[dnag]

Assuming the round circle was built of stones, plants growing over stones tend to develop stronger, more entangled roots. These plants become hardy than similar species of plants growing nearby over loose soil. Rock crevices also tend to accumulate water which these plants should be able to use when other adjoining plants are dying. I think this is the reason why the circle became apparent during this drought.

[masklinn]

Considering the wording, I'd expect the plants simply shoot their roots as deep and wide as they can, since those above stone can't sink theirs as far as the others they get less access to water and nutrient and thus mature slower for lack of resources. So they're still green while the others are already past that point.