Extreme drought and heat wave effects on Tuscan forests: forest dieback and tree mortality in summer 2017

The reports of forest die-off events triggered by dry spells have increased significantly in the last decades. During summer 2017, central Italy was hit by intense drought and heat waves, with temperature peaks over 40°C. Starting from mid July, impacts, such as leaf discolouration, desiccation and, in August, early foliar shedding in deciduous broadleaf tree species, and diffuse desiccation of leaves and branches in evergreen broadleaf species, were observed. This study describes the results of a preliminary analyses of these impacts in Tuscany (central Italy), based on field observations and remote sensing surveys. To this purpose, we adopted three different approaches: (i) mapping of drought-induced forest damage, by means of the analysis of canopy reflectance; the Normalized Difference Vegetation index (NDVI) and Sentinel 2 spectral bands (NIR, RedEdge 3, RedEdge 4) were evaluated for drought-affected and unaffected forest stands for each forest type studied; (ii) analysis of non-structural carbohydrates content in woody tissues (small branches and twigs) in drought-damaged and non-damaged trees for the most relevant species; (iii) evaluation of the resilience of evergreen sclerophyllous species, by analyzing their capacity to produce new shoots and leaves from the damaged crown. Beech (Fagus sylvatica L.), downy oak (Quercus pubescens Willd.), Turkey oak (Quercus cerris L.) and holm oak (Quercus ilex L.) forest stands, located respectively in the Apennines, hilly and Mediterranean areas of the region, were the forest ecosystems which suffered the most relevant impacts. The strongest impacts were observed in the sites at higher altitudes, south exposed and/or on poor substrata, especially on calcareous and serpentine soils. Remarkably, deciduous trees were affected by strong crown defoliation but, apparently, no mortality, whereas evergreen species showed foliar desiccation and large crown dieback. Crown defoliation, desiccation and tree mortality affected also drought-tolerant sclerophyllous shrubs, like Arbutus unedo L., Phillyrea latifolia L., Erica arborea L., and perennials herbs of the Mediterranean shrubland. The studied tree species, which mostly suffered of the severe dry spell in 2017, and their distribution in Tuscany, suggest a relevant role of the site conditions (slope aspect, bedrock, soil properties as depth and capacity of water retention) in the occurrence and diffusion of forest dieback. The analysis of these factors allows to map the sensitivity and vulnerability of forests to extreme climate events. This is the premise for the development and application of new management strategies aimed at decreasing climate-induced risk and promoting forest resistance.