Salix polaris growth rings are an unreliable proxy for satellite-derived tundra productivity on Svalbard

Abstract

The Arctic is warming nearly four times faster than the global average, with Svalbard among the regions experiencing the most extreme warming. This has likely driven widespread “greening” of the archipelago, yet in-situ data on tundra productivity in Svalbard remain scarce, and satellite-based measurements are challenged by cloud cover and low sun angles. This study assessed whether Salix polaris growth ring width can serve as a proxy for large-scale tundra productivity on Svalbard, quantified using NDVI. The results indicate that S. polaris growth rings are not a reliable indicator, as only two of nine sites returned a significant (p < 0.05) ring width-NDVI relationship, associations across sites were very weak (R² < 0.05), and predictive performance of ring-width timeseries for local interannual tundra NDVI based on unseen sites was very poor (Nash-Sutcliffe Efficiency < −2). These findings contrast with studies on other Salix species and small-scale biomass changes. S. polaris radial growth increased with summer temperature and thawing degree-days but decreased with freezing degree-days and delayed snowmelt at multiple sites, while other climatic effects were site-specific. NDVI responded more consistently to climate, increasing with summer temperature, thawing degree-days, and summer precipitation, and decreasing with delayed snowmelt and winter precipitation. Differences in how S. polaris radial growth and NDVI respond to climate, along with the influence of abundant non-vascular vegetation on NDVI, likely explain why S. polaris growth rings are mostly unsuitable as a proxy for large-scale tundra productivity in this High Arctic tundra region, as approximated by satellite-derived NDVI.