These tabular data were compiled to document how key plant trait values change during plant development, particularly seedling stages, and in relation to soil moisture. An objective of our study was to answer three main research questions: (1) Do seedling trait values differ across early to late stages of seedling development and do those trajectories vary among plant species and functional types (i.e., forbs vs. grasses)?; (2) Does water availability influence seedling ontogenetic trait variation? and, if so, does this variation affect plant species drought performance?; and (3) Do seedling trait values at early stages of development differ from complied trait database values for species? These data represent key trait values, including specific leaf area, root:shoot ratio, specific root length, and root dry matter content of 20 – 62-day-old seedlings grown under low and high levels of water availability. These data were collected from July-October 2019 at the Research Greenhouse at Northern Arizona University in Flagstaff, Arizona. These data were collected by U.S. Geological Survey staff and greenhouse assistants at Northern Arizona University using observations and destructive harvesting of plants in a greenhouse setting. These data can be used to understand variation in functional traits early in plant development to predict community level processes and ecosystem responses to environmental change and ecological restoration practices.