Widespread drought‐induced leaf shedding and legacy effects on productivity in European deciduous forests

Abstract

Heatwaves and droughts are becoming more common and severe in Europe, causing changes in tree phenology, disrupting the sequestration of carbon and causing tree mortality on a continental scale. The responses of leaf shedding to heatwaves and droughts remain uncertain, although temperate deciduous forests may shed their leaves if exposed to extreme heat and water stress. Little information, however, is available about the extent and recurrence of early leaf shedding induced by drought, likely because it occurs in small forest patches and can be discriminated only during a few weeks. We used highly spatiotemporal Sentinel-2 data as evidence of widespread drought-induced early leaf shedding in Europe during 2017-2021. We estimated the timing of leaf shedding from NDVI time series and a threshold-based method that extracts the end of the growing season. Then, we evaluated the heatwave and drought impacts at the end of season by analysing the z-score of Landsat-7 and -8 land surface temperature and the ERA5-Land air temperature and aridity index. The 10-m resolution Sentinel-2 data identified early leaf shedding not detected by the low-resolution (250 m) MODIS sensor. Early leaf shedding was observed across Europe during the entire study period and its occurrence was linked to preceding anomalously high temperatures and arid conditions. Our results also indicated that mean summer NDVI decreased significantly in the years following early leaf shedding, suggesting a legacy decline in vegetation productivity. Our study demonstrates that decametric satellite data can be used to monitor the responses of forests to extreme climate events at the canopy level and indicates that early leaf shedding associated with heatwaves and droughts is more widespread and frequent across the continent than previously thought.

Potential of Earth Observation to Assess the Impact of Climate Change and Extreme Weather Events in Temperate Forests—A Review

“…Both Descals et al [116] and Brun et al [38] demonstrated the impact of the severe 2018 drought in Central Europe on temperate forests and the resulting early leaf shedding. While Descals et al [116] confirmed that early leaf shedding occurred during the entire study period (2017-2021), increased in extent…”

“…Three studies investigated the extent to which drought events induced early wilting. Using multispectral timeseries from MODIS and Sentinel-2, changes in the NDVI or EVI signal could be attributed to early leaf shedding [38,116,117]. Both Descals et al [116] and Brun et al [38] demonstrated the impact of the severe 2018 drought in Central Europe on temperate forests and the resulting early leaf shedding.…”

Reviews and syntheses: Remotely sensed optical time series for monitoring vegetation productivity

“…In addition, satellite TS analysis shows an increased trend in the frequency and intensity of droughts, and illustrative studies have been conducted for northern Europe (e.g. Reinermann et al, 2019;Senf et al, 2020;Descals et al, 2023). Climate change is also increasing the frequency of biotic disturbances like insect outbreaks (Senf et al, 2017;Olsson et al, 2017).…”

Catastrophic climate change and the collapse of human societies

“…These climate projections could be even more detrimental if models would not neglect, as they currently do, feedback in the carbon cycle and potential tipping points that could generate higher GHG concentrations [ 4 ]. Examples include the apparent slowing of dampening feedbacks such as the natural carbon-sink capacity [ 5 , 6 ], the loss of carbon due to increasing frequencies and intensities of fire at northern latitudes [ 7 ], droughts and fires in the Amazon [ 8 ] or the thawing of Arctic permafrost that releases methane and CO 2 [ 9 ]. This feedback is also likely not proportional to warming, as is sometimes assumed.…”

“…This feedback is also likely not proportional to warming, as is sometimes assumed. Instead, abrupt and/or irreversible changes may be triggered at a temperature threshold [ 7 ]. Particularly worrying is a ‘tipping cascade’ in which multiple tipping elements interact in such a way that tipping one threshold increases the likelihood of tipping another [ 4 , 10 ].…”