• Secondary metabolites make leaves unpalatable for herbivores and influence decomposition. Site-specific differences are presented in phenolics and nitrogen in Betula nana leaves from dwarf shrub tundra at Abisko, northern Sweden, and from tussock tundra at Toolik Lake, Alaska, subjected to a decade of warming, fertilization and shading. • Nitrogen and a number of phenolics, including condensed and hydrolysable tannins, flavonoids, phenolic glucosides and chlorogenic acids, were analysed in B. nana leaves. • Phenolic concentrations showed marked between-site differences (e.g. condensed tannins were 50% higher at Abisko than at Toolik); responses to the environmental manipulations were more pronounced at Toolik compared with Abisko. Warming increased condensed tannins and decreased hydrolysable tannins at Toolik, but had no effect at Abisko, whereas fertilization and shading generally decreased concentrations. • Betula invests less carbon in phenolics at Toolik than at Abisko and shows a greater response to environmental changes by investing more carbon in growth and less to phenolic production. Hence, the Toolik population has a lower herbivore-defense level, which declines further if nutrient availability increases. By contrast, under warmer conditions, the increase in bulk phenolics and decrease in leaf palatability are greater at Toolik than at Abisko.