The physiological growth model BALANCE calculates the 3-dimensional development of individual trees or forest stands and estimates the consequences of environmental influences.

As an individual tree model BALANCE simulates growth responses on the single tree level, which enables also an estimation of the influence of competition, stand structure, species mixture, and management impacts because tree development is described as a response to individual environmental conditions and environmental conditions change with each individual tree development. Dimensional tree growth is calculated once a year based on the biomass increase of the woody tissue that has been accumulated during the last year by each single tree. Biomass of an individual tree is calculated from the dimensional variables tree position, tree- and crown base height, diameter, and crown radii. The increase in biomass is the result of the interaction of several physiological processes which depend on the physical and chemical microenvironment that is itself influenced by the stand structure.

The individual carbon-, water- and nutrient balances of the trees species beech, oak, spruce and pine are the fundamental processes for the simulation of growth. Each tree is structured in crown and root layers, which are in turn divided in up to eight crown- and root sectors. For each layer resp. each sector micro climate and water balance are calculated by using temperature, radiation, precipitation, humidity and wind speed measurements from climate stations. While these calculations are computed daily, the physiological processes assimilation, respiration, nutrient uptake, growth, senescence and allocation are calculated in monthly or decadal time steps (= 10-day periods) from the aggregated driving variables. This way, CO2-concentration, soil condition, competition between individuals, and stress factors, as for example air pollution and nutrition deficiency, can be considered besides the weather conditions when modelling the growth of trees. Based on the individual carbon balance, dimensional changes and mortality of a tree are computed annually.

BALANCE includes different approaches for the estimation of the stand climate for each individual tree. The Penman-Monteith equation is the base of the water balance calculations. Via the stomatal closure water balance is connected with photosynthesis, which is calculated in BALANCE by using the approach of Haxeltine and Prentice (1996). Carbon and nitrogen allocation into roots, branches, foliage and stem is computed according to functional balance and pipe model principles.

To depict the relationships between the environmental influences and growth the annual cycle of foliage development must be known. With the beginning of bud burst foliage, biomass and leaf area as well as light availability and radiation absorption change. Thus, the date of foliage emergence in a tree determines its assimilation and respiration rate but also affects the environmental conditions of the trees in its vicinity. In BALANCE the beginning of bud burst is modelled by using a temperature sum model (Rötzer et al. 2004), while foliage senescence is estimated in dependence of the respiration sum.

Because tree development is described as a response to individual environmental conditions and environmental conditions change with the individual tree development, environmental influences can be assessed in any kind of species mixture or stand structure.