Sapling is too generic to me. Two leaves in a 2 cm stick is a sapling but a juvenile 5 m high tree is also a sapling. Some trees grow really slow, some plants are not trees but have a 'similar' function in nature (bamboo for example) and a lot of trees begin as multitrunked shrubs. Is not trivial to count trees in a forest when several trunks share the same root system so we can expect a lot of troubles and variations. We should focus in measuring live biomass probably instead. To group different saplings of different species and try to cook a result from this is always a problem.
Maybe to make people understanding the consequences of deforestation we need to create a new system and talk instead of ecological categories like seed, seedling, 'treedler', tree and perhaps 'treelder', but I'm just having fun with the words.
Ash, for example, is equal to "a common tree" but people forget often that the morphospecies "a 500 years ash" is extinct in most forests (as all species that depend on it). This is like if we try to define and understand human societies just looking at "people from 20 years or less".
Only if it treeddles! If not, might sapling be a satisfactory existing word? Some dictionaries constrain sapling to 4 inches, which is ~10 cm.