Modeling the effects of recent land use change On the carbon cycle in the zhu jiang delta region Of southern china

Our medium estimate for carbon lost from phytomass is 566×104 tons; the majority (82%) of this was from natural vegetation (Table 3). Phytomass accounted for about 72% of the total of 792×104 tons of carbon released (medium estimate).

4. Discussion And Conclusion
Land use change in the Zhu Jiang Delta region between 1988 and 1996 affected the regional carbon cycle by reducing both the annual rate of NPP and the size of the terrestrial carbon reservoir. The dominant mode of land use change underlying this effect involved the conversion of vegetated land areas (natural and cultivated) to urban uses (Fig. 1). In some types of land use change where vegetation is removed from the landscape (e.g., deforestation, swidden agriculture), NPP can rebound through secondary growth. Vegetation-to-urban land use conversion, however, causes a sustained reduction in the capacity the landscape to assimilate atmospheric CO2. Thus the observed 7.5% decline in annual NPP is a measure of the reduction in the total photosynthetic capacity and carbon sequestration potential of terrestrial ecosystems in ZJDR.

Vegetation removal and soil disturbance in the expansion of urban areas reduced the amount of carbon stored in existing phytomass and soils by an estimated 792×104 t C. In the absence of a compensatory increase in NPP and biomass through secondary growth, this result represents a sustained reduction in the potential size of the terrestrial carbon pool in the ZJDR. The future course of the socio-economic driving forces that influence the rates and modes of land use change in the region (Seto, 2000), in combination with potential climate change, will determine whether these effects become exacerbated or diminished in the coming decades.



Figure 2. Estimated total amount of carbon released in the ZJDR study area as a result of fossil fuel energy use (1996) and land cover change (annual average, 1988-1996).

Our analysis shows that recent land use change caused significant modifications to the terrestrial ecosystem component of the regional carbon budget in the ZJDR. It is instructive to compare our estimates of carbon released from ZJDR ecosystems to carbon emissions from fossil fuel combustion for the same area and time (Fig. 2). We used energy consumption data to estimate the carbon emissions from fossil fuels for Guangdong Province in 1996 (SSB, 1998). We adjusted this figure to our study area based on the fraction of the 1996 Guangdong gross domestic product (GDP) generated within the area (K. Seto, personal communication). The results indicate that fossil fuel combustion in our ZJDR study area in 1996 emitted about 1.7×107 tons of carbon, which is two orders of magnitude greater than the average annual amount of carbon released as a result of land use change in the ZJDR (8.8×105 t C). Despite significant effects for the terrestrial ecosystems, the role of land use change in the overall carbon budget of the ZJDR is small (5.3%) relative to fossil fuel-based energy consumption.

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