You could TRY and ESTIMATE biomass by measuring seedling height or volume - e.g. by photographing them with a ruler and using ImageJ or similar SW. Far from perfect but will give you an estimate.

This is done on a regular base for mature trees - google "tree volume estimation"

At the community level, a variety of non-destructive approaches to biomass measurement have been developed, including techniques such as optical occlusion sampling, vertical and horizontal image analysis, radiation reflectometry, laser level band detection or NDVI satellite remote sensing

(https://watermark.silverchair.com/mcm009.pdf?token=AQECAHi208BE49Ooan9kkhW_Ercy7Dm3ZL_9Cf3qfKAc485ysgAAAr0wggK5BgkqhkiG9w0BBwagggKqMIICpgIBADCCAp8GC )

Here is a link to a search that might help youز

Please check it.

Chapter Methods of Estimating Forest Biomass: A Review