Spatial–temporal characteristics and influencing factors of farmland expansion in different agricultural regions of Heilongjiang Province, China

Farmland expansion, including area and pattern, has become one of the main manifestations of global land use change during the last century. Since the reform and opening up, Heilongjiang Province has experienced a constant expansion of farmland, mainly in Songnen Plain (SNP) and Sanjiang Plain (SJP). However, the spatial–temporal heterogeneity of farmland expansion intensity and influencing factors remains unclear. Based on land use change trajectory, this study explored the spatial–temporal characteristics of farmland expansion in SNP and SJP from 1980 to 2018, by using the farmland expansion intensity index (FEI), farmland expansion intensity differentiation index (FEDI), and revealed the influencing factors from the qualitative and quantitative perspectives. Results show that (1) The farmland area in SNP and SJP demonstrated an absolute expansion trend, and the scale in SJP was relatively more significant. The newly added farmland resulted from ecological land reclamation, distributed in SNP by infilling expansion, while in SJP by edge expansion. (2) The trend of farmland expansion intensity in these two plains fluctuated over time. Counties in SNP mainly had low- and medium-rate farmland expansion intensity, and those areas with medium-rate expansion were mainly located on the edge of plain. The farmland expansion intensity in SJP gradually increased from southwest to northeast, and those areas with high-rate expansion intensity were mainly located in the northeast and east. (3) Socio-economic context and national policies were the main factors driving farmland expansion. Slope is the main limiting factor with the widest range and the highest intensity, and 63.53% and 76.78% of the grid including the newly added farmland in SNP and SJP, respectively, were prominently influenced. The results reveal the change process and main influencing factors of farmland expansion in Heilongjiang Province, and provide reference for differential protection of farmland.