Der eichen-durchforstungsversuch Waldleiningen 88. Auswirkungen unterschiedlicher eingriffsstärken nach 65 Jahren Beobachtung

The trial plots 'Waldleiningen 88' were established by Professor FABRICIUS in 1934, when the stand age was 48 years. The plots are located near the city of Kaiserslautern in the southern part of the Pfälzer Forest. The site is characterized by favorable climate, average nutrient supply, and sufficient precipitation during the vegetation period. The yield potential of the investigated oak stands proved to be exceptionally high. Density measurements of unthinned stands showed maximum values of 40 m² ha^-1. The current growth rate is considerably higher than suggested by yield table estimates, which seems to be a stable or even an increasing phenomenon. It is striking that the annual ring growth increases continuously throughout the 65 years' observation period. The aim of the investigation was to quantify the development of crown- and stem-dimensions in dependence on thinning intensity. The degree of thinning in the early stages of the stands was most important for the further development. Given the same diameter at breast height (dbh), crown sizes are considerably larger on plots with intensive thinnings compared to unthinned controls. The diameter growth on intensively thinned plots is increased for the remaining trees by 10 to 20 %. However, we found that intensive thinning enhanced crown size development to a greater degree than stem diameter growth, because of a remarkable annual ring growth of more than 2 mm per year, even on the control plots. Also, the variation in annual ring growth between the years was larger on the plots subjected to intensive thinnings. Overall, it is concluded that the growth of the oak stands is increased, especially since the 80ties of the past century, and that this increase can be enhanced on a single tree basis by intensive thinnings. Using the measured dimensions in the year 1999 as initial values, we calculated the development of the economic value of the oak stands with the forest growth simulator SILVA 2.2, and estimated the effect of different thinning intensities. It is estimated that the mean dbh of intensively thinned stands will exceed that of the controls by 5 to 10 cm at the end of the rotation period. Assuming that the time of harvesting depends on a certain tree diameter, this means that the length of the rotation could be decreased by 30 to 50 years. In order to maintain a high economic value, however, it is necessary to keep a sufficiently high number of trees per area.