natural grade line

例句：

1. 1.The natural grade line is a crucial factor in determining the optimal growth conditions for the plant.
   自然等级线是确定植物最优生长条件的关键因素。
2. 2. The research team carefully studied the natural grade line to understand the ecological impact of the construction project.
   研究团队仔细研究了自然等级线，以了解建设项目对生态系统的影响。
3. 3. The natural grade line plays an important role in defining the boundaries between different ecosystems.
   自然等级线在界定不同生态系统的边界中起着重要作用。