Blue Rings reveal thermal thresholds of wood formation in Low Arctic tundra shrubs

Abstract Low Arctic tundra shrubs experience short growing seasons, prolonged cool conditions, and episodic cold extremes, but the effects of these events on wood tissue formation remain poorly understood. Here we identify thermal thresholds associated with lignification processes in two dominant shrub taxa ( Alnus alnobetula and Salix spp.) along a 68–69°N transect in northwestern Canada. Using wood anatomical indicators of incomplete secondary cell wall lignification (blue rings) and freezing-induced cell collapse (frost rings), we show that wood tissue development is sensitive to thermal stress at the beginning and end of the growing season. We identified thermal thresholds constraining shrub wood formation. Incomplete lignification resulting in blue rings was associated with cumulative August cold exposure below taxon-specific thresholds (~ 2 °C in Alnus , ~ 4 °C in Salix ) and contrasting sensitivities to cumulative cold exposure. Early growing season frost injury in some cases followed prior-year incomplete lignification. The results indicate that contrasting forms of growing-season cold stress constrain distinct phases of wood formation and highlight taxon-specific thermal limits controlling shrub growth. By quantifying climatic controls on lignification failure and frost injury, we link sub-seasonal cold extremes to specific phases of wood formation, which may shape tundra vegetation responses under ongoing Arctic climate change.