## 3 Results
### Sensors and Sensor Type  
The selection of sensors is primarily influenced by the focus of the study. Different sensor types are differently suited for the detection of the impact on forest of individual EWEs or climate change. In addition, the study area size is decisive for the choice of sensor. In recent decades, the number of available data have increased distinctly [57; 58; 75; 76; 78; 89; 90; 91; 92; 93; 94]. In general, several sensor types are available to detect the impacts of climate change or EWEs in temperate forests [90; 91; 94]. Basically, remote sensing sensors can be divided into two categories: active sensors, and passive sensors. Active sensors actively emit a signal and measure its reflection from the ground [95]. Passive sensors measure the electromagnetic radiation reflected from the surface. The radiation detected is typically measured over a range from visible light (VIS) to short-wave infrared (SWIR), and is particularly sensitive to atmospheric disturbances such as clouds, fog, and similar phenomena [95]. Passive sensors are further subdivided into multispectral, hyperspectral, thermal, and passive microwave sensors. Multispectral sensors have several bands in addition to the optically VIS RGB bands. These often include near-infrared (NIR), SWIR, and red-edge. Hyperspectral sensors often have more than one hundred bands with narrow bandwidths [96]. SAR and LiDAR are active sensors, but SAR (synthetic aperture radar) is unaffected by cloud cover, while LiDAR scans can acquire data only under cloudless conditions [95]. Figure 5 shows the different sensors, sensor types, and platforms used. Figure 5: Overview of the different remote sensing sensors, their platform, and the sensor type combination used in the reviewed articles. Abbreviations: A-thermal—Airborne thermal, AHS—Airborne Hyperspectral Sensor, ALS—Airborne Laser Scanning, AMS—Airborne Multispectral Sensor, AMS—Advanced Microwave Scanning Radiometer, AVHRR—Advanced Very High Resolution Radiometer, GLAS—Geoscience Laser Altimeter System, MODIS—Moderate Resolution Imaging Spectroradiometer, MODIS LST—Moderate Resolution Imaging Spectroradiometer Land Surface Temperature, OCO-2—Orbiting Carbon Observatory-2, PROBA-V—Project for On-Board Autonomy—Vegotation, SPOT—Satellite Four I’Observation de la Terre. The use of multispectral sensors clearly predominates in the reviewed articles. Multispectral sensors were used in 118 studies. Four studies each used thermal and hyperspectral sensors. Three studies used Sentinel-1 data, which belongs to the group of SAR sensors. Two studies used ALS data, and one study used passive microwaves. Among the multispectral sensors, the familiar ones clearly dominate. MODIS is by far the most frequently used sensor, with a total of 48 studies. Most MODIS data and products are available from the year 2000 onwards [64; 66; 67]. The spatial resolution ranges from 231 m to 1000 m, depending on the band. A major advantage over other sun-synchronous satellite platforms is the daily repeat rate north of 30\({}^{\circ}\) latitude [63]. The temporal resolution of the Landsat archive is less; the individual satellite systems have a repeat rate of about 16 days. However, the spatial resolution is distinctly better. The current versions Landsat 8 and Landsat 9 have a resolution of 30 m in the VIS, NIR, and SWIR. The Landsat archive additionally provides the longest timeseries available, with the first images dating back to 1972 [97]. A total of 30 studies have used the Landsat archive, mostly combining multiple sensors. The most commonly used sensors within that group were Landsat 5 and Landsat 8. Altogether, 20 studies were conducted with Sentinel-2. The multispectral satellite pair has been providing composites with a 5-day repeat rate since 2015. With a resolution of 10 m in the VIS and NIR, the sensor can be used for wide range of applications to provide multidisciplinary routine measurements for operational purposes [75]. AVHRR is one of the most widely used sensors, with 14 studies.