Effect of diurnal solar radiation regime and tree density on sap flow of Norway spruce ( <i>Picea abies</i> [L.] Karst.) in fragmented stand

Abstract The continuous threat of ongoing climate change and related weather anomalies challenge forest ecosystems. The phytosociological structure of forests plays a crucial role in determining their resilience to various abiotic and biotic stressors. Furthermore, stand density, which partly regulates the allocation of resources within individual trees, is a vital aspect of understanding forest functioning. This study was conducted in Norway spruce ( Picea abies [L.] Karst.) forests in the Czech Republic, where we investigated the influence of tree density on sap flow rates within three predefined directions corresponding to sun position during the morning (5:00–11:10 hours; East), noon (11:10–15:10 hours; South), and evening (15:10–21:10 hours; West) intervals. Tree density was calculated within a 10 m radius buffer around each sap flow measured tree using high spatial resolution aerial imagery acquired by Unmanned Aerial Vehicle. Sap flow was measured at 10-minute intervals for 25 selected trees during the nine hottest summer days in 2019. Our data reveals an inverse correlation between sap flow and tree density, underscoring the substantial impact of neighboring tree density on transpiration. This relationship was most pronounced during midday, followed by the evening and morning hours, suggesting higher heat loads on the trees and forest grounds during the midday. Our findings emphasize the critical role of stand density in modulating forest functioning and underscore the importance of maintaining specific canopy densities as part of effective silvicultural practices in the face of ongoing climate change.