Linking NDVI and Leaf Senescence in High Arctic Tundra Plants: Soil Moisture Control on Senescence Phenology Revealed by Direct Observations and NDVI Sensors

A comprehensive understanding of the drivers of tundra plant senescence phenology and climate-related changes in senescence timing across tundra landscapes is currently lacking. Near-surface remote sensing (NRS) holds the potential to bridge this gap by overcoming problems common to satellite remote sensing in the Arctic. We investigated senescence timing in six tundra vegetation communities in Svalbard in 2023 using field surveys and NRS-derived Normalized Difference Vegetation Index (NDVI). We mapped the influence of microclimate on the timing of field- and NDVI-derived phenophases and compared the results to assess the applicability of NRS NDVI for senescence monitoring. NDVI declined with senescence progression, and estimates of 50% senescence from NDVI aligned well with field-based estimates, although variation occurred among communities. However, caution is needed in wetland communities, where a 12-day mismatch was observed between field- and NDVI-derived end-of-season indicators. All phenophases (25, 50 and 75 % senescence) occurred later in wetter sites while the earliest phenophase (25% senescence) occurred earlier in warmer sites. Although based on a single, relatively warm and wet year, our results support the notion that microclimate shapes senescence phenology and demonstrate that NRS-based NDVI is a promising approach for monitoring tundra senescence phenology.