Defoliation, Recovery and Increasing Mortality in Italian Forests: Levels, Patterns and Possible Consequences for Forest Multifunctionality

Forest health and multifunctionality are threatened by global challenges such as climate change. Forest health is currently assessed within the pan-European ICP Forests (International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests) programme through the evaluation of tree crown conditions (defoliation). This paper analyses the results of a 24-year assessment carried out in Italy on 253 permanent plots distributed across the whole forested area. The results evidenced a substantial stability of crown conditions at the national level, according to the usual defoliation thresholds Defoliation > 25% and Defoliation > 60%, albeit with species-specific patterns. Within this apparent temporal stability, an increased fraction of extremely defoliated and dead trees was observed. Extreme defoliation mostly occurred in years with severe summer drought, whereas mortality was higher in the years after the drought. The results for singular species evidenced critical conditions for Castanea sativa Mill. and Pinus species, whereas Quercus species showed a progressive decrease in defoliation. Deciduous species, such as Fagus sylvatica L., Ostrya carpinifolia Scop. and Quercus pubescens Willd. suffer the loss of leaves in dry years as a strategy to limit water loss by transpiration but recover their crown in the following years. The recurrence of extreme heat waves and drought from the beginning of the XXI century may increase the vulnerability of forests, and increased tree mortality can be expected in the future.

Automated summary

This study analyzed the results of a 24-year assessment carried out in Italy, showing relative stability of tree crown conditions at the national level but with an increase in extreme defoliation and tree mortality due to extreme climate events. With the recurrence of extreme heat waves and drought, forest vulnerability may increase, leading to higher tree mortality in the future.