| dc.description.abstracteng | Forests are dynamic and complex ecosystems that play important roles for economic, ecological and social aspects. Besides that, they house the largest share of terrestrial biodiversity. Forests function as carbon sinks, provide natural resources and become more and more valuable. Hence, comprehensive knowledge about forests and their status is crucial. Optical multispectral remote sensing is one appropriate instrument to observe and monitor larger areas. In recent years, hyperspectral sensors have been developed that offer much more spectral details. Besides remote sensing, surveys made in the field are essential for sensor calibration and training data.
This study captures both aspects of remote and ground observations using hyperspectral airborne and non-imaging field data covering a spectral range of 400-2500 nm. It consists of four sub-studies and was conducted in the Hainich national park in central Germany, a beech (Fagus sylvatica) dominated broadleaved forest with large old-growth stands. The study considers present issues and shows the capability of spectral high-resolution information.
In addition to spectral reflectances, selected broad- and narrowband vegetation indices (VI) are calculated and used to describe differences among the considered species. The leaf optical properties of the main tree species were examined including reflectance, transmittance and absorptance. Repeated measurements of reflectances were covering two subsequent growing seasons, allowing insights in the seasonal phenology. Overall, the optical properties depend highly on the date and measuring method. Examining species differences, clear rules for separation are not apparent. Especially in the shortwave infrared, a triplet grouping of species could be observed. Beech and hornbeam (Carpinus betulus) showed similar appearances in shortwave infrared, as well as maples (Acer platanoides and Acer pseudoplatanus), and Ash (Fraxinus excelsior) and Oak (Quercus petraea). However, general assumptions about the response pattern related to species are hard to communicate due to high variation and changes in the order of reflectance values. This study also revealed the complexity of spectroscopy in forests.
In a next step, in-situ leaf and crown reflectances were compared with remotely sensed values using airborne sensors. This study incorporates a unique data set of simultaneously gathered measurements. Compared to crowns, sampled leaves show much higher reflectances. Differences of the various levels could be described with simple linear and logarithmic model approaches. For further comparison, VI and red edge position metrics including Red Edge Position Index (REPI) and spectral derivatives were calculated for each level and species. Some of the in-situ leaf level values were more similar to the remotely sensed data than to the in-situ crowns.
Within an area of 2.25 ha, the canopy light interception and crown porousness was assessed. Different approaches were compared including digital cover and hemispherical photography in both visible and near infrared light. Additionally, hyperspectral irradiances were measured below the canopy to retrieve the amount of intercepted light and corresponding extinction coefficients. Ground data was then compared to aerial hyperspectral imagery. From the calculated remotely sensed VI, the Photochemical Reflectance Index (PRI), followed by the Carotenoid Reflectance Index 1 (CRI1), showed the highest, albeit moderate, correlation with openness derived from hemispherical and near infrared cover photos. Fractional cover, derived from radiation measurements, was moderately correlated with Normalized Difference Lignin Index (NDLI) and Red Green Ratio Index (RGRI).
In the last sub-study, forest inventory data was combined with hyperspectral airborne data. Standard stand variables averaged per inventory plot were related to remotely sensed metrics. Basal area did not show any correlation with the derived spectral VI. Also in height, tree diameter at breast height (dbh) and stand density classes no clear trends could be observed. However, in near and shortwave infrared, there are tendencies of a relation between reflectance and dbh and density class. Comparing all calculated VI, the PRI had the highest, moderate, correlation with dbh and density classes as well as with mean tree height per plot. Again, the PRI showed promising results for future analyses. | de |