Plateau pika (Ochotona curzoniae, hereafter pika) is considered to exert a profound impact on vegetation species diversity of alpine grasslands. Great efforts have been made at mound or quadrat scales; nevertheless, there is still controversy about the effect of pika. It is vital to monitor vegetation species composition in natural heterogeneous ecosystems at a large scale to accurately evaluate the real role of pika. In this study, we performed field survey at 55 alpine grassland sites across the Shule River Basin using combined methods of aerial photographing using an unmanned aerial vehicle (UAV) and traditional ground measurement. Based on our UAV operation system, Fragmentation Monitoring and Analysis with aerial Photography (FragMAP), aerial images were acquired. Plot-scale vegetation species were visually identified, and total pika burrow exits were automatically retrieved using the self-developed image processing software. We found that there were significant linear relationships between the vegetation species diversity indexes obtained by these two methods. Additionally, the total number of identified species by the UAV method was 71, which was higher than the Quadrat method recognition, with the quantity of 63. Our results indicate that the UAV was suitable for long-term repeated monitoring vegetation species composition of multiple alpine grasslands at plot scale. With the merits of UAV, it confirmed that pika's disturbance belonged to the medium level, with the density ranging from 30.17 to 65.53 ha(-1). Under this density level, pika had a positive effect on vegetation species diversity, particularly for the species richness of sedge and forb. These findings conclude that the UAV was an efficient and economic tool for species monitoring to reveal the role of pika in the alpine grasslands.

Soil organic carbon (SOC) and total nitrogen (TN) stocks are usually calculated with samples collected using core samplers. Although the calculation considers the effects of gravel in soil samples, other coarse fragments such as stones or boulders in soil may not be collected due to the restricted diameter of core samplers. This would cause an incorrect estimation of soil bulk density and ultimately SOC and TN stocks. In this study, we compared the relative volume of coarse fragment and bulk density of fine earth determined by large size soil sampler with three core samplers. We also investigated the uncertainties in estimation of SOC and TN stocks caused by this soil sampler procedure in three typical alpine grasslands on the northeast edge of the Qinghai-Tibetan Plateau ( QTP), China. Results show that (1) the relative volume and size of coarse fragment collected by large size sampler were significantly ( p <0.05) higher and larger than those of core samplers, while bulk density of fine earth, SOC and TN stocks show opposite patterns in all grassland types; (2) SOC and TN stocks determined by core samplers were 17%- 45% and 18%- 46% higher than larger size sampler for three typical alpine grasslands; and (3) bulk density of fine earth, SOC and TN stocks exponentially decreased with the increasing of coarse fragment content. We concluded that core sampler methods significantly underestimated the volume occupied by coarse fragment but overestimated SOC and TN stocks. Thus, corrections should be made to the results from core samplers using large size samplers on regions with gravel and stone-rich soils in future studies.