SAP FLOW DYNAMICS IN THINNED AND UNTHINNED BOREAL FORESTS Effects of selection cutting on sap flow in a multi layered stand of Norway spruce and Scots pine

Sammanfattning

Understanding how thinning influences tree-level water use is essential for developing climate resilient continuous cover forestry (CCF) in boreal ecosystems. This study investigates sap flow dynamics in Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) in two adjacent multilayered stands at Remingstorp, Sweden. One of the stands were untreated and used as control and one stand was subjected to selection cutting (blädningsgallring) in 2015. Sap flow was measured using EMS 81 sensors on 20 trees across two growing seasons (2024-2025) and tree-level drivers were analyzed using linear mixed-effects models (LMM). Additional analyses examined tree size differences, temperature sensitivity and stand level-transpiration. Daily sap flow differed between species and treatments at the descriptive level with spruce in the control stand showing the highest mean sap flow (32.8 kg day-1) and pine in the thinned stand the lowest (14.9 kg day-1). However, these differences were not statistically significant. Temperature emerged as the dominant driver of sap flow across all models (p < 0.001), while structural variables (DBH, tree height and crown height) and thinning treatment had comparatively minor effects. Temperature sensitivity differed significantly between species x treatment groups, with thinning reducing sensitivity more strongly in spruce than in pine. Tree size did not differ significantly between stands. Stand-level transpiration was substantially higher in the control stand (20 400 kg ha-1 day1; 2.04 mm day-1) than in the thinned stand (8 800 kg ha-1 day-1; 0.88 mm day-1), primarily due to higher tree density rather than differences in basal area or tree size. Overall, the results show that species identity and temperature exert stronger influence on sap flow than thinning under the conditions studied. These findings are consistent with previous studies showing that low-intensity thinning often produces limited changes in tree-level sap flow. Together, the results contribute to understanding water-use dynamics in CCF and highlight the need for future research incorporating soil moisture, VPD and drought-period responses.