Populus Euphratica(胡杨) is the only tree species    1     (find)in the desert of North China. In Ejina(额济纳)in the west of Inner Mongolia, there's a Populus Euphratica. As the     2     (old) tree in this Populus Euphratica forest, it    3     (guard) this place for more than 800 years. The locals call it the "sacred tree". Dinosaurs     4     (die) out about 65 million years ago. Then a new species appeared along the ancient Mediterranean Sea. That's the Populus Euphratica. Seasonally flooded river beds(河床) in the Gobi desert are perfect for the Populus Euphratica.     5     there is water, the Populus Euphratica grows.

Populus Euphratica loves water,     6     it also highly resistant to drought. As long as the groundwater is at least 4 meters below, Populus Euphratica can live    7     ( comfortable). If the groundwater level drops below 9 meters, it will die.

Because the course of desert rivers changes frequently, Populus Euphratica has left its mark across the desert. It acts as a good windbreak, prevents sand erosion, and is salt resistant.     8     adult Populus Euphratica can discharge tens of kilograms of salt and alkali(碱) every year. When the wind is blocked, the sand is fixed, and the soil is improved. So the ecological environment of Ejina is protected.

To the south of the Populus Euphratica forest in Ejina, lies Badain Jaran, the third largest desert in China.     9     this 260 square kilometer Populus Euphratica forest, the Badain Jaran Desert, already 49,200 square kilometers in size, would spread northward,     10     (squeeze) the living space of humans. These Populus Euphratica trees form a golden natural barrier, guarding the ecology of Ejina year after year, for another 800 years.

2 . The number of trees larger than two feet across has declined by 50 percent on more than 46, 000 square miles of California forests, the new study finds. No area was spared or unaffected, from the foggy northern coast to the Sierra Nevada Mountains to the San Gabriels above Los Angeles. In the Sierra high country, the number of big trees has fallen by more than 55 percent; in parts of southern California the decline was nearly 75 percent.

3 . The connection between people and plants has long been the subject of scientific research. Recent studies have found positive effects. A study conducted in Youngstown，Ohio，for example, discovered that greener areas of the city experienced less crime. In another, employees were shown to be 15% more productive when their workplaces were decorated with houseplants.