Modelling the Afforested System: The Forest/Tree Model

Abstract

A forest/tree model has been developed of which the main growth processes are based on the CENW model. The model links the flows of carbon (C)), energy, nutrients and water in trees and soil organic matter. Modelled tree growth depends on physiological plant factors, the size of plant pools, such as foliage mass, environmental factors, such as temperature and rainfall, and the total amount and turn-over rates of soil organic matter, which drives mineraliZation of soil organic nitrogen (N). The forest/tree model has been developed as a generic model for coniferous trees. In addition, the model has been extended generically for deciduous trees by shedding of leaves in autumn and including growth buds in which C can be stored during winter time, and from which re-growth is initiated in spring. In spring the initial C gives the leaf activity an augmentation for photosynthesis. For the purpose of the available input parameters in the AFFOREST project it was needed to change the daily time step of the original CENW-model into a fortnightly time step. In relation to this, the original simple hump functions for N, water and temperature dependencies are replaced by smooth minimum/optimum/maximum functions. The model was validated against data obtained for Quercus robur (Oak) from the AFFOREST chronosequence in Vestskoven in Denmark. The different tree stands covered a chronosequence over a period of 35 years. The forest/tree model was successfully able to simulate all aspects of tree growth, which included water, N and C dynamics reflected in biomass production. An associated structure variable, i.e. mean tree height was also simulated successfully, which is an important input parameter for the amount of atmospheric N deposition in forests. The model includes essentially all relevant pools and processes that determine forest growth under a range of natural conditions.