A CROWN RATIO MODEL FOR AUSTRIAN FORESTS

A crown ratio model for individual trees is developed for all major tr ee species in Austria. The study is part of a comprehensive project de veloping a distance-independent single tree growth simulator for the f ull range of stand conditions in Austria. Data were obtained from the Austrian National Forest Inventory and consist of more than 42000 tree s growing on over 5000 permanent plots measured during 1981 and 1985, Crown ratio was predicted using nonlinear regression with a logistic f unction. The argument of the logistic was a linear combination of tree size characteristics, stand density measures, and topographic site fa ctors. The total variation explained by the model varied from 49% for larch to 17% for the 'other broadleaf species'. The model explained 41 % of the variation in crown ratio for the principal species, Norway sp ruce. The model explained less than a quarter of the variation for all the broadleaf species and for stone pine. The effect of the size vari ables is approximately equal in importance to the variables representi ng competition for the major species. The set of topographic site fact ors explained the least amount of variation, less than 10% in all case s. Because the height/diameter ratio, the most important size variable , can also be considered to be an integrator of past competition, the crown ratio model is dominated by competition measures. A validation t est using independent data from permanent research plots for the main species (Norway spruce, white fir, Scots pine, and beech) demonstrated that the models appear to be well behaved and robust for both pure ev en-aged and mixed uneven-aged stand types in Austria. Analyses of the residuals from permanent research plots representing a wide variety of thinning treatments indicate that the effect of management seems to b e adequately represented by the model.