Text S1
Climatological context: The first sampling, carried out in 2015, occurred in the context of an unusually warm and dry summer (Table S1 ). Summer air temperatures were >2 °C higher and precipitation amounts were 20-40% below the long-term climate norm (1981-2010) in much of Switzerland, but up to 45% above the long-term norm in the southern and western Swiss Alps (MeteoSchweiz, 2015). These unusual conditions raised questions of whether or not the patterns that we previously observed – specifically, that winter precipitation contributes strongly to summer evapotranspiration (Allenet al. , 2019a,b) – were likely to also hold in more typical summers. However, many subsequent years have also continued to be warm and dry, and thus our subsequent sampling has also been conducted in dry years. In particular, a small subset of sites were resampled in the summer of 2018, which was the most extreme drought on record in central Europe (Schuldt et al. , 2020). Precipitation was only 20 – 40% of the long-term norm in June 2018, and only 71% of the long-term norm for summer 2018 as a whole, leading to widespread observations of drought stress and canopy dieback, particularly among spruce and beech trees (BAFU, 2019; Brun et al. , 2020). A larger set of sites were resampled in the summer of 2019, which was similarly hot; however, precipitation was more normal (Table S1 ; MeteoSchweiz, 2019).