Sap flow velocity (SFV) in sapwood of a mature Norway spruce (Picea abies (L.) Karst.) was monitored in the northwestern part of Latvia by the heat ratio method during 2014–2015. The effect of temperature and soil water potential was assessed by a bootstrapped Pearson correlation analysis. Seasonal and daily variation of SFV was observed. The highest SFV was observed in late April and during summer months whilst the lowest SFV occurred in winter months. During most of the studied period, SFV correlated with temperature. A negative effect of temperature was observed in summer suggesting occurrence of water and/or heat stress. The effect of soil water potential also has been significant in summer supporting the occurrence of water deficit conditions. During the dormant period, SFV was low, but still it reacted to temperature, suggesting that tree has been physiologically active. Nevertheless, sap flow differed between the inner and outer part of sapwood likely due to the differences in area of foliage of different age.

Keywords: circulation of sap, Picea abies, dormancy, heat ratio method.