Quantitative evaluation of grazing effect on nomadically grazed grassland ecosystems by using time-lapse cameras

Abstract

Grazing is a key factor in grassland disturbance that affects plant biomass, soil properties, and carbon exchange in grasslands. The objective of this study was to quantitatively evaluate the effects of grazing on grassland ecosystems in Mongolia. To this end, we developed a new method for determining the actual and quantitative livestock effects. Fenced (ungrazed) plots were established in two nomadically grazed grasslands in Mongolia, Bayan-Unjuul (BU) and Baganuur (BN), and time-lapse cameras were installed at the plots for photographing outside the fences (grazed area) every 10 min. Using images collected for 3 years (2016 − 2018), we counted the number of livestock animals by species and calculated grazing intensity (GI). In addition, we investigated plant biomass; CO2 fluxes including gross primary production and ecosystem respiration; and soil properties during mid-August at the BU and BN sites. Our new method enabled us to quantitatively assess the seasonal and interannual variations and spatial differences in GI. Livestock used the BU site mainly in spring but the BN site in summer. Using the seasonal GIs, we performed linear regressions to examine grazing effects on plant biomass, CO2 exchanges, and soil properties. Our results indicated that, among the livestock species we monitored, grazing by goats most strongly affects the grassland function and structures. GI negatively correlated with aboveground biomass (AGB), CO2 fluxes, soil organic carbon (SOC), and total nitrogen (TN), suggesting that these parameters decreased with increasing GI. The effect size of grazing on AGB was negative for all years and sites, indicating that grazing consistently decreased AGB, whereas CO2 fluxes, SOC, and TN were positive when the GI was low. By using our new method, we obtained the certain values of GI at which the grazing effect shifted from positive to negative.