As in any ecological study, they can be quantified with sampling of frames of 1 m square. The number of samples will depend on the work area

Seedlings are usually plants less than 30 cm long, measured when extended. I find that it's possible to count all seedlings, perhaps under tree canopies, or use quadrats. But random small square quadrats only work if you find seedlings in them! So instead, you can use random strip quadrats :)

Individuals with height less than

https://www.forru.org/advice/field-performance-planted-trees