In the Netherlands the nitrogen availability in the forest is presently very high. This can be seen with data on nitrogen contents of foliage of trees. In the first half of this century, nitrogen usually was limiting growth, although few stands were deficient. At this moment, the nitrogen content in needles of Scots pine and Douglas-fir are higher than necessary (sometimes 2.5 % of the dry weight or even higher). The high nitrogen content is accompanied by low contents of other nutrients (e.g. P, K, Mg). The nutrition therefore, is unbalanced. The nitrogen deposition shows the same trend as the number of cattle in the area. This means that the rate of nitrogen deposition is 6 times higher as around 1950. The deposition in the forest is calculated at 50 to 200 kg ha1yr1. With also rising CO2-levels, a growth trend would be expected. At this moment the measured growth is higher than indicated in the yield tables, but this could be due to underestimations in the yield tables. Therefore, the growth data should be examined carefully to avoid effects of age differences in different periods.
Data from 80 plots of Douglas-fir, with 3 to 13 observations between 1900 and 1994, were used to analyze if a growth trend exists (860 observations, average top height, dbh, or basal area of the plots). Depending on how the data are plotted, positive or negative trends can be observed. The development of basal area growth (current increment) shows a negative trend between 1930 and 1990. However, if we look at the age distribution during that time interval, we see that the average age of the forest increased from 36 years (in the period before 1960) to 47 years (in the period after 1960). Therefore, the declining trend in the current increment of the basal area plotted over time during this century, is an overall aging effect of the Douglas-fir stands.
However, the general variation in the data is very high for the easy observation of a possible trend. A further selection of measurement data is necessary to reduce the variability in the graph. Results will be presented for selected plots per soil type or other site variables, to reduce variation in the data.
Key words: growth and yield, growth trends, permanent plot data.
Correspondence: A.F.M. Olsthoorn, Institute for Forestry and Nature Research (IBN-DLO), P.O. Box 23, NL-6700 AA Wageningen, Netherlands