Effect of climate variables on the modeling of vegetation net primary productivity in karst areas

Green plants play an important role in matter and energy transformations and are key messengers in the carbon and energy cycle. Vegetation net primary productivity (NPP) reflects the capability of plants to convert solar energy into photosynthate (fixed carbon). It can be used as a direct indicator of current status and future trend of ecological processes affected by human activities and climate variability, especially in fragile ecological environments such as karst areas where rock-desertification increasingly occurs. Using remote sensing and geographic information system technologies, research on spatio-temporal variations of NPP and its response to changes in climate variables (temperature, rainfall and solar radiation) in Guizhou province of China was conducted. Karst areas within the study area were delineated to explore the relationship between NPP and climate variables. The NPP model developed by Wang et al. (2010) was used. This model is a modification of the light use efficiency (LUE) model of Running et al. (2000), validated for this region in China. As input to the model, three climate scenarios were designed - one representing current conditions and two for hypothetical future conditions. Both future scenarios have a 2˚C increase in temperature and 6% increase in rainfall, applied spatially across the region using a constant scaling factor on the current condition scenario. The first future scenario has unchanged solar radiation; and the second has a 10% increase in solar radiation for investigating the effect of solar radiation on NPP. The model was run on a monthly time step for one year. Under the current condition scenario, the spatio- temporal patterns of monthly NPP were analysed using MODIS imagery, with the climate variables as model inputs. The relationships between NPP and temperature, and NPP and rainfall, were quantified using single and partial correlation. The hypothetical future condition scenarios were then used to explore the response of NPP to changes in these variables. This paper reports on the results of this modelling under all scenarios, with particular interest in the relationship between solar radiation and karst areas. Model results show that, under the current condition scenario, the mean annual NPP of terrestrial vegetation is 421gCm -2 with the NPP in karst areas being 11.9% lower than in non-karst areas. Maximum and minimum NPP values occur in July and January, respectively. The results show strong correlations between NPP and climatic variables: (1) Temperature is a key factor which significantly limits vegetation growth in the northwest. The correlation between NPP and temperature decreases from north to south, and is stronger in karst (than non-karst) areas. (2) The correlation between NPP and rainfall is most significant in the southeast and west in comparison with other areas, and is lower in karst (than non-karst) areas. Under the first future scenario, the total annual NPP decreases from 74×10 6 tC to 73×10 6 tC; whereas under the second future scenario, annual NPP increases from 74×10 6 tC to 79×10 6 tC. The results from both future scenarios indicate that the largest variation in NPP occurs in karst areas with severe rock-desertification, indicating solar radiation is a dominant factor affecting NPP in karst areas. There are obvious regional differences on spatio-temporal patterns of NPP between karst areas and non-karst areas. The analysis of the interactions among three climate variables - temperature, rainfall and solar radiation has contributed to our understanding of their contributions to NPP, especially in karst areas.