Differentiating Acute and Chronic Stress in Norway Spruce using Hyperspectral and Thermal Remote Sensing in Experimental Plots

Abstract. Norway spruce forests across Europe are increasingly threatened by acute and chronic stressors, particularly thermal exposure and drought, exacerbated by climate change. This study investigates the physiological and spectral responses of spruce trees under experimentally induced stress using high resolution thermal and hyperspectral remote sensing from UAV and aircraft platforms. Field experiments were conducted in Slovakia and the Czech Republic across three plot types: edge exposed (acute thermal stress), roofed (chronic drought stress), and unmanipulated controls.
By integrating remote sensing data with sap flow, VOC emissions, and soil microclimate measurements, we show that acute stress from sudden edge exposure triggers rapid physiological responses, including increased transpiration, elevated crown temperatures, and distinctive VOC profiles. In contrast, chronic drought stress, simulated through rainfall exclusion, leads to sustained reductions in sap flow, lower water availability, and declines in key spectral indices such as MRENDVI and NDWI. Bark beetle attacks occurred only in drought stressed trees, suggesting a link between prolonged physiological decline and pest vulnerability.
Generalised Additive Models revealed that crown temperature, soil water potential, and canopy structure strongly influence sap flow, while thermal traits were shaped by canopy density and solar radiation. Remote sensing data captured these relationships, enabling early detection of stress before visible symptoms emerged.
Our findings confirm that thermal and hyperspectral imaging, supported by ground truth data, can reliably distinguish between acute and chronic stress in Norway spruce. This multi sensor approach supports proactive forest health monitoring and early intervention against bark beetle outbreaks under changing climatic conditions.