"Hemeroby" is a method to estimate the man-made impacts of vegetation. It compares present vegetation with a reference vegetation system, which can be (1) pristine vegetation or (2) present potential natural vegetation (PNV). The present plant/forest communities are unanimously the result of long-term interactions between vegetation and site factors including man-made impacts. Hence it is nearly impossible to find pristine vegetation on the earth, especially in the developed regions with a long human history. In the "Cropland Conversion to Forest/Grassland Project", we may use PNV as a reference vegetation system and use the method "hemeroby" to estimate how far the rehabilitated vegetation to its PNV is? We may also use PNV as a guide line for the selection of vegetation types and plant species in this Project.

我国西部地区长期以来对自然资源的不合理开发利用，过伐，过垦，过牧，过采（集）等，使本来就脆弱的西部生态环境进一步遭到了破坏，因此，在西部地区进行退耕还林还草，恢复和重建当地生态环境，对西部的开发与持续发展诚为重要。如何科学地退耕还林还草以及如何评价退耕还林还草地上重建的植被与其“潜在的自然植被＇的距离，是本文试图探讨的问题。