Assessing structures in mountain forests as a basis for investigating the forests' dynamics and protective function

Stand structures were studied in 200 plots in subalpine spruce dominated forests in various parts of Switzerland. A typology of structures was created by evaluating the data obtained from these plots. A key for assessing 22 forest structure types stereoscopically was made and a forest area of 4.5 km(2) near Davos was mapped and terrestrially verified with aerial photographs (CIR, 1:9000). Structure types could be assessed from aerial photos with an accuracy of more than 85%. In order to increase the objectivity of the forest-structure maps, attempts were made to map the forest structures automatically through image processing. Promising results were obtained in identifying single trees and in automatically assessing the most important structure types. The majority of the study area was also mapped retrospectively using aerial photographs from 1930. A geographical information system (GIS) was used to calculate the transition-probabilities between the different structure types in the period from 1930 to 1997. The area was stratified in order to examine and model forest dynamics under different site conditions. Stands showing open structures in 1930 had frequently developed into dense, uniform stands by 1997, especially on warmer, unexploited sites. Harder climatic conditions and/or cattle grazing facilitated the development of open, multilayered stands. The risk of avalanche release within the forest zone was modeled and mapped in a GIS-based application of the assessment methods. A statistical evaluation of data on stand structure and terrain at 150 real avalanche starting zones provided the basis for these models. A comparison of the models deduced from the 1930 and 1997 map showed an increase in the area of forests, which performed a direct protective effect in the study area, from 101 to 127 ha. (C) 2001 Elsevier Science B.V. All rights reserved.