Abstract
Remote sensing is increasingly widely used in nature conservation management. The research focuses on developing an optimal set of airborne raster data for the identification of the invasive alien species Spiraea tomentosa L. The plant species selected for the purposes of this study poses a serious threat to peat bog plant communities, moist coniferous forests, and meadows in Central Europe. The impact of the data acquisition time on the accuracy of classification and the percentage cover limit required for correct identification of a target species using the developed method were also investigated. The study area is located in the Lower Silesian forests in Poland and is protected as a Natura 2000 site. Airborne hyperspectral and laser scanning data were simultaneously acquired two times in the growing season (August and September 2016) parallel to on-ground reference data collection. The 1 m resolution HySpex images with spectral range of 0.4–2.5 μm were corrected atmospherically, radiometrically and geometrically. Airborne Laser Scanning (ALS) data acquired at 7 points/m2 were used to generate several products, e.g. Canopy Height Model (CHM), rasters representing morphometric features of the area (Multiresolution Index of the Ridge Top Flatness or Valley Bottom Flatness—MRRTF, MRVBF), wetness relations (Topographic Wetness Index—TWI) and the availability of light (Total Insolation—TI), intensity of laser pulse reflection and geometric relations of vegetation points (i.a. Vegetation Cover, Vegetation Mean Intensity). The Random Forest (RF) classification and different raster datasets were used to identify the target species. As a result, the highest accuracy was obtained for the scenario based on HySpex images acquired in September. The accuracy (f1 score) for the target species achieved 83%. The developed method for the identification of Spiraea tomentosa has a great potential for application and can be used for monitoring peat bogs threatened by invasion of alien plants.
Highlights
Encroachment of invasive alien species on protected areas of high nature conservation value has been observed all over the globe
Of the five raster datasets tested, the highest accuracies for both measurement dates (P1: Kappa = 82.35%, F1 for the target species = 82.96%; P2: Kappa = 76.53%, F1 for the target species = 77.25%) were obtained for the set consisting of only 25 MNF transformation bands (Scenario 1)—Table 5, which means that adequately processed information of hyperspectral images HySpex with a spectral range of 0.4–2.5 lm is sufficient to correctly identify the species Spiraea tomentosa
Based on the results reported in this paper, it may be assumed that the use of HS data and the Random Forest (RF) classification method is a good approach for the identification of the invasive plant Spiraea tomentosa, especially in large and difficult-to-access study areas
Summary
Encroachment of invasive alien species on protected areas of high nature conservation value has been observed all over the globe (de Poorter 2007; Foxcroft et al 2013). Wetlands, which play a key role in biodiversity conservation (Howard 1999), are a important group of ecosystems exposed to invasion of alien species (Dajdok and Pawlaczyk 2009). In Europe, the contribution of alien species to the flora of some protected areas is up to 40% (Foxcroft et al 2013) Due to such a large contribution as well as the adverse impact of invasive species on native ecosystems, many authors emphasize the necessity of early detection of such species In Central Europe, Spiraea tomentosa is one of the alien species that poses a threat to wetland communities (Dajdok et al 2011, Wiatrowska et al 2018).
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