Modeling fractional shrub/tree cover and multi-temporal changes using high-resolution digital surface models and CIR-aerial images

The objective of this paper is to assess increase and decrease of forest area and estimate shrub encroachment between 1997 and 2002 in open mire land using CIR-aerial images, DSMs derived from it and LiDAR data. The present study was carried out in the framework of the Swiss Mire Protection Program, where changes of forest area are a key issue. The study area is located in the Pre-alpine zone of Central Switzerland. In a first step, highquality DSMs were automatically generated from CIRaerial images of 1997 and 2002. This DSM generation is based on high accuracy, intelligent matching methods developed at ETHZ which are able to produce very dense and detailed DSMs that allow a good 3D modeling of both deciduous and coniferous trees and shrubs, and multi-temporal analysis of their growth pattern. In a second step, tree layers from both years were then generated combining canopy height models derived from the DSMs and LiDAR DTM with a fuzzy classification of spectral information (NDVI) of CIR aerial images. In a third step, on the basis of these tree layers fractional tree/shrub covers were generated using explanatory variables derived from the DSMs and logistic regression models. Finally, bias was estimated by analyzing the distribution of the fractional model differences. The corrected models reveal a decrease of tree/shrub probability. This indicates a decrease of forest and other wooded areas between 1997 and 2002. On the other side, the models also indicate real shrub encroachment and tree growth in open mire land. The study stresses the importance of high-resolution and high-quality DSMs and highlights the potential of fractional covers for ecological modeling.