Monitoring the effects of extreme drought events on forest health by Sentinel-2 imagery

Abstract

Global climate change is expected to result in more frequent and intense drought events, especially during the warm season. In such perspective, it is crucial to assess the forest stands vulnerability to extreme climatic events, such as drought, even for Mediterranean forest tree species, commonly considered resistant to dry spell. To test the capability of multitemporal imagery derived by Sentinel-2 (S2) in detecting the impacts of extreme drought events on forest health assessed as crown dieback, some forest stands in Tuscany (central Italy) were analyzed. Vegetation indices (VIs) and ancillary digital terrain model-derived data have been collected in 118 observational samples distributed along an ecological gradient. VIs detected a reduction of trees of photosynthetic activity in August 2017. S2 data have allowed the observation of the different response strategies of the tree species considered in this study to the extreme climatic event that occurred. The case study presented shows that S2 can be applied for monitoring climaterelated processes providing a synthetic overview of forest conditions at regional scale.

Widespread Crown Defoliation After a Drought and Heat Wave in the Forests of Tuscany (Central Italy) and Their Recovery—A Case Study From Summer 2017

“…The subsequent autumn and winter months (2017-2018) were wet and rainy (Figure 1), favoring a substantial recovery from drought. In June 2018, the foliar mass of F. sylvatica forests appeared almost completely restored (data confirmed by airborne observations, Puletti et al, 2019). Quercus pubescens showed a similar pattern.…”

“…In sclerophyllous species at the Mediterranean areas, subjected to branch desiccation, the crowns are restored with of the activity of the adventitious buds. In this case the recovery processes are slow, and the evidences of the drought impact were detected in the year 2018 with a remote sensing survey (Puletti et al, 2019).…”

Special issue in honour of Prof. Reto J. Strasser - The JIP-test as a tool for forestry in times of climate change

“…Vegetation dynamics models predict transformation of forests as the climate changes, meaning that new strategies will be required to maintain functional forests. Apart from the expected increase in average temperatures, a key stressor of European forests will be the increasing probability of extreme summer drought associated with extreme heat (Carnicer et al 2011), like it occurred in Central Europe in 2003 (Bréda et al 2006, or in Italy in 2017(Puletti et al 2019, and with an exceptional heatwave in early summer 2019. These stressful conditions are widely degradation and erosion.…”

Exploring the Use of Sentinel-2 Data to Monitor Heterogeneous Effects of Contextual Drought and Heatwaves on Mediterranean Forests

“…Thus, forest ecosystem responses to drought and their adaptation depend on the drought intensity and duration [20,21]. Moreover, some tree species are more sensitive to drought-triggered growth decline and damage [21][22][23][24] and sometimes the response of forests to drought can be exacerbated by local environmental and site conditions [24][25][26][27][28].…”

“…The launch of the new Sentinel-2 satellite, one of the core missions of the Copernicus Earth Observation Programme of the European Union, provided opportunities for new research activities into the assessment and monitoring of forests. Sentinel 2, freely available, is characterized by a wide swath (290 km), a short revisit time (5 days with 2 satellites under cloud-free conditions which results in 2-3 days at mid-latitudes), 13 spectral bands (4 bands at 10 m of spatial resolution; 6 bands at 20 m of spatial resolution, and 3 bands at 60 m of spatial resolution) at a wider scale [50] that guarantee consistent time series minimizing any artifacts introduced by atmospheric variability and allowing for the systematic monitoring of the land surface [20]. The resolution (10 m) of this sensor could be sufficient to retain most of the details of the local response of forests to external stresses accounting both for damaged crowns and understory vegetation.…”