Climate data
Hourly air temperatures measured at 2 m above ground approx. 3 km southwest of the research site at ClimateBasis Disko (69.24 °N, 53.53 °W, 1 m a.s.l.; Fig. 1b) covering the period 23.09.1993 – 31.12.2019 were downloaded from the GEM database (https://data.g-e-m.dk accessed on 9.3.2021; ). Daily mean temperatures were calculated based on these hourly measurements. Days with fewer than 16 hourly measurements were not further processed. Mean monthly temperatures were calculated based on the daily means for months with at least 20 daily means. This resulted in a mean monthly temperature series from October 1993 to December 2019, with gaps in 1993 (December), 1995 (May-July), 1997 (January, September – December) and 1998 (January). These gaps were filled and the monthly temperature record was extended back to 1880 with temperatures from the homogenized monthly temperature dataset from Ilulissat (; https://crudata.uea.ac.uk/cru/data/greenland/ accessed on 19.03.2021). Monthly mean temperatures from this dataset were transferred using linear regression between this dataset and the ClimateBasis monthly means to match the level of the latter. These records are strongly correlated for each month (0.79≤r≤0.99, p<0.0001), with on average slightly lower temperatures recorded in Ilulissat.
Monthly precipitation sums (1901-2019) were extracted from the monthly gridded dataset CRU TS4.04 for the grid 69-69.5 °N, 53.5-53 °W (https://crudata.uea.ac.uk accessed on 14.5.2021; ). This gridded dataset was extended through regression with monthly precipitation sums calculated from daily data measured at Ilulissat (1880-1991; 69.22 °N, 51.1 °W, 39 m a.s.l.) as obtained from the Global Historical Climatology Network (GHCN accessed on 10.3.2021; ). Precipitation sums from these records are strongly correlated (0.80≤r≤0.97, p<0.0001) with slightly higher values recorded in Ilulissat than suggested for the grid matching the research site. The resulting monthly temperature and precipitation records span the entire period of shrub growth (1892-2013) studied here.