Terrestrial Carbon Dynamics of Southern United States in Response to Changes in Climate, Atmosphere, and Land-Use/Land Cover from 1895 to 2005
Type of DegreeDissertation
Forestry and Wildlife Sciences
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Historical human activities and environmental changes have strongly modified the global carbon cycle, which can lead to both an energy shortage and environmental problems such as global warming due to elevated atmospheric CO2. North American terrestrial ecosystems, especially in the Southern United States (SUS) were suggested to be important carbon sinks. In this dissertation a dynamic land ecosystem model (DLEM) was applied to assess the carbon storage of SUS terrestrial ecosystems, and to study the ecosystems’ responses to historical climate change, atmospheric change (i.e. elevated CO2, elevated nitrogen deposition, and elevated tropospheric ozone stress), and land-use change (cropland conversion, urbanization, and reforestation). First, a series of landscape-level case studies were conducted on three different types of Southern terrestrial ecosystems: the natural ecosystem of Great Smoky Mountain National Park (GRSM), the semi-natural ecosystem along a rural-urban gradient in west Georgia, and urban ecosystems in SUS. Results from these studies suggested that the undisturbed Southern forest ecosystem has potential to store large amounts of carbon (as high as 15.9 kg m-2 in GRSM) which is very sensitive to disturbances, especially changes in land use. Historical cropland conversion has resulted in significant carbon emissions in SUS, while the vast cropland abandonment since the mid-20th century has made many regions in the SUS net carbon sinks. The impacts of urbanization on SUS carbon balance became more and more important since the 1970s. A high resolution spatial database throughout thirteen Southern states was developed as a model input to study the regional carbon balance of SUS in response to multiple stresses in the past 110 years. The model output suggests that the total terrestrial ecosystem carbon (TOTEC) storage of the SUS is about 20.26 P g C (1 P = 10 15), 55% of which is stored in soil, 39% in plant biomass, and about 7% in litter pools. Forests account for 84% of the ecosystem carbon storage in SUS. Our model estimation, which is comparable to the results of other studies, indicated that since 1950, the terrestrial ecosystem of SUS was a carbon sink of 46.4 T g C / year. Before 1950, however, the region had acted as a net carbon source of 1.56 P g C since 1895. Historical land-use change, elevated CO2 and elevated atmospheric nitrogen deposition were among the most important factors controlling the Southern carbon balance. Temporal patterns were generally controlled by the impacts of historical land-use change, while the long-term CO2 and nitrogen fertilization effects due to atmospheric change enhanced the carbon sequestration capacity of Southern ecosystems. All the environmental factors together resulted in a net carbon sink of about 0.9 P g C in SUS from 1895 to 2005.