2016年12月英语六级（第3套 · 含听力）选词填空

The tree people in the Lord of the Rings — the Ents — can get around by walking. But for real trees, it's harder to uproot. Because they're literally rooted into the ground, they are unable to leave and go __26__.
When a tree first starts growing in a certain area, it's likely that the __27__ envelope — the temperature, humidity, rainfall patterns and so on — suits it. Otherwise, it would be unable to grow from a seedling. But as it __28__, these conditions may change and the area around it may no longer be suitable for its __29__.
When that happens, many trees like walnuts, oaks and pines, rely __30__ on so-called "scatter hoarders," such as birds, to move their seeds to new localities. Many birds like to store food for the winter, which they __31__ retrieve.
When the birds forget to retrieve their food — and they do sometimes — a seedling has a chance to grow. The bird Clark's nutcracker, for example, hides up to 100,000 seeds per year, up to 30 kilometers away from the seed source, and has a very close symbiotic (共生的) relationship with several pine species, most __32__ the white bark pine.
As trees outgrow their ideal __33__ in the face of climate change, these flying ecosystem engineers could be a big help in __34__ trees. It's a solution for us — getting birds to do the work is cheap and effective — and it could give __35__ oaks and pines the option to truly "make like a tree and leave."