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【C1】______.

When a tree first starts growing in a certain area, it’s likely that the【C2】___envelope—:the temperature, humidity, rainfall patterns and so on—suits it. Otherwise, it would be unable to grow from a seedling. But as it【C3】, these conditions may change and the area around it may no longer be suitable for its【C4】___.

When that happens, many trees like walnuts, oaks and pines, rely【C5】___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【C6】___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【C7】______the whitebark pine.

As trees outgrow their ideal【C8】___in the face of climate change, these flying ecosystem engineers could be a big help in【C9】trees. It’s a solution for us—getting birds to do the work is cheap and effective—and it could give【C10】___oaks and pines the option to truly “make like a tree and leave. “

A)ages I)legacy

B)breathing J)notably

C)climatic K)offspring

D)elsewhere L)replanting

E)exclusively M)subsequently

F)forever N)vulnerable

G)fruitful O)withdraws

H)habitats

【C2】