Diameter and Basal Area Increment in Permanent Growth and Yield Plots in Switzerland

Abstract

The development trend of the diameter increment and the basal area increment of single trees and of stands is examined by means of 25 permanent trial plots of the growth and yield research. A number of these plots have been periodically measured for more than 100 years. The plots are not systematically distributed over Switzerland. The periodic resolution of the measurements lies between 4 and 10 years. In the study, the diameter increment is used for single trees and tree classes, the basal area increment for stands. These two quantities are preferred over other quantities such as height and volume because of their simple and reliable measurability. The assessment of a possible trend is done by means of a comparison of the growth with either the mean of the whole observation period, or, in those cases where it is permissible, with yield tables. Despite increasing age, a constant or, as is the case in the most recent observation period, an increase of the diameter and basal area increment can be shown for various tree species and for various forest structures in different areas of Switzerland. Relationships between the observed basal area increment and the development of the climate parameters or the silvicultural treatment can be ascertained only in some cases. Apparently the site productivity has changed or is in the process of changing. On the basis of the available data, none of the possible hypotheses - climate change, change of the CO₂, content in the air, influx of substances, extension of silvicultural management etc. - can be either accepted or rejected.