PROGNAUS – A Forest Growth Model for Inventory Data Update
Following a North American trend of the early 70ies, individual-tree growth models have been developed in Central and within the last two decades. These models are considered as alternative management tools replacing the classical yield tables. Advantage: stand development is predicted by aggregating the growth data of the individual trees and not by a direct prediction of area-related or mean stand attributes (e.g. stem number, basal area or volume per hectare or mean diameter, stand height or top height). Therefore, individual-tree growth models can describe the impact of a multitude of silvicultural treatment methods, tree species compositions and age combinations, thus providing detailed information on stand structure development (DBH-distribution, stand layers, tree species composition). Considering the trend towards individual-tree oriented forest management systems, this approach becomes increasingly important.
The Institute of Forest Growth Research of the Austrian University of Natural Resources and Applied Life Sciences (BOKU) in Vienna has developed the distance-independent individual-tree growth model PROGNAUS. The Federal Research and Training Centre for Forests, Natural Hazards and Landscape (BFW) has supplied data from the continuous Austrian National Forest Inventory (ANFI) for the parameterisation of the model. The forest growth simulator is available as an executable WINDOWS-application working under the version PROGNAUS for WINDOWS 2.3.
In all its components PROGNAUS runs as a distance-independent individual-tree growth model. This means that it does not need spatial positions of individual trees. In addition, it is not based on the potential growth concept, but it rather supplies tree species specific, direct estimates of increment (diameter and height) and mortality, depending on tree dimensions, competition variables, stand density, and a set of site factors as identified by the ANFI. The minimum diameter of all sub-models is 5 cm. By means of an ingrowth model the number, tree species and dimensions of those trees are estimated which exceed the minimum diameter within five years. In addition, the simulator contains various auxiliary models for the prognosis of stem damage, log size and log quality assortments.
In order to start a simulation run, diameter, height and tree species of all trees must be entered into the simulator. Furthermore, the simulator needs the plot size as well as the following site parameters: elevation, slope, exposition, relief, soil depth, humus thickness, soil type, soil humidity, vegetation type, and growth district (see Fig. 1, flow chart).
Based on the optional settings of a user, specific components or routines can be activated or deactivated (e.g. mortality model, ingrowth model, harvesting interventions, and models for stem quality predictions). Thus, according to the objective more or less complex prognosis runs are possible. The harvesting interventions may be both interactively (Fig. 2) or automatically executed, the results shown both graphically (Fig. 3) or in the form of a stand table.
With a few exceptions, the simulator has been used so far only for research and education at the Austrian University of Natural Resources and Applied Life Sciences in Vienna (BOKU). Among other matters it has been studied if PROGNAUS, in combination with continuous forest inventories, could be used to control sustainability at the district or enterprise level. Moreover, various management strategies in peeled stands were studied concerning their impact on the log quality distribution. In the future the Federal Research and Training Centre for Forests, Natural Hazards and Landscape (BFW) intends to use the simulator for training and experimentation purposes.
Basically, PROGNAUS can be used for all production forests in due to the representativeness of its parameterisation data. Nevertheless, the simulator should be subject to a verification, and – if necessary – to a local calibration whenever this is possible (e.g. if data of a continuous forest inventory are available). PROGNAUS is predisposed for updating relatively small sample plots of forest inventories due to the sampling method of its parameterisation data, its distance independency and its ability to process routine inventory data. Therefore, it is possible to run scenario simulations not only at the stand level but also at the enterprise, regional and national level what is the basic advantage of PROGNAUS.
Examples of a successful application on the basis of continuous forest inventories are the timber supply study for and the prediction of growing stock and increment in the production forests of the Austrian Federal Forestry Company (ÖBf AG). Moreover, it has been used several times to identify the annual allowable cut in a forest enterprise which is in a transition period between age class and single tree selection system.
In PROGNAUS the site potential is solely described by different continuous and non-continuous site variables, i.e. the site index of a tree species is not needed. This is the reason why this growth model is suitable to assess the growth potential of tree species that have not existed at a given site before, but could be a possible alternative at this site.