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ŠUMARSKI LIST 9-10/2014 str. 50     <-- 50 -->        PDF

image analysis (OBIA) (Blaschke, 2010) with a combination of WV2 imagery and LiDAR data allows for successful classification of individual crowns of five different tree species in the dominant layer of natural, mixed, heterogeneous urban forest in Ljubljana, Slovenia. The forest in this research is facing various macro and micro scale conflicts due to its general conservation status (Odlok, 1984, 2010a), protected habitat locations (ZGS, 2007), intensive and various recreation versatile (Verlič and Pirnat, 2010; Smrekar et al., 2011) as well as small-scale (mostly) private ownership (ZGS, 2007).
2. Materials and methods
Materijali i metode
2.1. Study area – Područje istraživanja
More than 60 % of urban forests within the City of Ljubljana are primal forests and have continuously offered a natural forest ecosystem experience to the citizens (Hladnik and Pirnat, 2011). Since 2010 most of the forest area is protected (Odlok, 2010b), due to its highly emphasized social and ecological forest functions. This protected remnant of natural mixed forest lies within the Tivoli, Rožnik and Šišenski Hill Landscape Park (Fig. 1) located in the city centre. In 1984, the 459 hectare area was declared a natural landmark (Odlok, 1984). A study conducted in 2010 (Smrekar et al., 2011) estimated 1,750,000 visits to this forest per year.
Geological substratum is predominantly of Permo-Carbonian shales and sandstones, where dystric brown soil, ranker and luvisols developed. The highest peaks are the Rožnik area (394 meters above sea level) and Šišenski hill (429 meters above sea level).
On the whole area of Rožnik there are three prevailing forest communities: Blechno-Fagetum (mixed forests dominated by beech, oak and chestnut, thriving mainly on northern and eastern slopes, where the soil is deeper and humid; Vaccinio-Pinetum (acidophilic pine forest covers in particular sunny exposures, in the southern and western slopes); Alnetum glutinosae (alder located on the marshy part of the valley on the north and northwest side) (Marinček and Čarni, 2002). According to the share of the growing stock of the forest management unit Ljubljana is dominated by the following tree species: Scots pine (Pinus sylvestris) (22 %), Sessile and Pedunculate oak (Quercus petraea and Quercus robur) (15 %), European beech (Fagus sylvatica) (21 %), Norway spruce (Picea abies) (19 %) and Sweet chestnut (Castanea sativa) (8 %) (ZGS, 2007).
2.2. Remote sensing data – Podaci dobiveni daljinskim istraživanjima
The acquisition of data was guided by phenological stages of the forest tree species and recommendations by some prior studies, which tested seasonal differences for classification for tree species (e.g. (Voss, 2008)). Aerial imagery and laser scanning data were recorded simultaneously on 24.3.2011 from a helicopter on a windless day to avoid errors in fused images due to moving tree crowns, as warranted by Puttonen et al. (2009). The time of data acquisition was before the full unfolding of tree leaves, which was a compromise between scanning tree crowns and penetration to the ground (Đurić, 2011). Additional aerial images were taken in summer, when tree leaves were fully leafed-out and thus enabled a visual distinction among the tree species. The images were processed into a true orthophoto imagery which was used in the field work for detailed identification of individual trees, in the manual digitisation of tree-crown polygons of trees selected in the field, as well as for the visual verification of the classification results.
The distance between ten flight lines was 250 m, which provided sufficient overlap of aerial images along and across the flight lines. Furthermore, because the width of individual lidar strips is approximately 550 m, it also doubled the density of laser points of a combined dataset from 10 pts/m2 to about 20 pts/m2 (Đurić, 2011), with 0.33 m average point spacing in a single flight line. A Riegl LMS-Q560 laser scanner was used.
A digital elevation model (DEM) with the 0.5 m resolution was obtained through a combination of adaptive triangulated irregular network densification – ATIN ((Axelsson, 2000); as implemented in Terrasolid Terrascan 11) and repetitive interpolation – REIN (Kobler et al., 2007). The REIN algorithm uses a two-step approach to calculate a raster DEM. It was developed for steep forested terrain and was considered most suitable for the model area of this study. The first phase employs a geomorphological filter to eliminate all echo points that lie under the terrain and most, but not necessarily all above terrain points. The second phase removes the remaining non-terrain points and computes a raster elevation model. We have modified the procedure by replacing the first phase, i.e. the geomorphological filter, with ATIN filtering, because it has a big advantage in non-continuous surfaces characteristic for urban areas (Sithole and Vosselman, 2004). By doing so, we combined the benefits of both algorithms – effective operation in build-up areas (ATIN) and on sloped forested terrains (REIN). A raster DEM produced via repetitive triangulation is also superior for further spatial analyses compared to directly rasterized DEM, as REIN uses several estimations of elevation values at a certain raster pixel. REIN was also used to calculate the digital canopy model (DCM, (Fig. 2)).
The WorldView-2 (WV2) satellite image used in the study was acquired on August 1, 2010, in the peak of vegetation period. Obviously, the satellite imagery and aerial data were not from the same period; however this compromise had