WEHAM (“Waldentwicklungs- und Holzaufkommensmodellierung“: forest development and timber resource modelling) is a forest growth model specifically designed to meet the requirements of and work with the data provided by the German “Bundeswaldinventur” (BWI, National Forest Inventory). It is used as the core instrument for nationwide future forest resource assessments, sustainability checks and for various ecological, economic or political planning purposes.
The model consists of three parts. The growth simulator extrapolates tree dimensions (diameter at breast height (DBH) and height). Repeated measurements from trees included in at least two inventories are used to generate growth curves describing the development of DBH over age. Each tree sampled in a plot is referenced to a growth curve representative of / correlated with its DBH and age. The diameter increment over the period of interest is then derived from this growth curve. Heights are not modelled but calculated from tariff functions. Otherwise, especially at higher ages, height growth was overestimated.
The management simulator describes, for every plot, the timing and intensity of thinning and cutting activities by using tree species, federal state, thinning type, thinning intensity (recurrence interval and target stand basal area), rotation period length, and target DBH as parameters. In addition, type of ownership is also taken into account in some federal states where differences in management between ownership types are known.
The grading module sorts every stem cut into size classes customary in the trade. It uses federal state, tree species, and top diameter of the stem as controls. In a first step, the timber volume of a given tree or stand is calculated as cubic metre solid volume over bark, then bark, normal losses due to harvest and cut-to-length division are subtracted.
WEHAM projects the development of single trees, thus results can be aggregated at various levels, from plot to stand, region or state level, by species or species groups, by age classes, and at any combination of these classifications. By default, all projections are done in five years steps and the state of the forest (plot, single tree) at the beginning and at the end of this period are given, also as mean annual changes during these five years. The calculation of carbon stocks is done using the same equations and tools as used for Kyoto and UNFCCC reporting, e.g., in the NIR 2010.
The control parameters used in the management simulator and the grading module describe the management of forests as was conducted and planned in 2000 – 2003. Actual management might differ from this due to changes in the economic environment or in owner preferences.
All results are based on estimates of future forest management and are thus not to be understood as predictions but as one plausible possibility. The overall time horizon was set to 40 years because the model does not take changes in growing conditions, silvicultural treatments or economic or socio-political framework conditions into account.