Spatiotemporal Variations and Driving Factors of Net Primary Productivity Across Different Climatic Zones in Cambodia and China

Vegetation plays a crucial role in nature-based carbon neutrality solutions, exhibiting a strong correlation with climatic factors. This study employed a modified CASA (Carnegie–Ames–Stanford Approach) model to estimate Net Primary Productivity (NPP) across Cambodia, as well as Baise, Guilin and Fenyang from China representing diverse climatic zones—from 2000 to 2020. Spatiotemporal NPP patterns and their underlying mechanisms were investigated using Theil–Sen median analysis, the Mann–Kendall test and land use change matrices. The results indicate that: (1) Mean annual NPP from 2000 to 2020 was 753.68 gC·m−2·a−1, 960.58 gC·m−2·a−1, 768.11 gC·m−2·a−1 and 334.20 gC·m−2·a−1 for Cambodia, Baise, Guilin and Fenyang, respectively. While Cambodia showed a non-significant downward trend, the other regions exhibited upward trends. (2) Cambodia’s NPP demonstrated elevated values in the eastern and southwestern regions. Baise and Guilin exhibited higher NPP values at the periphery with lower central values, while Fenyang displayed a northwest–southeast gradient in NPP. (3) Forestland and cultivated land dominated the study areas with a unimodal relationship between elevation and vegetation NPP. (4) Temperature primarily influenced the NPP of Cambodia, Baise and Guilin, and precipitation was the dominant factor in Fenyang. Cambodia (tropical area) and Baise/Guilin (subtropical area), benefiting from favorable hydrothermal conditions, maintained high annual NPP. Fenyang (temperate area), with less favorable conditions, showed a strong positive correlation between precipitation and NPP. Positive correlations existed between NDVI (Normalized Difference Vegetation Index) and annual mean NPP across all study regions. (5) Annual mean NPP in the study area initially increased with elevation but declined beyond a certain altitude. These findings enhance understanding of vegetation carbon cycling across diverse climatic zones, informing accurate regional carbon sink assessments.