NTT checklists include tree species only and this results basically from two limitations: the author's knowledge and checklist availability in the literature, particularly during the first fifteen years of the database history. This obviously had to face the difficulty of defining a tree. Firstly, because this growth habit has no natural clear boundary and there are many proposed definitions in the literature, some of which either include or exclude particular cases, such as those of palms, tree-ferns and woody bamboos and rosettes. Secondly, because many species may grow here as trees and there as other growth forms, particularly shrubs, lianas and hemi-epiphytes. On top of this, descriptions of growth habits on herbarium labels often bring erroneous information (e.g. showing 'tree' for a liana collected from the forest canopy). NTT, therefore, adopted a 'liberal' definition, that is, tree species (arboreal and arborescent) are plants able to grow taller than 3 m in stature without climbing or leaning against other plants (free standing stems).

Circumscribing the growth habit becomes a minor problem when it comes to the ever-changing taxonomic circumscription of the many thousand tree species of the Neotropics. This is an inescapable reality and seeking for novel literature and establishing a constant interaction with specialists became routine in order to eliminate doubtful or flagrantly erroneous identifications and occurrence records. The NTT endeavor would not be possible without the help of specialists and knowledgeable vegetation scientists. Among the many contributors, the following scientists deserve mention for the time and energy devoted to help building up NTT: Marcos Sobral, Eve Lucas, Gwillim Lewis, Toby Pennington, Haroldo Lima, James Ratter, William Milliken, Daniela Zappi, Lulu Rico, Rafaela Forzza, João Renato Stehmann e Alexandre Salino.