Changes in Net Primary Productivity in the Wuyi Mountains of Southern China from 2000 to 2022

Forest carbon sinks have faced significant challenges with the accelerating warming trend in the 21st century. Net primary productivity (NPP) serves as a critical indicator of the carbon cycle in forest ecosystems and is intricately influenced by both human activities and climate change. This study focuses on the subtropical Southern Forests of China as the research object, using the Wuyi Mountains as a representative study area. The positive and negative contributions of ecologically oriented human activities driven by China’s forestry construction over the past few decades were investigated along with potential extreme climate factors affecting the forest NPP from an altitude gradient perspective and regional-scale forest NPP changes from a novel viewpoint. MODIS NPP, climate, and land use data, along with a vegetation type transfer matrix and statistical methods, were utilized for this purpose. The results are summarized as follows. (1) From 2000 to 2022, NPP in the Wuyi Mountains exhibited a high distribution pattern in the northeastern and southern areas and a low distribution pattern in the central region, with a weak overall increase and an average annual growth increment of only 0.11 gC·m−2·year−1. NPP increased with altitude, with a mean growth rate of 5.0 gC·m−2·hm−1. Notably, the growth rate of NPP was most pronounced in the altitude range below 298 m in both temporal and vertical dimensions. (2) In the context of China’s long-term Forestry Ecological Engineering Projects and Natural Forest Protection Projects, as well as climate warming, the transformation of vegetation types from relatively low NPP types to high NPP types in the Wuyi Mountains has resulted in a total NPP increase of 211.58 GgC over the past 23 years. Specifically, only the altitude range below 298 m showed negative vegetation type transformation, leading to an NPP decrease of 119.44 GgC. The expansion of urban and built-up lands below 500 m over the 23-year period reduced NPP by 147.92 GgC. (3) The climatic factors inhibiting NPP in the Wuyi Mountains were extreme nighttime high temperatures from June to September, which significantly weakened the NPP of evergreen broadleaf forests above 500 m in elevation. This inhibitory effect still resulted in a reduction of 127.36 GgC in the NPP of evergreen broadleaf forests within this altitude range, despite a cumulative increment in the area of evergreen broadleaf forests above 500 m over the past 23 years. In conclusion, the growth in NPP in the southern inland subtropical regions of China slowed after 2000, primarily due to the significant rise in nighttime extreme high temperatures and the expansion of human-built areas in the region. This study provides valuable data support for the adaptation of subtropical forests to climate change.