The complexity of forest ecosystems and the special situation in Valais, where the Scots pine forests have been strongly influenced by human activities for centuries explains the presence of several causes of the mortality of the pines in the 'Telwald'. Based on our dendroecological studies we can exclude overaging as explanatory factor, because the trees are only about 200 years old. Rigling and Schweingruber (1997) showed that on comparable sites Scots pine can reach ages up to 450 years.
Competition as explanatory process for mortality:
The dead pines diffusely distributed in stand 1 and 3 have the same age and have smaller DBH compared to the living pines. This can be explained by increasing competition from Norway spruce (stand 1), together with high stand densities (stand 3), which are consistent with an advanced stage in the natural succession of the forests. This leads to a shortage of light and water and finally to death. The dead pines show a continuous decrease in radial growth prior to death. The year of death varies from one individual to another, and in most cases is determined by an additional stress factor
Insect attack as explanatory parameter for mortality:
The diffuse and clustered dead pines in stand 2 have the same age and have a similar DBH distribution as the living pines. Competition seems not to be high, as shown by the low stand density and the little percentage of Norway spruces in the undercover. Initially dead pines have normal growth rates, followed by an abrupt growth reduction which led to death. In the living pines rings are missing exactly at the time of beginning of the growth reduction.
Pine shoot beetle as explanatory parameter for mortality:
The whole stand 3 is infested by pine shoot beetles. Signs of their shoot feeding are visible in every crown. The clustered dying Scots pines show an abrupt growth reduction, starting between 1990 and 1993, and very often trees fail in forming tree rings All the pines, despite their individual situation within the stand are declining.