This file contains isotopic and environmental data from seven individuals of Larix decidua, sampled at two sites located at an elevation of 1300 meters above sea level in the Lötschental valley, Switzerland, during the period of 2012-2013. Within this file, you will find two datasets. The first dataset, labeled 'd18Ocellulose,' comprises individual intra-annual d18O cellulose measurements, along with corresponding temperature, vapor pressure deficit, and relative humidity data. These measurements are aggregated for the period during which each tree-ring section was in the cell wall formation phase. The second dataset, identified as 'd18ONW-XYL,' includes individual d18O measurements for both xylem and needle water, in conjunction with temperature, vapor pressure deficit, and relative humidity data.