The confounding effect of snow cover on assessing spring phenology from space: A new look at trends on the Tibetan Plateau

Highlights: • Four satellite indicators are tested to estimate spring phenology trends over the Tibetan Plateau. • The confounding effect of snow cover is evaluated. • The Normalized Difference Phenology Index provides best results for snow conditions. • Normalized Difference Vegetation Index SOS trends are biased by snow trends. The Tibetan Plateau is the highest and largest plateau in the world, hosting unique alpine grassland and having a much higher snow cover than any other region at the same latitude, thus representing a "climate change hot-spot". Land surface phenology characterizes the timing of vegetation seasonality at the per-pixel level using remote sensing systems. The impact of seasonal snow cover variations on land surface phenology has drawn much attention; however, there is still no consensus on how the remote sensing estimated start of season (SOS) is biased by the presence of preseason snow cover. Here, we analyzed SOS assessments from time series of satellite derived vegetation indices and solar-induced chlorophyll fluorescence (SIF) during 2003–2016 for the Tibetan Plateau. We evaluated satellite-based SOS with field observations and gross primary production (GPP) from eddy covariance for both snow-free and snow covered sites. SOS derived from SIF was highly correlated with field data (R² = 0.83) and also the normalized difference phenology index (NDPI) performed well for both snow free (R² = 0.77) and snow covered sites (R² = 0.73). On the contrary, normalized difference vegetation index (NDVI) correlates only weakly with field data (R² = 0.35 for snow free and R² = 0.15 for snow covered sites). We further found that an earlier end of the snow season caused an earlier estimate of SOS for the Tibetan Plateau from NDVI as compared to NDPI. Our research therefore adds new evidence to the ongoing debate supporting the view that the claimed advance in land surface SOS over the Tibetan Plateau is an artifact from snow cover changes. These findings improve our understanding of the impact of snow on land surface phenology in alpine ecosystems, which can further improve remote sensing based land surface phenology assessments in snow-influenced ecosystems.