The tree-ring stable carbon and oxygen isotope chronologies from two forest sites located in the Forni Glacier forefield (Italy)—one along the glacier stream (GL) and the other toward the valley slope (SL)—were analyzed with the aim of disentangling the precipitation and glacier meltwater inputs in source water δ¹⁸O, as reflected by the tree-ring cellulose δ¹⁸O. The cellulose δ¹⁸O from the GL trees has a negative correlation with winter and summer temperatures, whereas the cellulose δ¹⁸O from the SL trees has a positive correlation with precipitation δ¹⁸O. The isotopic signature of the source water at the GL site is also influenced by waters of glacial origin, as confirmed by the ¹⁸O-depleted glacier meltwater inputs (GMWI_δ¹⁸O) estimated by means of an isotope model. The GMWI_δ¹⁸O values are consistent with the mean difference measured between the δ¹⁸O in the glacier stream and in the precipitation and the winter and summer temperature explains up to 37 percent of the GMWI_δ¹⁸O variance. Our results show an increasing influence of glacier meltwater throughout the past decade for the GL site. Our analysis opens new opportunities to reconstruct changes in water regimes of the glacier streams by means of the tree-ring cellulose δ¹⁸O.