It refers solely to trees, and Vallombrosa is the place "where th' Etrurian shades / high overarch't imbow'r."
这个比喻指的就是树木,地点是瓦隆布罗萨,“在瓦隆布罗萨,厄特鲁利亚的光辉逐渐暗淡“
弥尔顿课程节选 : 耶鲁公开课
2
This is the initial part of the simile that describes the leaves: "thick as Autumnal Leaves that strow the Brooks / in Vallombrosa, where th' Etrurian shades / high overarch't imbow'r."