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

Abstract. The summer of 2021 brought a severe drought to southern Finland. We explored how a drained boreal peatland forest responds to drought by combining a wide range of in situ monitoring and Unmanned Aerial Vehicle (UAV) remote sensing. A dataset combining eddy-covariance (EC) fluxes, sap-flow sensors, UAV mapping, soil and weather data was collected. Spruce stand reaction to drought is examined at sub-daily to seasonal time scales, and its temporal and spatial features are identified separately for a recently thinned Continuous Cover Forestry (CCF) block and a control block. Sap flow data showed that the CCF-harvested block shows greater tree resilience to high vapour pressure deficit (VPD) than the control block, likely due to the higher soil water availability during the rainless and hot period. At the same time, both the control and CCF harvest blocks had notably 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.