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

Abstract

Abstract The continuous threat of ongoing climate changes and related weather anomalies pose a significant challenge to forest ecosystems. The phytosociological structure of forests plays a crucial role in determining their resilience to various biotic and abiotic stressors. Moreover, stand density, which regulates the allocation of resources within individual trees, is a vital aspect for comprehending forest functioning. This study was conducted in Norway spruce forests located 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 10m radius buffer around each tree using high spatial resolution aerial imagery acquired by Unmanned Aerial Vehicle (UAV). We measured the sap flow in 10-minute intervals for 25 selected trees during the nine hottest days of the summer in 2019. We normalized sap flow measures using the abundance of tree foliage, which was qualitatively evaluated in the field as a reverse estimate of defoliation. The normalized data were used in further statistical analyses. Our findings reveal a strong negative correlation between sap flow and tree density, underscoring the substantial impact of neighboring tree density on tree transpiration. This relationship was most pronounced during midday, followed by the evening and morning hours, suggesting that sparser stands experience greater water deficit. The interaction between stand density and incoming solar radiation may constitute a crucial factor allowing forests to endure and adapt to climate change and other stressors such as bark beetle infestations.