The desert was once a green这句,为什么and后还要加个that?

that 指代前面“they were cut down”,
that指代前面的词汇、短语、句子,以避免重复.
这边that指代前面的介词短语作为定语从句的主语.
that 引导定语从句时 叫 关系代词 代替先行词 引导定语从句 在从句中起代词作用.
再问： 还是不大清楚啊，能否再深入些？去掉那个that也挺通啊
再答： 去掉那个that就变成了They were cut down and resulted..., 如果是这种用法因为and的原因就可以变成两句话，They were cut down 。They were resulted in.... 后面一句话根本就不通，语法已经错误了（resulted 没有被动态），如果要连起来不能用and ，要换句式换成Their cutting down resulted in..... 所以这句话是为了精简语句而写的定语从句，在这句话里面that不能省略。