Question

这个英语长句子的结构！高手来

Immediately adjacent to the timberline, the tundra consists of a fairly complete cover of low-lying shrubs, herbs, and grasses, while higher up the number and diversity of species decrease until there is much bare ground with occasional mosses and lichens and some prostrate cushion plants.

译文：
由于紧挨着林木线，苔原上比较完整地覆盖着矮灌木、药材和草地。随着海拔的增加物种的数量和多样性会逐渐减少，直到出现大片的空地，这些空地仅仅伴有零星的苔藓和地衣或者一些伏地植物。
好，我要问得是 while higher up the number and diversity of ···中的higher up 是什么词性类型 ，总觉得 是不是没谓语？

Answer 1

亲你可以这么拆这个句子
"Immediately adjacent to the timberline, the tundra consists of a fairly complete cover of low-lying shrubs, herbs, and grasses."这算一半，有自己的主谓宾语
然后后面的
"higher up the number and diversity of species decrease until there is much bare ground with occasional mosses and lichens and some prostrate cushion plants."是另一半，读着不顺可以在higher up后面加一个逗号，意思一样，所以higher up在这里应该是状语，the number & diversity of species主语，decrease谓语，decrease后面那堆"until..."都是状语，甚至可以在decrease后面加个逗号，把"until..."全部移到"higher up"之后，整个句子意思不变~

Answer 2

the number and diversity of species 主语，decrease谓语，higher up是状语，