The oak thinning experiment Waldleiningen 88. Growth reactions in dependence on different thinning regimes after 65 years' observation

The trial plots 'Waldleiningen 88' were established by Professor FABRICUS i n 1934, when the stand age was 48 years(') The plots are located near the c ity of Kaiserslautern in the southern part of the Pfalzer Forest. The site is characterized by favorable climate, average nutrient supply, and suffici ent precipitation during the vegetation period. The yield potential of the investigated oak stands prooved to be exceptiona lly high. Density measurements of unthinned stands showed maximum values of 40 m(2) ha(-1). The current growth rate is considerably higher than sugges ted by yield table estimates, which seems to be a stable or even an increas ing phenomenon. It is striking that the annual ring growth increases contin uously throughout the 65 years' observationperiod. 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 devel opment. Given the same diameter at breast height (dbh), crown sizes are con siderably larger on plots with intensive thinnings compared to unthinned co ntrols. The diameter growth on intensively thinned plots is increased for t he remaining trees by 10 to 20 %. However, we found that intensive thinning enhanced crown size development to a greater degree than stem diameter gro wth, 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 betwe en the years was larger on the plots subjected to intensive thinnings. Over all, it is concluded that the growth of the oak stands is increased, especi ally since the 80ties of the past century, and that this increase can be en hanced on a single tree basis by intensive thinnings. Using the measured dimensions in the year 1999 as initial values, we calcul ated the development of the economic value of the oak stands with the fores t growth simulator SILVA 2.2, and estimated the effect of different thinnin g intensities. It is estimated that the mean dbh of intensively thinned sta nds will exceed that of the controls by 5 to 10 cm at the end of the rotati on period. Assuming that the time of harvesting depends on a certain tree d iameter, this means that the length of the rotation could be decreased by 3 0 to 50 years. In order to maintain a high economic value, however, it is n ecessary to keep a sufficiently high number of trees per area.