2.2 | Environmental data
Environmental data included half-hourly relative humidity (RH, %), photosynthetic photon flux density (PPFD, µmol m-2s-1), ambient temperature (Ta, °C) and soil water content (SWC, m3 m-3), and daily precipitation (mm). These data were measured at both plots. All abbreviations, their units, and values of constants are summarised in Table 1. Gaps in the meteorological data, due to instrument failure, were filled using measurements from the Svartberget forest, which is located about 8 km from the study site. During the period 1981-2010, mean annual temperature and precipitation at Svartberget was 1.8 °C and 614 mm, respectively (Laudon et al., 2013).
The temperature data were used to define the “thermal growing season” which estimates the period theoretically suitable for vegetation growth for a given year (Cornes, van der Schrier, & Squintu, 2018; Linderholm, 2006). The thermal growing season was defined to begin after the occurrence of five consecutive days with mean daily temperature > 5 °C and the end was defined as the occurrence of five consecutive days < 5 °C (Mäkelä et al., 2006). According to this definition, the 2012 growing season lasted from 14th of May to 10th of October and, in 2013, from 8th of May to 14th of October.
Atmospheric CO2 concentration and δ13C (δ13Ca, ‰) were both collected from the National Oceanic and Atmospheric Administration database using the nearest sample station, at Pallas-Sammaltunturi in Finland (White, Vaughn, & Michel, 2015). This was necessary to account for pronounced seasonal and annual variation in these variables at our high latitude.