Very high-resolution true color leaf-off imagery for mapping Taxus baccata L. and Ilex aquifolium L. understory population

Abstract

According to the “Habitat” Directive 92/43/EEC, the conservation status of natural habitats depends on the occurrence of populations of their typical species. For some forest habitats, typical species do not occur as canopy dominant trees but are found in the understory. This is the case with the priority habitat ’‘Apennine beech forests with Taxus and Ilex’’ 9210*. Taxus baccata L. and Ilex aquifolium L. are evergreen tree species and occur as isolated trees or groups in the understory of beech dominated forests. Accordingly, the knowledge of the spatial pattern of populations of typical species is fundamental for habitat monitoring goals. In this perspective, this study aims to evaluate the potential of very high-resolution, true color, leaf-off imagery (pixel size = 0.11 m), supplied by Google Earth, for mapping these populations. Understory layer detection has been accomplished through an object-oriented approach, based on multiresolution segmentation. The classification was developed with thresholds based on spectral and geometric properties, as well as on textural and contextual information. The main critical issues are represented by site conditions, where shadowing can prevent crown detection. The thematic accuracy of the target species map resulted in a Producer Accuracy of 0.77 and a User Accuracy of 0.80. The proposed procedure offers a good methodological foundation with which to map the actual spatial extent of forest broadleaved deciduous habitat types, characterized by low abundance and patchily distributed populations of yew and holly.