题目内容：

With seasonal temperatures breaking records almost every year, tree species 1. (expect) to adapt to the changes by slowly shifting their population centers northward. But 2. recent survey shows the trend toward westward movement is even 3. (strong) than expected — in some cases, species have shifted their ranges to west by as much as 73 percent.

The survey shows that roughly three-quarters of the 86 tree species 4. (survey) have shifted their population centers westward 5. 1980.

Obviously, trees aren’t uprooting 6. (them) and moving elsewhere. The survey provides insight into general population trends as young trees continue to take root in westward land, while some of the older 7. (plant) of the species in the eastern areas are slowly dying out. In this way, the center of a species, range can gradually shift over time.

Though scientists aren’t sure 8. is causing this change, the publishers of the study think that it’s connected to rainfall. Rainfall totals across the United States have changed,9. (cause) areas such as the Southeast to experience significantly less rain annually, while the Great Plains is getting far more than its historical average. For this reason, most trees are moving toward the 10. (increasing) watery plains.