[This dataset is embargoed until December 31, 2026]. This dataset provides physiological, morphological, and functional trait measurements for twelve African savanna tree species belonging to the Vachellia and Senegalia genera (Fabaceae family). The data encompasses six species classified as encroaching (increasing in density in African savannas) and six non-encroaching species, facilitating comparative analysis of traits facilitating encroachment ability. The variables capture sapling responses to a factorial combination of environmental conditions, specifically atmospheric carbon dioxide concentration, water availability, and interspecific grass competition. The dataset details plant performance under elevated CO2 versus ambient CO2 and contrasts well-watered and water-limited conditions. Furthermore, the data includes comparative metrics for trees grown with and without competition from the grass species Themeda triandra. All measurements were derived from saplings grown for 11 months in large pots containing local savanna soil within open-top chambers (Rhodes University Elevated CO2 Facility, Makhanda, South Africa), culminating in physiological assessments and destructive harvesting for morphological analysis. The work was supported by the Natural Environment Research Council (Grant NE/T000759/1). Full details about this dataset can be found at https://doi.org/10.5285/f7b8e72f-9bbe-4a03-8140-fada73e95b8c