In place of the great trees which had been growing there for centuries,patches of green had begu

In place of the great trees which had been growing there for centuries,patches of green had begun to appear in the blackened soil
里面的patches of 前面为什么有逗号？还有这个句子主句是不是In place of the great trees patches of green had begun to appear in the blackened soil谁能细说一下？

In place of the great trees which had been growing there for centuries，p
atches of green had begun to appear in the blackened soil

代替那颗已经长了一个世纪的树，一片片绿色开始出现在发黑的土壤里
逗号的使用同中文的一样，表示停顿间隔。
which had been growing there for centuries是定语从句，修饰the tree
但是这句的主句其实是
patches of green had begun to appear in the blackened soil

期待看到有用的回答！