The use of satellite data to detect forest areas impacted by extreme events, such as droughts, heatwaves, or fires is largely documented, however, the use of these data to identify the heterogeneity of the forests’ response to determine fine scale spatially irregular damage is less explored. This paper evaluates the health status of forests in southern Italy affected by adverse climate conditions during the hot and dry summer of 2017, using Sentinel-2 images (10m) and in situ data. Our analysis shows that the post-event—NDVI (Normalized Difference Vegetation Index) decrease, observed in five experimental sites, well accounts for the heterogeneity of the local response to the climate event evaluated in situ through the Mannerucci and the Raunkiaer methods. As a result, Sentinel-2 data can be effectively integrated with biological information from field surveys to introduce continuity in the estimation of climate change impacts even in very heterogeneous areas whose details could not be captured by lower resolution observations. This integration appears to be a successful strategy in the study of the relationships between the climate and forests from a dynamical perspective.

<p>Increasing drought severity can affect the healthy status of forests and determine changes in structural and ecophysiological responses to such extreme climate events. Reduced canopy cover, productivity and tree growth and recent dieback phenomena are widespread responses to drought. However, favourable climatic conditions can improve the post-drought recovery capacity of forests, but also make them vulnerable to drought damage through structural overshoot by altering the root to shoot ratio due to wet conditions. Due to the lack of integrated and retrospective field data, the patterns and responses of forests to wet-dry climate variability are still poorly understood. In this work we used remote sensing data (NDVI) to characterise the canopy conditions and combined them with field and tree-ring width data to assess the effects of the summer 2017 drought on Mediterranean tree species in southern Italy (<em>Fraxinus ornus, Quercus pubescens, Acer monspessulanum, Pinus pinaster</em>). By comparing radial growth and resilience indices we found that growth responses to drought depended not only on tree species but also on site conditions. Overall, the growth decline due to drought was followed by a rapid recovery, while negative legacies to drought were found at lower quality sites, which corresponded to sites with the lowest NDVI values. Indeed, trees at these sites showed high growth rates before drought, in response to wet winter-spring conditions, and then suffered more from drought stress. Our results demonstrated how structural overshoots predisposes to drought damage and induced negative legacies. Specific knowledge on the effects of drought overshoot over time is important for analysing and understanding current forest responses and dynamics.</p>

Forests around the world are facing climate change. Increased drought stress and severe heat waves in recent decades have negatively impacted on forest health, making them more vulnerable and prone to dieback and mortality phenomena. Although the term vulnerability is used to indicate an increased susceptibility of forests to climate change with a worsening of their vigour status that can compromise their ability to respond to further climate extreme events, there are still uncertainties on how to evaluate it. Indeed, evaluation of forest vulnerability is complex both because of some critical issues in the estimation methods used and because of the multiple factors influencing the response of forests to ongoing climate change. A way to assess the vulnerability to environmental stresses is by combining remote sensing and dendroecological data. However, these two approaches entail multiple uncertainties, including growth/photosynthetic relationships, carbon allocation dynamics, biases of tree-ring data and noisy remote sensing data, which require further clarification for proper monitoring of pre- and post-drought forest trajectories. Our review aims to create an overview of the current literature and knowledge to understand the critical issues, needs and possible solutions that forest vulnerability research is addressing. We focus on Mediterranean forests located in a climate warming hotspot and showing a high vulnerability to increased aridification.

<p>Forest dieback phenomena occur all over the world and have been mainly attributed to extreme droughts and heatwaves. These phenomena are more pronounced in the Mediterranean basin, causing a significant impact on the structure, composition, and productivity of forests. Understanding the response of Mediterranean forests to extreme climate events is of paramount importance to assess their vulnerability to such phenomena. The present study combined both radial growth data and remotely sensed indices (i.e., Normalized Difference Vegetation Index, NDVI) to analyse the vegetation status of selected deciduous forest stands impacted by the summer 2017 heatwave. We surveyed several sites of the Southern Apennines mountains range in order to: I) investigate the growth response of forest vegetation in terms of resilience and resistance and II) characterize their ability to recover after extreme climatic events. Overall, we observe a significant reduction in radial growth after 2017 in all studied stands. Moreover, the vegetation shows clear signs of "recovery" that are strictly dependent on species-specific and site-specific conditions. In this study we will highlight how these stands responded to the heatwave occurred in the 2017 and how these events can affect the future vegetation dynamics either in terms of growth and evolution of Mediterranean ecosystems.</p>