Flash drought alters immediate growth rather than tree water relations: A case study from central Italy.

The ecological resilience of Mediterranean forests is increasingly challenged by the rising frequency, severity, and unpredictability of drought events driven by climate change (IPCC, 2021). In recent years, flash droughts, characterized by their rapid onset and short duration, have emerged as a significant climatic stressor. Understanding how different tree species respond to these abrupt drought events is essential for predicting forest vulnerability and resilience under future climate scenarios and for translating physiological responses into effective forest conservation and management strategies.

This study aims to: (1) analyze the temporal characteristics of flash drought events, including their onset, duration, and intensity, using atmospheric indicators; and (2) assess species-specific responses of stem radial growth (SRG), tree water deficit (TWD), and stem water transport (sap flow) during and after flash drought episodes, with a comparison between coniferous and broadleaves species.

The research was conducted in the Piegaro Forest, located in the Umbria region of central Italy (42.96°N, 12.06°E; 430 m a.s.l.). The study site is dominated by deciduous broadleaved stands, mainly oaks (Quercus cerris and Quercus petraea) and wild cherry (Prunus avium), alongside conifer plantations of Douglas-fir (Pseudotsuga menziesii) and Scots pine (Pinus sylvestris). An IoT-based monitoring platform, the TreeTalkerCyber device, was installed on selected trees to continuously record individual tree physiological functioning and microclimatic conditions.

Our findings show that flash drought period significantly suppressed stem radial growth in conifer species, whereas this suppression was not significant in broadleaved species, despite that all species maintaining sap flow during the flash drought. Notably, sap flow played a critical role in sustaining growth during flash drought periods. However, the effects of drought stress were more pronounced in the post-drought period, with reduced stem growth and sap flow compared to pre-drought conditions. Overall, stem radial growth emerged as the most sensitive and responsive indicator, revealing persistent internal water stress that extended beyond drought termination.

These results provide valuable insights into species-specific drought resilience and have important implications for sustainable forest management and silvicultural practices under increasing climate variability and the intensification of flash drought events.