On the definition of tree seedlings

Seedlings are the most critical bottleneck in plant recruitment, yet there is no practical and widely accepted definition of what a seedling is. Typically, the end of the seedling stage has been defined as the point when it stops being dependent on seed resources, or at the point of maximum growth rate. However, these definitions are of limited use for ecological studies based on field surveys. I analyzed 25 years of literature and 1766 publications to provide a first quantitative assessment of how tree and shrub seedlings are defined in forest field studies. I counted 282 unique definitions of seedlings, and I found that 24.4% of studies did not provide a clear definition. The most widely used definitions were “newly emerged” seedlings, followed by individuals less than 50 cm and less than 100 cm tall. Only 6.9% of the studies cited previous work to justify their choice. The most used definitions were generally consistent between tropical and extra-tropical studies, and between single versus multiple species studies. Three simple steps, if adopted by authors, could help to improve the clarity of what is meant by seedlings: (1) to always provide a clear definition of seedling, (2) to refer to previous studies, and (3) to adopt the most widely used definitions reported here (i.e., < 50 cm or < 100 m tall) whenever possible.