Fig. 3. Timing of the summer-regime relative to the snow-free period for selected fluxes and components of the surface energy budget (SEB).

Number of days difference between the start of the summer-regime period and the spring snow-free date (left panel, x-axis) as well as between the end of the summer-regime period and the autumn snow-onset date (right panel, x-axis). The summer-regime timings are derived from the smoothed seasonalities (mean ± s.e., see Fig. 2) of selected surface energy fluxes (y-axis), for different vegetation types (n = 5 per surface energy flux), colored dots: prostrate-shrub tundra (light red), graminoid tundra (yellow green), wetland complex (light blue), erect shrub tundra (green), boreal peat bog (blue). We excluded latent heat fluxes since they are >0 Wm⁻² all year in most cases. Summer-regime is defined as the time when surface energy fluxes: >0 Wm⁻², when surface temperature >0 °C, or when albedo <mean of annual minimum and maximum value, respectively (Methods). Results are based on the vegetation subset of the data for the period 2000–2021 and excluding barren vegetation type because of missing net radiation data; nr. sites = 28, nr. site years = 217. Note: flux direction convention is positive away from the surface for heat fluxes (i.e. H, LE and G). Source data are provided as a Source Data file.