Summary Green alder (Alnus alnobetula (Ehrh.) K. Koch) is a tall, multi-stemmed deciduous shrub widespread in the Central European Alps across the treeline ecotone. On Mt. Patscherkofel (2246 m a.s.l.; Tyrol, Austria; 47°12’31” N, 11°27’38” E) three A. alnobetula stands were selected and sap flow density was continuously recorded from June 21 throughout September 9 during the growing seasons of 2023 and 2024, respectively. Sap flow density was monitored by means of the heat dissipation approach (n=11–15 sap flow records per year). The plots included a site situated somewhat below the forest limit at 1940 m a.s.l. (FL) and two other sites located close to the treeline. The latter differed in terms of contrasting environmental conditions with regard to the duration of snow cover, i.e., one site facing south-east (TR-S, 2140 m a.s.l.) and one facing north (TR-N, 2150 m a.s.l). The age of selected A. alnobetula stands within the treeline ecotone ranged from 20 to 30 years. The data file (data_environm_sapflow.xlsx) contains sap flow density data of all samples and environmental records during the study period 2023-2024 at all study plots within the treeline ecotone.