Drought offsets gross primary productivity benefits from the afforestation initiatives-induced greening in China

Over the past few decades, China has implemented large-scale Forest Ecological Engineering Projects (FEEPs) aimed at restoring and enhancing ecosystem functions. However, global warming has exacerbated the frequency, intensity, and duration of droughts, which may undermine the positive effects of these ecological engineering programs on greening and carbon sequestration. This study utilizes remote sensing indices, namely the Normalized Difference Vegetation Index (NDVI) and Gross Primary Productivity (GPP), to represent vegetation greenness and productivity, respectively. To extract long-term vegetation change trends across eight FEEPs in China, we apply the Ensemble Empirical Mode Decomposition (EEMD) method. Additionally, a multi-scale Standardized Precipitation Evapotranspiration Index (SPEI) is used to assess the sensitivity and response of vegetation greenness and productivity to drought conditions. The results reveal a monotonic increasing trend in both NDVI and GPP across the eight FEEPs, but the rate of increase in GPP in regions such as the Shelterbelt Program for Liaohe River (SPLR), the Afforestation Program for Taihang Mountain (APTM), the Shelterbelt Program for Pearl River (SPPR), and the Coastal Shelterbelt Program (CSP) is significantly lower than that of NDVI. Before 2000, changes in NDVI and GPP followed relatively consistent trajectories. However, a divergence between these two indices became evident after 2000, particularly during the prolonged drought period from 2000 to 2009. The opposite trend between greenness and productivity in humid ecosystems during drought periods mainly caused this trend difference. In humid ecosystems, short-term drought promotes vegetation greening, while long-term drought has no significant impact on greenness. Our findings highlight the complex dynamics of vegetation growth in the context of climate change and underscore the challenges posed by drought in maintaining the effectiveness of afforestation and greening efforts.