Effect of the Qinghai-Tibetan Highway on the β diversity of grassland plant communities in the northern region of the Qinghai-Tibetan Plateau

The Qinghai-Tibetan Plateau forms a key genetic resource for plant flora in China, while also providing an important base for localized animal production. From an ecological perspective, its current degraded state has caused much concern amongst researchers and many factors contribute to this, particularly the Qinghai-Tibetan Highway. In this research, an altitudinal transect was established along the Qinghai-Tibetan Highway from the Xidatan (94°10.016′E,35°43.063′N) to the southern Tanggulashan pass (33°07.120′E,91°52.670′N), and 75 sampling zones in 32 locations were identified at vertical intervals of 100 m between 4300 and 4930 m above sea level in the permafrost region of the Qinghai-Tibetan Plateau. The β diversity of the plant communities was measured by using the Cody index and the Whittacker index . β diversity is often used to analyze the diversity between communities; it reflects species replacement along an environmental gradient and is particularly useful in areas where an obvious ecological gradient exists. The results of this study show that the Cody and Whittacker indices were different in sampling areas with natural plant communities or plant communities that had been recently established after road construction. The Whittacker index decreased in proportion to the increasing sampled area measured; however it remained steady when the sample areas were greater than 8 m~2. Conversely, the Cody index increased with an increasing sample area, and remained steady when the sample area was 16 m~2. Thus, sampling areas greater or less than 16 m~2 provided an ideal scale for β diversity measurement. The Cody index and Whittacker index mostly displayed opposite trends when compared between natural plant communities and recently restored plant communities. The Whittacker index of natural plant communities was determined to be smaller than that of recently restored plant communities. A similar trend was identified for the Cody index of natural plant communities in both lowly elevated regions(4 320~4 420 m)and highly elevated regions (4 820~4 920 m). However, the Cody index of natural plant communities was far greater than that of restored plant communities in regions of other elevations. ,Previous studies showed that the Cody index should be able to express the relationship amongst both natural and restored communities at different elevations. While the greatest differences in the Cody index were found between plant communities in areas sampled at 4620~4720 m, the differences in plant communities and their respective Cody index decreased at lower elevations. Such findings are further explored in this paper, and a discussion presented of the merits and shortcomings of the Cody and Whittacker indices, based on an analysis of the study results.