Synergistic effects of climate change and grazing on net primary production in Mongolia: Integrating livestock population dynamics into an ecosystem model
The Mongolian Steppe (41.6 – 52.2oN and 87.6 – 119.9oE) is one of the largest remaining grassland ecosystems encompassing a region of considerable ecological importance. Mongolia currently faces two major challenges: First temperature has increased by 2.14oC since the 1940s compared to the global average of 0.55oC. Second, 30- 40% of the people in Mongolia are nomadic herders. So, any changes in the Steppe ecosystems are going to have a substantial impact on the country as a whole. While there is general consensus among the scientific community about the observed grassland degradation in Mongolia, it is still not well known on how grassland degradation is related to climate or overgrazing. In addition, current ecosystem models do not explicitly consider the effect of livestock and are calibrated to some benchmarks with grazing. In this study, we developed a livestock population dynamics model which has been coupled with an ecosystem model (the Dynamic Land Ecosystem Model) to capture the interactions among water, carbon, and nitrogen cycles and to understand the interactions, responses, and feedbacks among climate, grasslands and livestock.
Our results indicate that moisture limitation exert an important control on grassland productivity in Mongolia. Grazing (without grazer population dynamics) resulted in an overall reduction in grassland productivity ranging from 2% to 15.4%, with the largest reduction occurring at dry sites. Across 6 sites, grazing had a detrimental effect on grassland productivity at dry compared to wet sites indicating that the associated feedbacks between moisture limitation and grazing resulted in a larger reduction in grassland productivity at dry sites compared to wet sites where moisture is not as limiting. In addition, we found compensatory growth responses to grazing intensity among years where the ability of plants to partially compensate for grazing damage was higher in sites receiving high precipitation. However, explicit representation of livestock population dynamics in an ecosystem model is key to understanding the potential drivers of livestock population dynamics and the associated feedbacks among climate, grasslands, and livestock.