Spatiotemporal changes in net primary productivity before and after the development of unused land in the hilly areas of Hebei, China

Net primary productivity (NPP) plays an important role in the carbon cycle of an ecosystem. To explore the impact of unused land development on NPP, this study adopted an improved Carnegie Ames Stanford Approach (CASA) model to analyze the changes in NPP before and after the development of unused land in Tang County, Hebei Province, in 2000, 2007, and 2018. The results showed that, due to the changes in land use types from unused land, forestland, arable land with high NPP values to urban and rural residential land, traffic land with low NPP values, and the changes in precipitation and temperature, the NPP in the study area showed an overall trend of decreasing first and then rising from 2000 to 2018. Before the development of unused land in 2000, the total NPP was 38.45×10(10) g C. After the development in 2007 and 2018, the total NPP was 36.44×10(10) g C and 41.05×10(10) g C, respectively. The NPP of each land type in 2018 was arable land (1046.18 g C m(-2)) > forestland (464.42 g C m(-2)) > unused land (356.34 g C m(-2)) > grassland (343.77 g C m(-2)) > waters (182.56 g C m(-2)) > urban and rural settlements (120.86 g C m(-2)) > traffic land (120.70 g C m(-2)). The distribution of NPP was generally high in the north and low in the south before and after development. NPP was mainly concentrated in the interval of 300 g C m(-2) yr(-1)–400 g C m(-2) yr(-1), and the range of NPP change was mostly within 100 g C m(-2). The influence of elevation, temperature and precipitation on the spatial distribution of NPP was significant. Elevation and precipitation were positively correlated with NPP, while temperature was negatively correlated with NPP. The increase in NPP mainly originated from the conversion of unused land to forestland and arable land. The loss of NPP was mainly due to the conversion from forestland with high vegetation productivity to a land use type with low vegetation productivity, such as the conversion from forestland to urban and rural residential land. The results can provide references for making reasonable land planning decisions and ecological environment construction.