Key Words: net primary production, biomass, carbon cycle, land use change
Abstract: We investigated the effects of recent land use change (1988-1996) in the Zhu Jiang Delta region, People's Republic of China, on two components of the terrestrial carbon cycle: net primary production (NPP) and ecosystem carbon storage. The analysis employed a mechanistic model of NPP in combination with satellite-derived and reported environmental and ecological data. Our analysis shows that land use change in the 9-year study period significantly altered the terrestrial ecosystem component of the regional carbon cycle. The annual amount of atmospheric carbon assimilated into phytomass through NPP was reduced by approximately 155×104 t C (-7.5%). More than half of this reduction is attributed to the loss of cultivated land. Vegetation removal and soil disturbance by the expansion of urban areas reduced the amount of carbon stored in terrestrial ecosystems by 792×104 t C, of which approximately 72% came from phytomass. The carbon released by land use change, however, is only 5.3% of the carbon emissions from fossil fuel combustion for the same area and time, suggesting relatively small significance for the overall carbon budget of the ZJDR.
1. Introduction
Terrestrial ecosystems mediate the exchange of carbon between the atmosphere and the terrestrial biosphere through photosynthesis, respiration, decomposition, and combustion. They also comprise a major carbon reservoir by storing carbon in biomass, decaying organic matter, and soils. Land use and land use change by humans can alter ecosystem performance in these roles, which may alter regional and global carbon budgets.
Recent satellite-based studies indicate that land use change in the People's Republic of China, as in many parts of the developing world, is occurring at unprecedented rates. The land use changes result from a combination of driving forces, including population growth, migration, economic development, and government policies (e.g., Seto, 2000; Kikuchi et al., 1997). In the Zhu Jiang Delta region (ZJDR) of Guangdong Province during the period from 1988 to 1996, urban areas increased by over 300%, while natural and agricultural land areas declined by approximately 6% and 10%, respectively (Fig. 1). We investigated the effects of these changes in the ZJDR on two components of the terrestrial carbon cycle: net primary production (NPP) and ecosystem carbon storage. Our analysis employed a mechanistic model of NPP in combination with satellite-derived and reported environmental and ecological data.
Figure 1. The spatial extent of major land use types in Zhu Jiang Delta region for years 1988 and 1996 as determined by the satellite analysis of Seto et al. (2000). Labels refer to natural vegetation (NAT), agriculture (AGR), urban (URB), and water (WTR) land use classes.
2. Background And Methods
2.1 Description of the Study Area
The study area encompasses the Zhu Jiang River Delta region (ZJDR) in Guangdong Province, People's Republic of China. The boundaries correspond to the Landsat Thematic Mapper scene used in the land use change analysis (Seto et al., 2000). The area covers 2.7 x 106 ha, or approximately 15% of the total area of Guangdong Province. The natural, potential vegetation of the region is predominantly evergreen broadleaved forest (Zhao, 1994).
Over much of the study area, the observed vegetation deviates substantially from the climatic potential. The differences reflect current and historical patterns of intensive land use by humans (Li and Liu, 1994). We focussed our analysis on two of the three non-water land use classes in the Seto et al. (2000) study: natural vegetation (NAT) and agriculture (AGR). We assumed that NPP in the urban (URB) land use class was negligible.
2.2 Modeling NPP with Satellite Data
We used a simple light-use efficiency (LUE) model to estimate NPP (e.g., Waring and Running, 1998; Sheriff et al., 1995). The basic form of the LUE model can be expressed as:
NPP = f·e·FPAR·S. (1)
The S term is the amount of PAR incident above vegetation canopy (MJ m-2). FPAR is a dimensionless term that accounts for the efficiency with which PAR is absorbed by the green, photosynthetic elements of the canopy. The product of S and FPAR determines the amount of vegetation-absorbed PAR (APAR, MJ m-2). The e parameter equals the gross or net amount of plant dry matter produced per unit of APAR (g MJ-1). The f term is a dimensionless factor that accounts for any reduction in e resulting from environmental constraints (e.g., Prince, 1991).
