## 5 Discussion
### Impacts of LUCC on GPP  
Wuhan is an excellent example of a human-natural system, where forests and wetland have been well preserved in recent decades as urbanization rapidly occurred. The results showed that in addition to the conversion from cropland to bare land and water bodies, the greatest loss of GPP was caused by the conversion from cropland to built-up areas that occurred in the suburbs and the earubs due to urban expansion. This result was in agreement with findings reported by several studies that the substantial loss of primary production was due to the conversion from cropland to urban areas in the suburbs of cities in China [6; 10]. Although built-up areas encroached cropland at a fast pace, the increasing relative fraction of forests that were more photosynthetically productive offset a certain amount of GPP loss. A study to analyze the impacts of LUCC on terrestrial carbon sinks and emissions in Guangzhou, China also reported that the carbon sink gained through the changes from the low carbon sink (e.g., grassland) to the high carbon sink (e.g., forests) [41]. Furthermore, they found thatthe proportion of urban land influenced the changes in carbon sinks, which is similar to our findings that the change in vegetation composition affected the interannual variation of GPP.  
The conversion from cropland to water bodies led to a great loss of GPP that was even larger than the loss caused by the conversion from cropland to built-up areas. However, a certain amount of transition between cropland and water bodies may be due to the change from farmland to paddy fields, as the rotation between winter wheat/oilseed rape and paddy rice is common in the study area. Nonetheless, driven by economic benefits, the conversion from cropland to ponds for fisheries and aquatic plants led to the loss of GPP. Land cover maps were mostly derived from images of June, July, or August, but the date of the image used for classification varied from year to year. Hence, the transition between cropland and bare land may be either caused by the crop rotation or urban expansion.  
The map of the GPP_ref change rate illustrates the spatiotemporal dynamics of GPP over 14 years. The significant decreases in GPP were found around the metropolitan area to be due to urban expansion, and around several major lakes because of the preferences of building apartments and houses near lakes and rivers. The greenbelt along the highway contributed to the interesting increasing trend in GPP. In the southeast of the study area, cropland was replaced by woods for economic reasons, leading to the increase in GPP. In the northwest of the study area, cropland was transformed to ponds, leading to the sharp decrease in GPP. The detailed map of the change rate demonstrated the ecological consequence of the characteristic land cover changes in a city.  
China is currently promoting the concept of an urban ecosystem service [44]. In this study, the quantitative analysis of vegetation GPP changes over 14 years caused by LUCC may provide guidance to adjust land-use/cover structures in Wuhan in order to ensure minimized carbon loss in the future. Carbon consequences should be considered before land management policies and activities are put forth. Integrating more photosynthetically productive species in the limited area may largely mitigate the carbon losses that are associated with urbanization. Additionally, the method can be applied to other major cities, and combined with economic methods for policy-makers to establish carbon trading or offsetting rates policies [45].