Indicator-dependent vegetation trends and interannual variability across Asia–Oceania from NDVI, vegetation optical depth, and model based GPP during 2000–2021

Abstract. Vegetation change has been widely monitored using satellite-derived vegetation indices, particularly the Normalized Difference Vegetation Index (NDVI). However, different vegetation indicators represent different aspects of ecosystem conditions and may provide contrasting views of long-term vegetation change. In this study, we compared long-term trends and interannual variability among multiple indicators including two NDVI datasets, Vegetation Optical Depth (VOD), and gross primary production (GPP) simulated by a process-based ecosystem model VISIT across the Asia–Oceania region during 2000–2021. Long-term trends were estimated using Sen’s slope, and interannual variability was evaluated using detrended and standardized annual anomalies. Both NDVI datasets showed widespread positive trends across East Asia and northern Eurasia, whereas VOD exhibited weaker and more spatially heterogeneous trends, particularly in high-latitude regions. East Asia showed relatively consistent positive trends among NDVI, VOD, and GPP, suggesting coherent increases in vegetation greenness, biomass, and ecosystem productivity. In contrast, substantial discrepancies were observed in Siberia, where strong NDVI greening was not consistently accompanied by increases in VOD or GPP. Comparisons of interannual variability also revealed regional differences in agreement among indicators, with stronger relationships among NDVI, VOD, and GPP in East Asia and some semi-arid regions than in boreal ecosystems. These results indicate that assessments of vegetation change depend strongly on the indicator used and that different indicators capture different aspects of ecosystem dynamics. Combining optical, microwave, and model-based indicators therefore provides a more complete understanding of vegetation change across the diverse environments of Asia–Oceania.