Capturing temporal shifts in forest soil-atmosphere feedbacks: evidence from a Mediterranean catchment

This study explores the complex interactions among eco-hydro-meteorological (𝐸𝐻𝑀) variables, such as soil moisture (𝑆𝑀), precipitation (𝑃 ), vapor pressure deficit (𝑉 𝑃𝐷), and sap flow velocity (𝑆𝐹), in the Mediterranean climate, where water scarcity and the effects of climate change are becoming increasingly pronounced. While previous studies have identified thresholds controlling runoff generation in Mediterranean catchments, it remains unclear how lead-lag dynamics among EHM variables govern ecological responses. This study addresses this gap by using wavelet analysis to explore the controls and dynamics of 𝐹𝑆𝑀−𝑃 and 𝐹𝑆𝑀−𝑉 𝑃𝐷 feedbacks across different time and frequency scales, investigating whether EHM interactions exhibit evident thresholds that trigger feedback switches in response to SM availability and atmospheric demand. These dynamics were studied during the 2021 growing season at two topographic positions (riparian and hillslope) on an experimental hillslope in a Mediterranean forested catchment, central Italy. Our results revealed that SM in the hillslope position showed rapid, event-driven responses to P, with soil water recharge processes that differed significantly between the wet and dry seasons. This sensitivity to climatic forcings was greater in the hillslope SM than in the riparian one, which was characterized by more uniform and smoothed responses. Furthermore, the results highlighted the activation of proactive water-saving strategies adopted by trees located in the hillslope position, as evidenced by a critical SM threshold of 0.15 𝑚3∕𝑚3 below which not only was P ineffective, but also SF was severely limited. In contrast, the constant availability of soil water in riparian areas allowed trees to maintain higher transpiration rates, showing a more passive response to atmospheric demand. In conclusion, although P is the main source of soil water recharge, its effectiveness on water availability and tree transpiration depends on complex interactions within the soil-plant-atmosphere continuum, with topography acting as the dominant controlling factor. These insights are critical for improving our understanding of the resilience of forest ecosystems and for guiding more effective water and forest management strategies.