We determined δ26Mg values in the dominant tree species and organic layer along the 127 year-old Hailuogou glacial retreat chronosequence (southwest China), which is characterized by rapid ecosystem development from bare soil to a mature, conifer-dominated forest, in order to assess whether stable Mg isotope ratios reflect the increasing incorporation of Mg into biotic cycles with advancing vegetation succession. The dataset was complemented with measurements of major (N, Ca, Mg, and K) and trace element concentrations (V, Co, Rb, Sr, Ba, Fe, and Ni). Samples were collected from five ecosystem succession stages, which were exposed for 37, 47, 59, 87, and 127 years since glacier retreat. Each ecosystem succession stage was sampled in triplicate (i.e., 15 plots in total), as described in Basdediós et al. (2022). At each plot, we collected fresh leaves, 1 yr-old needles, 3 yr-old needles, branches, and roots of the dominant tree species between August and October 2017. To ensure representative sampling, a minimum of three trees were randomly selected from each plot and combined into a single composite sample. Four species were sampled along the chronosequence, including Hippophae tibetana Schltdl (37 yr-old site), Populus purdomii Rehder (47 yr-old site), Abies fabri (Mast.) Craib (59 and 87 yr-old site), and Picea brachytyla (Franch.) E. Pritz (127 yr-old site). Additionally, we collected leaves/needles of P. purdomii at the 59 yr-old site, and A. fabri at the 47 and 127 yr-old sites, which were the second most abundant tree species at those sites. For Mg isotope analysis, aliquots of the organic horizons (Oi, Oe and Oa) were thickness-weighted to a composited sample per plot.