Early Detection of Forest Drought Stress with Very High Resolution Stereo and Hyperspectral Imagery

Authors:

J. Tian – Remote Sensing Technology Institute (IMF), German Aerospace Center (DLR), Oberpfaffenhofen, Germany – jiaojiao.tian@dlr.de

T. Schneider – Technical University of Munich (TUM), Freising, Germany – tomi.schneider@tum.de

C. Kempf – Remote Sensing Technology Institute (IMF), German Aerospace Center (DLR), Oberpfaffenhofen, Germany – christian.kempf@dlr.de & Technical University of Munich (TUM), Freising, Germany – christian.kempf@tum.de

Y. Xia – Remote Sensing Technology Institute (IMF), German Aerospace Center (DLR), Oberpfaffenhofen, Germany – yuanxin.xia@dlr.de

M. Lusseau – Trier University (UTr), Trier, Germany – lusseau@uni-trier.de

J. Hill – Trier University (UTr), Trier, Germany – hillj@uni-trier.de

E. Jachmann – Technical University of Munich (TUM), Freising, Germany – emanuel.jachmann@tum.de

P. Reinartz – Remote Sensing Technology Institute (IMF), German Aerospace Center (DLR), Oberpfaffenhofen, Germany – peter.reinartz@dlr.de

Abstract: The project ‘Application of remote sensing for the early detection of drought stress at vulnerable forest sites (ForDroughtDet)’ is funded by the German Federal Agency of Agriculture and Food and aims to detect drought stress in an early phase using remote sensing techniques. In this project, three test sites in the south and middle part of Germany are selected. Three levels of observation and analyses are performed. In the first level, close-range stereo images and spectral information are captured with a research crane in Kranzberg forest. In the second level, three study sites are imaged twice in three years by airborne hyperspectral and stereo cameras. In the third level, the drought stress detection approach will be transferred to regional scale by satellite image. In this paper, we will briefly report our results from the first and second levels. In the first level, 3D models of the forest canopies are generated with the MC-CNN based dense matching approaches, with which the 3D shapes of the stressed and healthy trees are analysed. In addition, for the spectral analyses, different chlorophyll-sensitive indices are calculated and compared for the stressed and healthy trees. In order to further analyse the tree drought stress in the second level, a novel individual tree crown (ITC) segmentation approach is proposed and tested on the airborne stereo dataset.

The PSR+ Spectroradiometer was one of the instruments used during this study:

• Fast, full-spectrum UV-VIS-NIR measurements (350 – 2,500 nm)
• High Resolution Field Portable Spectroradiometer with 512 element Si array and two 256 element extended InGaAs arrays
• Various optics ranging from 1° to 25° for reflectance, radiance and irradiance measurements

Field-based operation:
• The PSR+ spectroradiometer is powered by batteries and connected to a rugged tablet which provides GPS, photo tagging, and voice notes
• Our self-developed software allows in-field mineral identification and classification
• Usage of handheld contact probe allows field measurements on outcrops or mapping open pits even at cloudy conditions

Laboratory operation:
• Best signal-to-noise ratio for improved reflectance values by using full range tungsten lamps
• Detailed analysis of field samples in order to build Arctic spectral libraries from natural covers

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