Background  The spatio-temporal variation of biomass in the alpine grassland is very large due to the complex terrain. However, such studies are rare till now. The aims of this study is to quantify the variation of aboveground biomass in alpine grassland with slope aspect, slope position, and time in growing season, to provide a scientific basis for its rational protection, utilization, and ecosystem service evaluation.

Methods  This study was carried out in the representative region of eastern Qilian Mountains and at the semi-arid Zhuaxixiulong township of Tianzhu county of Gansu province. Four plots of 1 m×1 m were set up at the down-, middle-and up-slope on a shady slope and a sunny slope without grazing and human disturbance. In addition, as a contrast experiment, 4 plots of 1 m×1 m were set up at a level ground where grazing and human disturbance existed. Within the growing season of 2016 (June-Sept.), the aboveground biomass were investigated monthly.

Results  The average aboveground biomass in growing season on the entire shady slope (292.4 g/m²) was slightly higher than that on the sunny slope (282.6 g/m²), and both were far higher than that on the level ground (163.5 g/m²). The aboveground biomass on both sunny and shady slope decreased with rising slope position on the whole, with the growing season averages of 365.76, 268.61 and 213.30 g/m² for the down-, middle-and up-slope on sunny slope, and 323.08, 278.61 and 275.42 g/m² on shady slope, respectively. The aboveground biomass first increased rapidly in the early growing season, and then increased with a declining and nonlinear rate, which was in accordance with the logarithmic function. The concrete growth rate varied with slope aspect, slope position and phenological time.

Conclusions  In general, affected by the differences in water and heat conditions and soil thickness induced by slope aspect and slope position, the aboveground biomass of alpine grassland shows an obvious and complex spatio-temporal variation.