## 3 Results
### Review of Thematic Foci on Extreme Weather Events and Climate Change
#### 3.6.5 Recurrent Extreme Events  
The category of recurring extreme events can be divided into two subcategories. Of the 23 publications in this category, 5 dealt with various recurring extreme events [114; 161; 160; 164] and 18 dealt with recurring drought events [165; 166; 74; 167; 168; 74]. Of these eighteen studies, three studies focused on changes in the productivity of forests: two in China, and one on the Iberian Peninsula. [PERSON] et al. [180] employed a 12-year MODIS timeseries to demonstrate that planted forests exhibited heightened sensitivity to recurrent drought events relative to natural forests in China. [PERSON] et al. [178] utilized AVHRR data spanning 30 years to investigate the growth variability along the drought gradient in China. Both studies revealed a negative correlation between tree growth and water deficit. The same sensor and the same timeframe were used in a study by [PERSON] et al. [175] to analyze productivity changes in a different study area (the Iberian Peninsula). Here, the focus was on the relationship between remote sensing or tree-ring data and various drought indices. It was found that tree-ring growth appears to be a more reliable indicator of the response of forests to drought. As with single drought events, mortality was the most frequently associated forest response category for recurring droughts/events. The research by [PERSON] et al. [171] on dieback in forest ecosystems and spatial variability of drought impact focused on the Iberian Peninsula. They found that the eastern forests of Spain and Portugal were more affected by droughts than forests in other regions of the peninsula. This research utilized a commonly used method for assessing the impact of recurrent droughts on temperate forests, namely, relationship analysis. The correlation between climate variables and remote sensing signals in the context of forest dieback has been studied on several occasions [172; 174; 179]. These studies demonstrated a correlation between forest decline and summer temperatures or the severity of drought or water stress. [PERSON] et al. [179] reached the same conclusion as the previously mentioned study by [PERSON] et al. [180], namely, that planted compared to natural forests in China react more strongly to the effects of drought. In contrast, [PERSON] et al. [88] employed a relationship analysis approach to ascertain and justify changes in phenology. The study of the entire Northern Hemisphere has demonstrated that an earlier start of the season is more dependent on temperature, while the later end of the season is influenced by both temperature and precipitation. The occurrence of extreme droughts, such as those experienced in Western Europe in 2003 and the USA in 2012, have resulted in a shift of more than 20 days in the timing of the start and end of the season in a predominant portion of these regions. The effects of recurring severe droughts and the associated forest dieback have been widely studied in Europe [74; 167; 168; 169; 170]. [PERSON] et al. [168] have quantified the forest mortality caused by droughts with the help of Landsat timeseries and correlation analyses. According to this, 500,000 ha of forest were destroyed by droughts alone between 1987 and 2016. This does not even include the most severe droughts of the recent past. A noteworthy aspect of studies on recurring droughts in Europe is the year 2018, which demonstrated a clear correlation with widespread canopy loss in Central Europe. [PERSON] et al. [74] calculated the loss across Germany using the saturation line-based Disturbance Index. [PERSON] et al. [169] were able to use aerial photographs to show how canopy mortality increased during droughts in Luxembourg. [PERSON] et al. [170] demonstrated that the drought signal in the canopy of German beech forests is only detectable with a delay to the meteorological drought peak. [PERSON] et al. [167] used MODIS timeseries combined with tree heights for Europe to show that shorter forests are less resilient to drought due to the deeper tree roots.