Applicability of the forest stand growth simulator prognaus for the Austrian part of the Bohemian Massif

Abstract

Our objective is to examine the applicability of the basal area increment model in the prognaus forest stand growth simulator using independent permanent plot data. The simulator is designed to forecast the development of both pure even-aged and mixed-species uneven-aged stands in Austria using distance-independent individual tree methodology. The primary model is for the basal area increment (BAI), which is predicted from size (diameter, crown ratio), competition (basal area of larger trees, crown competition factor), and site descriptors. Because the model must be able to simulate the development of both uneven- and even-aged stands, site index and age are intentionally not used as predictors. Available for testing is an independent data set of 22 permanent plots in mixed Scots pine ( Pinus sylvestris L.)-Norway spruce ( Picea abies L.) stands in the Austrian part of the Bohemian Massif, northeastern Austria. The validation plots were remeasured for three 5-year periods between 1977 and 1992. The prediction interval (95% confidence) for the basal area increment over the full 15-year period was 62–157%, with a mean of 99% of the observed increment. A plot-specific adjustment of the model's intercept using past increment did not improve predictions for Norway spruce, but did improve predictions for Scots pine. Thus, important site-specific variation not captured by the Scots pine model can nevertheless be accounted for by using increment calibration. A time trend towards increasing underestimation of the increment was detected in the later growth periods of the validation data. This trend agrees with a frequently reported increase in the site potential of central European stands, possibly caused by changing weather conditions, nitrogen deposition, and abandoning of litter raking. For future model development, as many growth periods as possible should be used to parameterize increment models, although even this cannot account for a long term change in the site potential.