Cambiamenti climatici in atto: osservazioni sugli impatti degli eventi siccitosi sulle foreste toscane / Climate change in progress: observations on the impacts of drought events on Tuscan forests

The empirical observation of the impacts of drought and heat waves that occurred between 2017 and 2022 in several forest areas of Tuscany allowed to describe various processes and effects on forest trees and shrubs. The most severe impacts were observed on the evergreen Mediterranean tall woodlands and the aged coppices (on holm oak trees), causing defoliation and even mortality. In most cases, the attempts of resprouting from dormant buds (from the base of the tree trunk or in the crown), following the drought events, were not successful. Deciduous broad-leaved trees (beech, downy oak) suffered a strong summer defoliation, especially on steep slopes with shallow soils and south-facing aspect. These species were generally able to replenish the foliage the following year. In the most unfavorable conditions, however, we observed extensive attacks of opportunistic parasites, such as the fungi of the genus Biscogniauxia, which were favored by drought and were the ultimate cause of mortality. Turkey oak showed mortality in small groups or individual trees. Heat and drought also affected several minor tree species, with defoliation, desiccation and, in the most serious causes, death of the tree. Some mesohygrophilous species that are restricted with relict populations to specific microclimatic sites within the Mediterranean and sub-Mediterranean habitats were also impacted, with consequences on biodiversity, regeneration, and production of fruits for wildlife. Increasing drought and heat will trigger regressive dynamic processes that will lead in many sites to the transformation of holm oak forests into bushy forms of scrub, and of scrublands into sparse garrigues or even steppes. A more intense natural thinning of aged coppice stands with especially Turkey oak is also occurring, due to the combined effect of drought and intra- or inter-specific competition. The implementation of a regional monitoring system is recommended, based on the integration of past and existing experiences with new initiatives, to quantify the dynamics, extent and severity of the impacts. A crucial point is the need to adapt forestry, and especially utilization of coppice woods, to the changed ecological conditions to avoid increasing the impact on them.