Evaluating the effects of extreme drought events on forest health by multitemporal Sentinel 2 satellite imagery

During summer 2017 central Italy has been hit by an intense drought and heat wave, with unusual mean daily temperatures, in many cases higher than 35°C, for several weeks. Extensive damage and tree mortality were observed on forest ecosystems. The health conditions of forests in Tuscany during and after this extreme climate events were assessed and mapped by means of multispectral and multitemporal Sentinel 2 imagery. A total of 87 observations combining field surveys and Sentinel 2 multi-temporal imagery of both healthy and unhealthy forest stands have been carried out. Beech (Fagus sylvatica L.), downy oak (Quercus pubescens Willd.), Turkey oak (Q. cerris L.), and holm oak (Q. ilex L.) forests have been considered as ecosystems with major impacts caused by drought and heat weaves. Drought-induced early foliar shedding in deciduous broadleaved tree species and diffuse desiccation of leaves and branches in evergreen broadleaves have been observed. The strongest impacts were detected in sites located at the higher altitudes, south exposed and/or in poor soil conditions (especially on calcareous and serpentine soils). Field surveys have revealed that deciduous trees were affected by strong crown defoliation without mortality, whereas foliar desiccation and large crown dieback characterized evergreen species. Red edge channels and NDVI demonstrated their ability in supporting detection and map of drought effects on forests and can be hopefully considered in future algorithms to estimate GPP from space using Sentinel 2 imageries.