Interactive Effects of Climate Change and Grazing on Ecosystem Productivity and Greenhouse Gas Balance at Multiple Scales from Landscape to Global
Type of DegreePhD Dissertation
Forestry and Wildlife Science
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The Anthropocene is facing several critical challenges in the 21st century due to increasing impacts of human activities such as deforestation, urbanization, agriculture and fossil fuel burning. Human activities alter surface aerodynamic, thermodynamic, radiative, hydrological and vegetative properties of the terrestrial biosphere with substantial impact on biogeochemical cycles and the global climate system. It is essential to understand the processes and dynamics that affect land transformation and how they mediate complex ecosystem processes in order to examine the impact of human activities on the terrestrial biosphere and their feedback to the global climate system. This study focuses on the impact of human land use (grazing and livestock production) and climate on terrestrial ecosystems and feedback of livestock production to the climate system. Although numerous studies have examined the role of grazing and climate on terrestrial ecosystems, few, if any have simultaneously investigated the impact of grazing on carbon storage and their contribution to greenhouse gas balance using an integrated modeling approach. In this study, I used a process-based (The Dynamic Land Ecosystem Model) and an empirical model to investigate the impacts of grazing and climate on terrestrial carbon budget at site, regional and global scales. In addition, I examined the consequences of increasing livestock production on global greenhouse gas balance. Results show that livestock grazing has a significant impact on aboveground NPP (-12%) and heterotrophic respiration (-12.4%), but did not alter net ecosystem productivity and evapotranspiration across different sites in the Northern Hemisphere. In Mongolia, historical grazing reduced aboveground NPP by up to 15.4%, with larger reduction in dry sites compared to wet sites. Regional simulation across the Mongolian Plateau show that 83% of the grassland area has experienced decline in aboveground NPP, largely due to climate change (-61.4%) and grazing (-23.2%) since the 1990s. In addition, climate change increased livestock mortality by 28% due to combined drought and extreme winter condition. At a global level, livestock grazing and climate change reduces soil organic carbon (SOC) at the rate of 13.4 PgC/yr and 2.0 PgC/yr, respectively (p < 0.05). Likewise, grazing reduced net primary productivity (NPP) by 4.3 PgC/yr, while climate change increases NPP by 1.6 PgC/yr. The results also indicate that livestock production plays an important role in regulating the concentration of greenhouse gases globally. Methane (CH4) emissions from the global ruminant livestock sector accounted for 47-54% of all non-CO2 GHG agricultural emissions. Since the 1890s, CH4 emission from the ruminant livestock sector has increased by 2.06 Gt CO2-eq (332%), with the largest contribution from dryland ecosystems (347%). Global drylands has 36% higher emission intensity (CH4 emissions/km2) compared to non-drylands in the recent decades. In addition, developing countries experienced the largest percentage increase in CH4 emissions from 51.7% in the 1890s to 72.5% in the 2010s. Nitrous oxide (N2O) emissions from the global grasslands increased significantly from 1.41 Tg N2O-N/yr in 1961 to 1.89 Tg N2O-N/yr in 2014. Managed pastures dominated N2O emissions contributing to up to 68% of the total grassland emissions. Among different sources, manure left on pastures contributed to ~40% of the total emissions, followed by manure applied to pastures (20%) and fertilizer applied to pastures (10%). Regionally, Asia dominated N2O emission contributing to 31% of the total emission, followed by North America (25%), Europe (20%) and Africa (13%). The results also indicate a positive cumulative impact of three greenhouse gases (CO2, CH4 and N2O) in the global grasslands. Grassland ecosystems were a net source of greenhouse gas of about 434 Tg CO2-eq/yr during 1960-2014. CH4 and N2O emissions contributed to 60% and 40% of the total GHG sources, respectively. Across different continents, southern Asia was the major source of GHG, followed by North America and Europe. In particular, managed pastures in southern Asia, Europe and North America dominated N2O emissions, which largely controlled the net GHG balance in these regions. Overall, the results of this study demonstrate that increasing livestock production and climate change (increasing heat waves, higher temperatures) have profound impacts on climate and the environment. In order to mitigate CH4 emissions from the livestock sector, direct and indirect approaches that relies on animal (improving feed quality, feed additives, animal productivity) and land (grazing optimization, transition to extensive system) based mitigation approaches and policy (imposing tax on conventional ranching) efforts can promote sustainable intensification. Likewise, improvement in genetic production potential of livestock and their feed composition can reduce nitrogen losses via urine and feces and application of manure to croplands and pastures based on specific plant nitrogen demand could potentially reduce N2O emissions.