Measuring seedling area
Q: We need to "measure all the seedlings area (by dividing by the number of them I can get indication to their area and structure, and other parameters I can get)... The area covered is 80*80cm."
The screenshot shows the results of my experiment. Here the red contours surround the darker areas (vegetation) and green surround the lighter areas inside. So, the total coverage is 60 - 9 = 51%. Unfortunately, I was unable to separate the seedlings from the rest of the vegetation.
There is also work with ecological researchers to measure vegetation coverage but mostly with horizontal shots.