This is the dataset used in "Young mixed planted forests store more carbon than monocultures: a meta-analysis" published in Frontiers in Forests & Global Change. The dataset contains carbon or biomass data for mixed and monoculture planted forests from 21 sites with a global coverage. We provide summary data necessary to conduct a meta-analysis: mean, standard deviation, and sample size, for each unique mixed and monoculture treatment at each study site. We indicate whether the values provided are aboveground carbon or biomass. For each treatment we also provide the species richness, age and species. We provide the longitude, latitude and country for each study site. Our study also assessed the effect of study design (experiment vs existing plantation), species origin (native vs non-native/mixed), and presence of nitrogen fixer in the mixture (N fixer present vs absent), we record the value of each of these factors. Finally, we categorised a subset of monocultures as commercial species monocultures based on the species use in that location.