Drought responses of a Norway spruce forest on drained peat soil: combining sap-flow sensors, eddy-covariance, meteorological, soil and UAV data

Abstract

The study explores the drought effects on boreal peatland forests, a problem that has not been sufficiently addressed to date. Deeper knowledge of how these stands respond to drought is essential for refining forest management practices and improving climate modeling. We conducted a holistic investigation of the ecophysiological and meteorological responses to drought in a southern Finnish Norway spruce-dominated, drained boreal peatland forest during the hot and dry spells of the summers of 2020 and 2021. The study utilized a comprehensive set of ecosystem monitoring products: eddy-covariance (EC) CO 2 and H 2 O fluxes, surface energy balance, sap-flow sensors, UAV-based leaf temperature and NDVI orthomosaics, soil moisture and water table level, and weather data. Norway spruce reaction to drought was examined at sub-daily to seasonal time scales in a recently thinned Continuous Cover Forestry (CCF) block and a control block, revealing divergent reactions to drought. The stand-scale response was most directly reflected in the sap flow data, which indicated higher stress in the control block, likely caused by higher inter-tree competition for soil water. At the same time, both the control and CCF harvest blocks showed signs of physiological stress, manifested as reduced net ecosystem exchange (NEE) and increased Bowen ratio, particularly on high VPD days. The UAV surveys indicated that trees in the CCF block tend to have higher canopy temperatures and lower Normalized Difference Vegetation Index (NDVI) than in the control block, implying their possible susceptibility to more extreme drought. The study confirms the expectation of stronger drought response in the denser stand and confirms the benefits of UAV remote sensing and sap flow measurement for tree drought stress detection.