Abstract
An anomalous event of drought and heat occurred in central Italy during the summer of 2017. Based on the SPI (Standardized Precipitation Index) and data from the European Space Agency, this event started in November 2016 and was characterized by a strong reduction of precipitation and soil moisture, especially in lowland areas with Mediterranean climate. The aim of this case report were to describe the impact of this event on representative forest communities in central Italy, to analyze the different responses of deciduous and evergreen tree and shrub species in contrasting environmental conditions and to assess their subsequent capacity of recovery or, if not, mortality. Trees suffered severe impacts consisting of widespread crown defoliation, leaf desiccation, crown dieback and whole tree mortality. Deciduous tree species (Fagus sylvatica, Quercus pubescens, Quercus cerris) shed their leaves during the summer, but apical buds and twigs were preserved. This allowed these species to produce new shoots in the following year (2018) and to restore the canopy closure of the stands. Mediterranean evergreen broadleaves, such as Quercus ilex and Phillyrea latifolia suffered of total or partial crown desiccation with wilting leaves and branch dieback. These species partially resprouted in 2018 from axillary and latent buds. The case presented here is discussed within the wider context of the impacts of climate change on Mediterranean forests. Future research directions should include an effective forest monitoring system that combines terrestrial and remote sensing surveys, ad hoc field climate change experiments and silvicultural trials from the perspective of proactive management for the adaptation of forests to future climatic conditions.
Highlights
Events of extensive tree dieback and mortality related to drought have been detected across the world and described in several papers (Gitlin et al, 2006; Allen et al, 2010, 2015; Anderegg et al, 2013, 2015; Choat et al, 2018)
Evergreen sclerophyllous tree and shrub species of the Mediterranean areas in Southern Europe are commonly considered resistant to drought and other environmental stressors typical of this region, such as high temperatures, high solar irradiation, UV radiations etc. (Bussotti et al, 2014)
The Mediterranean sclerophyllous forests can restore the “before event” conditions by regenerating the crowns due to the resprouting ability of axillary and suppressed buds at the stump and branches (Del Tredici, 2001). This ability evolved in plants subjected to recurrent environmental disturbances, like fire and severe drought (Pausas and Keeley, 2014) and represents a strategy to rapidly restore the closure of the canopies
Summary
Events of extensive tree dieback and mortality related to drought have been detected across the world and described in several papers (Gitlin et al, 2006; Allen et al, 2010, 2015; Anderegg et al, 2013, 2015; Choat et al, 2018). A more accurate analysis of the effects of extreme summer events on the vegetation was carried out on Mediterranean evergreen forests since they were subjected to stronger and long-term drought stress with respect to the deciduous forests (Figures 1A–C) The purpose of this analysis was to provide baseline data and to start following the subsequent phases of recovery or decline in the long-term. Defoliation (i.e., loss of leaves as compared to a reference tree with a completely foliated crown) and damaged and dead leaves (partially and completely dry leaves, respectively) were visually evaluated for each plant by well-trained crews, in FIGURE 2 | Images of drought impact and recovery on Mediterranean vegetation in Tuscany (Italy) after the drought and heat event of summer 2017. Tree diameter was positively related (P < 0.05) to resprouting from the stem (epicormic shoots) and stump
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