The seedling stage is generally the most important bottleneck for the successful regeneration of trees in forests. The traits of seedlings, particularly biomass allocation and root traits, are more easily quantified than the traits of adults. In this study, we tested the hypothesis that seedling traits vary and trade-off tracking the changing environment during secondary succession. We measured the major morphological traits of 27 dominant species and the major environmental factors in a chronosequence (30-yr-old fallow, 60-yr-old fallow, and old growth forest) after shifting cultivation in a tropical lowland rain forest on Hainan Island, China. The 30-yr-old fallow had higher light and nutrient availability, and the older forests had higher soil water content. Redundancy analysis based on species abundance and environmental factors revealed groups of seedlings that dominate in different stages of succession. Seedlings in different stages of succession had different strategies of biomass allocation for harvesting resources that varied in availability. Species characteristic of younger forest had higher allocation to roots and higher specific leaf area, while species characteristic of older forest had higher allocation to leaves. Our study suggests that the variations and trade-offs in the major functional traits of tree seedlings among successional classes may reflect changes in environmental conditions during succession.