Vegetation health is closely tied to climate, but tracking its response over large areas remains difficult. Traditional optical indices such as Normalized Difference Vegetation Index (NDVI) can be affected by cloud cover and atmospheric conditions, while microwave-based Vegetation optical depth (VOD) is more sensitive to changes in vegetation water content and biomass. Even so, scientists have not fully understood how VOD products from different frequencies and retrieval algorithms compare under changing climate conditions. This question is especially important in China, where vegetation greening has accelerated since 2000 and where ecosystems range from humid forests to arid and semiarid landscapes. Based on these challenges, deeper research is needed into how multi-frequency VOD products capture vegetation responses to climate change.
Researchers from Beijing Normal University, Tongji University, INRAE, the University of Montana, NASA Goddard Space Flight Center, Southwest University, and Chalmers University of Technology published the study (DOI: 10.34133/remotesensing.1028) on February 2, 2026, in the Journal of Remote Sensing. The work tackles a practical challenge in Earth observation: selecting the right microwave vegetation signal to monitor how ecosystems respond to heat, atmospheric dryness, and soil water shortage. The findings offer a clearer basis for choosing remote-sensing tools for drought assessment, ecosystem monitoring, and climate-impact studies across China.
The team analyzed seven VOD products spanning X-, C-, and L-band microwave frequencies and found that retrieval frequency mattered more than algorithm choice in explaining differences among products. Across plant functional types, all seven products showed stronger responses to atmospheric and soil water stress than to air temperature, highlighting VOD’s strength for monitoring vegetation water status. X- and C-band VOD products generally captured faster canopy responses and stronger seasonal variation, while L-band products, which penetrate deeper into vegetation, were more sensitive to woody structure and longer-term biomass-related signals. The study also showed that VOD could detect climate carry-over effects, especially in water-limited ecosystems.
The analysis covered China from 2012 to 2022 and focused on seven major plant functional types, including temperate forests, subtropical forests, temperate non-forests, alpine grassland, and three cropland classes. The researchers compared two X-band products, two C-band products, and three L-band products using growing-season anomalies against air temperature, vapor pressure deficit, and surface soil moisture. Among all vegetation types, temperate non-forests stood out as especially water-limited and showed strong positive soil-moisture relationships in X- and C-band VOD products. Across China as a whole, LPDR-X showed the highest positive correlation with soil moisture at r = 0.16, while MCCA-SMAP showed the strongest negative response at r = −0.40, underscoring how differently products can behave. The team also found that including climate conditions from the previous month improved many VOD–climate relationships, particularly for soil moisture, revealing clear one-month carry-over effects in arid and semiarid regions.
Suggested expert quote for press use: “Our results show that VOD is not a single universal indicator. Different frequencies capture different parts of vegetation behavior, and the strongest signal often comes from water stress rather than temperature alone. This means future climate and ecosystem studies can be more precise if they choose the VOD product that best matches the ecological question.” This wording is adapted from the authors’ conclusions.
The study combined seven freely available satellite VOD datasets from AMSR-E, AMSR2, SMAP, and SMOS, covering X-, C-, and L-band microwave observations. All datasets were resampled to 25 km and analyzed during the growing season from April to September. The researchers calculated anomalies relative to monthly climatology and used Pearson correlations to test how VOD changed with temperature, vapor pressure deficit, and soil moisture. They also added a one-month lag analysis to detect carry-over effects from previous climate conditions.
The findings could improve how scientists monitor drought, vegetation water stress, ecosystem resilience, and climate impacts using satellite data. They also suggest that no single VOD product should be used for every application: higher-frequency signals may be better for rapid canopy stress detection, while lower-frequency signals may be more useful for deeper structural changes. As climate extremes intensify, selecting the right microwave product could help refine regional ecological assessment, carbon-cycle research, and early warning systems in vulnerable drylands and agricultural zones.
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References
DOI
10.34133/remotesensing.1028
Original Source URL
https://spj.science.org/doi/10.34133/remotesensing.1028
Funding Information
This work was funded by the Major Program of the Natural Science Foundation of China (42090013) and the General Programs of the Natural Science Foundation of China (42471352).
About Journal of Remote Sensing
The Journal of Remote Sensing, an online-only Open Access journal published in association with AIR-CAS, promotes the theory, science, and technology of remote sensing, as well as interdisciplinary research within earth and information science.