ForGEM is a spatially explicit, individual tree model on genetics, ecology and management of forests. It includes detailed descriptions of the life-history of trees on:
- production and dispersal of seeds;
- the establishment, growth and competition for light of seedling cohorts or
- optionally individual seedlings;
- light interception by individual crowns;
- photosynthesis; (v) growth, including the allocation of the net primary production
- over the plant components, and the increment of the tree’s height, diameter and stem
- volume; and
- mortality of individual trees.
The model can simulate an understory that consists of grass and herb species that are distributed over, usually 20x20m, grids though grid size is variable. In the soil, water content soil organic matter and nitrogen availability is simulated:
In somewhat more detail, light interception can in ForGEM either follow a raytracing approach, similar as is used in the model SORTIE (Pacala et al. 1993) or horizontal light extinction only per light grid (that may deviate in size from the soil grid), where the integration interval is determined by the top and bottom of each individual crowns present at the grid.
For photosynthesis the standard Farquhar model is being used (Farquhar et al. 1980; Von Caemmerer and Farquhar 1981), including nitrogen-dependency of Jmax and Vcmax Forstreuter; (Forstreuter 2002; Leuning et al. 1995) and including the Leuning model of stomatal conductance (Leuning 1995). For the interaction of nitrogen content of the plant component with exchange of CO2 and H2O between the trees and the atmosphere and the soil we used the principle of optimization developed by Dewar (1996) (see Appendix), replacing the hyperbolic function of daily gross photosynthesis with both intercepted light and internal CO2 concentration with the Farquhar model. The model is validated on independent flux data of Quercus ilex in the MIND project (EVK2-CT-002-00158). Allocation of assimilates to the different plant components is done empirically and is presented in detail in Kramer (2001).
The soil sub-module used in ForGEM is based on the CENTURY dynamic terrestrial ecosystem model. CENTURY has been developed, tested and used over the past fifteen years to simulate the major pathways of carbon and nitrogen cycling of a multiple compartment organic matter sub model. (Parton et al., 1987, 1988, 1993, 1994; Kelly et al., 1993, Burke, 1997). The plant production sub model of CENTURY was replaced by the production sub-model of ForGEM. Water availability and flow through the system is simulated using a simplified water budget model which is mostly determined by soil texture and depth.
The reproduction of trees is described by: 1) the maximum production of seeds by an individual tree; 2) the variability of the production of seeds between years; 3) the seed dispersal distance and 4) survival and fraction of viable seeds.
Details on the production and dispersal of seeds of F. excelsior, and Q. robur and F sylvatica can be found in (Kramer 2004; Kramer et al. 2006).
Kramer K, Buiteveld J, Forstreuter M, et al. (2008a) Bridging the gap between ecophysiological and genetic knowledge to assess the adaptive potential of European beech. Ecological Modelling, 216, 333-353.
Kramer K, Vreugdenhil SJ, van der Werf BDC (2008b) Effects of flooding on recruitment and survival of riparian tree species: a field and modelling study on the floodplains of the river Rhine. Forest Ecology and Management, 255, 3893–3903.
Schelhaas MJ, Kramer K, Peltola H, Werf DCvd, Wijdeven SMJ (2007) Introducing tree interactions in wind damage simulation. Ecological Modelling, 197-209.
Schelhaas, M.J. (2008). The wind stability of different silvicultural systems for Douglas-fir in The Netherlands: a model-based approach. Forestry, 81 (3). - p. 399 - 414.