological properties into forest growth modelling, I. Effects on water balance and gas exchange

Grote, R.: Integrating dynamic morphological properties into forest growth modelling, II. Allocation and Mortality

Grote, R., Erhard, M.: Simulation of tree and stand development under different environmental conditions with a physiologically based model

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