Integrated Proximal Sensing and Ground-Based observations for drought and heat stress monitoring in a Mediterranean holm oak savanna

We enhance the observation capabilities of existing high-grade ICOS flux tower site by implementing continuous proximal remote sensing and tree scale measurements to build a unique integrated multiscale, high frequency (10-30 min) observational system to address critical knowledge gap regarding how Mediterranean tree species respond and recover from climate extremes such as compound heat and water stress. This issue is particularly relevant for Quercus ilex, the dominant species in Mediterranean area, which combines drought tolerance with limited understanding of its threshold responses to stress.

The main objective is to explore the potential of these combined observations in monitoring and understanding the mechanistic processes that regulate the functional response of Mediterranean holm oak open woodlands to heat and water stress, where structural heterogeneity and fast stress responses remain poorly captured by current limitations of remote sensing based Earth Observation systems (medium spatial resolution, revisit frequencies of days to weeks) and by standard monitoring approaches, insufficient to capture short-term adaptive physiological responses occurring at hourly or minute timescales, such as stomatal closure, water transport regulation, photoprotective mechanisms, and xanthophyll cycle dynamics.

The flux tower and related infrastructure deliver continuous ecosystem scale measurements of turbulent fluxes of energy, evapotranspiration (ET) and CO₂ (NEE, GPP), together with a comprehensive suite of meteorological variables and enhanced dense soil water observations (i.e. multiples soil water content and soil water potential profiles), complemented by point dendrometers, micro-tensiometers and sap flow measurements providing detailed information on tree water status and water transport dynamics at tree scale, and an innovative integration of state-of-the-art proximal remote sensing techniques: Thermal Infrared Imaging (TIR) coupled to a multispectral camera to resolve spatial patterns of vegetation surface temperature variations; Short-Wave Infrared Spectroscopy (SWIR) with novel Fabry-Perot micro-spectrometers for monitoring vegetation water content; Sun-Induced Fluorescence (SIF) and Visible-NIR Reflectance via FLOX system to distinguish active photosynthesis from photoprotective responses; and LED-Induced Fluorescence (LEDIF) to measure basal photosynthetic state and plant recovery.

In addition to provide comprehensive information to assess physiological and functional vegetation response to drought and heat stress, the integrated observational dataset is foreseen to be used for: (i) improving TSEB/3SEB evapotranspiration models to better characterize hydraulic and physiological constraints on water and energy fluxes and to enhance model performance; (ii) contribute to FLuorescence EXplorer (FLEX) mission Cal/Val activities by testing upscaling strategies in heterogeneous dehesa ecosystems within SPAFLEX project.