Satellite data track spatial and temporal declines in European beech forest canopy characteristics associated with intense drought events in the Rhön Biosphere Reserve, central Germany

Abstract

The increasing intensity and frequency of droughts under climate change demands effective ways to monitor drought impacts. We sought to determine how different satellite remote sensing sources influence our ability to identify temporal and spatial impacts on European beech forest canopy health during intense drought events.• Imagery from three satellite series (MODIS, Landsat and Sentinel-2) was used to observe changes in canopy health during the intense droughts of 2003 and 2018 in the Rh€ on Biosphere Reserve, central Germany. Monthly normalized difference vegetation index (NDVI) anomalies were calculated for each satellite between 2000-2020 and compared against temperature, precipitation and the standardized precipitation evapotranspiration index (SPEI).• Severe canopy impacts in 2003 and 2018 were associated with low NDVI in August and September. At the stand-scale, Sentinel-2 data allowed a spatially detailed understanding of canopy-level impacts, while MODIS provided the clearest temporal progression of the drought's impacts on the forest canopy. Low NDVI values were not exclusively associated with extremes of either temperature and precipitation individually; however, low canopy NDVI in August was associated with SPEI values below À1.5.• Although the intense drought of 2018, as defined by meteorological parameters, peaked in July, canopy NDVI did not decline until August, highlighting that our ability to detect canopy impact during drought events is sensitive to the timing of image acquisition. No single satellite sensor affords a full picture of the temporal or spatial progression of drought impacts. Consequently, using sensors in tandem provides the best possible representation of canopy health during intense drought events.

Influence of climatic conditions on Normalized Difference Vegetation Index variability in forest in Poland (2002–2021)

“…However, in this paper, the annual NDVI was prepared as the median of the maximum monthly NDVI values of the growing season for the year in question. Similar approach was used in, for example, Romania (Pr av alie et al, 2022) and Germany (West et al, 2022). The rationale behind using median is that it represents the situation of the vegetation over the whole growing season better than, for example, sole maximum value.…”

Responses of European forests to global change‐type droughts

“…The authors analysed the pan-European ICP Forest crown defoliation dataset for the last 25 years and found species-specific increasing trends in mortality rates for major coniferous, major broadleaved and a pooled dataset of minor European timber species related to decreasing soil moisture. West et al (2022) observed the development of extreme droughts, such as the 2018 event, on satellite-derived canopy vitality by assessing the normalized vegetation index (NDVI), which quantifies reductions in vegetation greenness. According to their results from a typical beech forest in central Germany, canopy responses lagged behind the onset of the 2018 drought by up to 2 months, showing peaks in August and September.…”

“…West et al . (2022) observed the development of extreme droughts, such as the 2018 event, on satellite‐derived canopy vitality by assessing the normalized vegetation index (NDVI), which quantifies reductions in vegetation greenness. According to their results from a typical beech forest in central Germany, canopy responses lagged behind the onset of the 2018 drought by up to 2 months, showing peaks in August and September.…”

“…In this special issue, a broad range of observational and experimental studies are provided that relate to tree health, drought and heat stress resistance and recovery at multiple levels, including causes and consequences of drought-induced tree mortality. The studies use a wealth of approaches, covering remote sensing West et al 2022), ground-based monitoring (Frei et al 2022;George et al 2022), different management regimes (Bose et al 2021;Meyer et al 2022), including species mixture effects Santamarina et al 2022), field studies on wood formation plasticity (Larysch et al 2022), acclimation potential at the leaf level (Weithmann et al 2021;Petrik et al 2022) and recovery potential at the tree level (Kn€ uver et al 2022). These are supported by field- (Guan et al 2022;Kahmen et al 2022;Kn€ uver et al 2022) and lab-based (Kunert et al 2021;Petruzzellis et al 2021;McAdam et al 2022) ecophysiological studies that will increase our understanding of the impacts and consequences of global change-type droughts.…”

Extreme droughts and heatwaves endanger temperate forests

“…This special issue, edited by Schuldt & Ruehr (2022), evaluates tree and forest responses to the 2018 European hot drought, one of the most severe droughts ever recorded in Central Europe. It illustrates and differentiates viewpoints across scales, from acclimation potential and regulatory mechanisms at the leaf level (Kunert et al 2021;Weithmann et al 2021;McAdam et al 2022;Petrik et al 2022), embolism and wood formation plasticity (Petruzzellis et al 2021;Larysch et al 2022;Meyer et al 2022), impacts of different management regimes (Bose et al 2021;Meyer et al 2022) and species interaction effects in mixtures Santamarina et al 2022), to results from remote sensing West et al 2022).…”