Forest growth trends can be recorded and interpreted by means of various data sources. Permanent plots of growth and yield research, not systematically distributed over the whole country and with observation periods of up to 100 years, can deliver growth trends of single trees and of stands. The chronological resolution of this information varies between four and fifteen years. For single trees there are also annual resolutions which were collected by means of stem disks. This data enables us, in spite of advancing age, to prove constant or increased diameter and basal area increment for various tree species and forest structures in different areas of Switzerland. Hypotheses to the reasons can be made by means of long-term records and with the help of available climatic and ecological data which can be related to the permanent plots.
In the course of the forest condition inventories in the 1980s, trees were bored on a systematic permanent sample plot net in parts of Switzerland. This data enables statements concerning the development of the annual increment of single trees, partly over a longer period of time, to be made. Increment changes can already be shown for the early 1980s by means of cores. New analyses of stem disks from permanent growth and yield sample plots allow a comparison of single tree growth with the stand growth.
The Swiss National Forest Inventory was conducted from 198385 for the first time and since 1993 the second inventory is being carried out. In these surveys, boring or the cutting of stem disks is not done. This does not allow these data to be used for statements concerning growth trends, at best they allow for a comparison between two dates. On the other hand there is an area, in which there are both permanent growth and yield plots as well as three forest management inventories with permanent sample plots. This data seems to confirm the observations made by means of single studies. Moreover they allow a representative quantification of the ascertained growth trends.
The object of this study is to interpret growth trends by means of the aforementioned data sources and to elaborate a synthesis from the different results. The elaborated indications should allow statements about actual growth trends in Swiss forests to be made, as well as deliver the beginnings for method development of growth and yield plots and representative surveys.
Key words: basal-area-increment, diameter-increment, growth and yield, tree-ring, inventory.
Correspondence: Andreas Zingg, Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Zürcherstrasse 111, CH-8903 Birmensdorf, Switzerland
Telefax +41-1 739 22 15
E-mail: andreas.zingg@wsl.ch