Drought reduces the effectiveness of ecological projects: Perspectives from the inter-annual variability of vegetation index

Abstract

To alleviate grassland degradation and improve ecosystem functions, the Chinese government has implemented some ecological projects since 2004 across the Tibetan Plateau, such as ‘Return Grazing Land to Grassland Project’ and the ‘Compensation and Rewards to Herders for Natural Grassland Conservation’. Meanwhile, the frequency and intensity of drought events have strengthened in this plateau. However, how grasslands respond to these projects and drought remains controversial. Here, taking the northern Tibetan Plateau (NTP) as the study area, we first quantified the spatiotemporal changes in the annual maximum grassland NDVI (NDVImax) after the implementation of the ecological projects (2005–2015). Then, we assessed the effects of drought (Standardized Precipitation-Evapotranspiration Index, SPEI) and ecological projects on the dynamics of NDVImax. The results showed that (1) the grazing intensity decreased significantly across the NTP after the start of ecological projects, but grassland NDVImax did not increase as expected. Statistically, 53.7% of grasslands showed a decreasing NDVImax, of which 5.7% had a significant decrease (P < 0.05), mainly in the central and eastern NTP. (2) NDVImax positively correlated with SPEI at the pixel, county, and regional scales, suggesting that drought was the driving force for the decreasing NDVImax. The severe and extreme drought events in 2015 contributed to 9.4% and 17.7% reduction of grassland NDVImax, respectively. (3) The residual analysis demonstrated that there were 4.4% of the grasslands experienced human-induced degradation. However, on the whole, the effects of ecological projects on vegetation changes of the NTP were positive. Our findings suggested that the positive effects of the ecological projects on vegetation growth were relatively mitigated or offset by drought. Thus, we question the validity of evaluating the effectiveness of ecological projects only according to the trend of vegetation productivity derived from remote-sensing based vegetation index.