Detecting forest drought stress from above and from below

Connecting environmental conditions with plant growth and stress is an important part of ecosystem management in the context of a rapidly changing climate. Our understanding of how varying growing conditions (e.g., soil water availability, meteorological conditions) translate into plant stress and recovery continues to be thwarted by technical limitations in the monitoring of environmental conditions at the appropriate spatio-temporal scale and signs of stress and recovery at the plant and ecosystem scale. One of the most limiting factors to plant growth is water availability and an important stressor is drought. During drought, physiological changes induce a reduction in photosynthesis and thus plant growth. However, intensity and duration of water stress conditions determine the plants physiological response. Under mild water stress, plant regulation of water loss and uptake still allows the plant to maintain its water status with little change in photosynthetic efficiency. However, severe water stress leads to effects ranging from inhibition of photosynthesis and growth to xylem embolism, leaf wilting and loss of key pigments and thus irreversible damage to the photosynthetic and water transport machinery. Several in situ measurements and remote sensing technologies have been developed to quantify plant stress and ecophysiological response to drought, each with their own strengths and limitations. For example, dendrometers can measure very small changes in stem diameter and thus record daily growth rates and water status variations , while sap flux measurements help quantifying the amount of transpired water. While these techniques are useful for quantifying individual tree responses to stress in terms of mass fluxes and plant water status, they are difficult to apply to whole forests or agricultural fields. Quantifying radiation budgets is another approach for measuring plant stress and response to droughts. Thermal infrared (TIR) and hyperspectral (visible, near-, and shortwave infrared reflectance (VNIR)/SWIR) approaches (besides sun-induced fluorescence) are widely used remote sensing techniques for the detection of plant water stress. An important advantage of remote sensing is that it can be applied to a broader spatial scale. However, the spatial resolution is often coarse and the interpretation in relation to in-situ processes can be complicated by phenological dynamics. Here we present results from a European beech stand in Luxembourg, where we analysed continuous in situ measurements of dendrometer, sap flux, TIR canopy temperature, meteorological variables and soil moisture. We compare water stress indices derived from sap flux and dendrometer data with a TIR-based crop water stress index (CWSI) recently developed for crops (Ekinzog et al. 2022). Results are put into context with a leaf and canopy energy balance model and implications of drought stress for short and long-term carbon and water fluxes are discussed. Literature: Ekinzog, E. K., Schlerf, M., Kraft, M., Werner, F., Riedel, A., Rock, G., and Mallick, K.: Revisiting crop water stress index based on potato field experiments in Northern Germany, Agricultural Water Management, 269, 107664, https://doi.org/10.1016/j.agwat.2022.107664, 2022.