Varying climate response across the tundra, forest-tundra and boreal forest biomes in northern West Siberia

Satellite studies using the normalized difference vegetation index (NDVI) have revealed changes in northern Eurasian vegetation productivity in recent decades, including greening in tundra and browning in the boreal forests. However, NDVI trends and relationships to climate depend on the temporal and spatial sampling, data and methods, underscoring the complexity of understanding the variability of vegetation productivity. Here utilizing Moderate Resolution Imaging Spectrometer (MODIS) data (2000–16), we perform a consistent analysis of NDVI and climate across four bioclimatic zones (tundra, forest–tundra, middle taiga and northern taiga) in northern West Siberia, further stratified into eight forest–landcover types. The study reveals that positive trends are most prevalent in the unforested and in the needleleaf deciduous forest (larch) within the tundra and the forest–tundra biomes, whereas negative trends are more prevalent in the coniferous forests in the taiga biome. Correlation analysis of NDVImax against SAT and precipitation averaged across each biome found two statistically significant results – between tundra and SAT, and middle taiga and precipitation. Spatial composites of NDVI and climate variables were effective for revealing insights and patterns of relationships. There was tundra greening in warmer summers, except when moisture is severely limited, e.g., 2016 record warmth coincident with negative anomalies in precipitation and soil moisture. The middle taiga has low NDVI in negative precipitation anomalies regardless of temperature, and high NDVI in moist and warm summers, i.e., higher temperature does not result in forest browning if moisture is adequate – no direct temperature stress. The transitional biomes share characteristics of their respective proximal biomes when greening, i.e., forest–tundra responds positively to higher temperatures, and northern taiga responds positively to precipitation. Spatial heterogeneity of NDVI patterns indicates divergent reactions of specific types of vegetation as well as local effects are clearly important on the background of regional climate response.