Looks like it works sort of like a peat bog. Trapped vegetation on the bottom layer decomposes and tries to float, but the mud/fencing keeps it down, and that’s used as a substrate for farming on top. Iirc bog islands like that aren’t exactly stable but some of them stick around long enough to grow small trees. Sounds like plenty of time for a few seasons of farming. Maybe it’s more like creating an artifical marsh?
Rather than looking at the illustration in the OP as three separate structures, I think it may be depicting a single structure in three stages of development, going left to right.
Looks like it works sort of like a peat bog. Trapped vegetation on the bottom layer decomposes and tries to float, but the mud/fencing keeps it down, and that’s used as a substrate for farming on top. Iirc bog islands like that aren’t exactly stable but some of them stick around long enough to grow small trees. Sounds like plenty of time for a few seasons of farming. Maybe it’s more like creating an artifical marsh?
Rather than looking at the illustration in the OP as three separate structures, I think it may be depicting a single structure in three stages of development, going left to right.
Looking at it again, I think you’re absolutely right. The tree growth is meant to show time passing. Neat!