Cr10x

Relative humidity was measured using a capacitive hygrometer (HMP35A Vaisala, Oy, Helsinki, Finland) and air temperature with copper-constantan thermocouples placed in a ventilated radiation shield at the center of the greenhouse. Photosynthetic photon flux density (PPFD) was also measured by means of PPFD sensors placed above each experimental bloc within the greenhouse. All data were stored in a datalogger (CR10X, Campbell Scientific Ltd.), with measurements taken every 30 s and an average calculated over 15 min. The data are summarized for each experiment in Supplementary Table S1.
Thermal time (TT) was calculated as the integral of a non-linear beta function of temperature (T) as proposed by Zaka et al. (2017 (link)):
$\begin{array}{l}f(T)={(\frac{T-T_{\text{min}}}{T_{ref}-T_{\text{min}}})}^q.(\frac{T_{\text{max}}-T}{T_{\text{max}}-T_{ref}})\end{array}$
$\begin{array}{l}TT={\displaystyle \underset{t0}{\overset{t}{\int }}}\text{max}\left[0,(T_{ref}-T_{base}).f(T)\right].dt\end{array}$
The equation has three parameters: the minimum (T_min) and maximum (T_max) temperatures at which development occurs and q, a shape parameter. In addition, T_ref accounts for a fixed reference temperature (20°C) and T_base (5°C) for a common base temperature used to scale time units from equivalent days at the reference temperature into degree days (°Cd). The parameters used for the different species were derived from the literature and are presented in Supplementary Table S2.

At each site, a datalogger (CR10X, Campbell Scientific, Utah) and sensors was used to record air temperature, relative humidity (Humitter 50Y, Vaisala, Finland), and photosynthetically active radiation (PPFD) (LI-190, Licor, Utah) every 15 minutes over 24 h prior to sample collection. Metadata was also collected for each sample tree, these included: GPS coordinates (WGS84 (G1762); degree minutes), elevation, tree height, and the basal diameter of the tree base at 10 cm off the ground. For each of the three branches collected per tree, height of the branch above ground and aspect (0–360°) were also recorded.

Ice nucleation, propagation, and supercooling within and around vegetative buds of P. abies were monitored with a digital infrared camera (T650SC, FLIR Systems, Danderyd, Sweden). This infrared camera has a thermal resolution of 0.2 mK. We used a close‐up lens (magnifying factor: 1.5×, working distance: 46 mm) to obtain a spatial resolution of 25 μm. Infrared images were recorded at an interval of 10 frames per second. Subsequent processing of the images by infrared differential thermal analysis (IDTA; Hacker & Neuner, 2008; Neuner & Kuprian, 2014) was conducted with FLIR ResearchIR Max (version 4.20.2.74, FLIR Systems). Image overlay was done with After Effects (Adobe Systems Inc., San Jose, USA).
A longitudinal section of the terminal part of a twig of P. abies, bearing three vegetative buds, was mounted on a small lift table by double‐sided adhesive tape. In the image field of the camera (16 mm × 12 mm), the three buds were arranged in a focal plane. Additionally, six thermocouples were mounted close to the buds. Thermocouple output was recorded every 10 s by a data logger (CR10X, Campbell Scientific).

Soil CO₂ concentration measurements were made during the growing season of 2009 using Vaisala CARBOCAP solid‐state CO₂ sensors (model GMT 221, Vaisala, Helsinki, Finland) at depths of 10 and 5 cm and the Li‐8150 system (Li‐Cor, Lincoln, NE, USA) at 0 cm. At the same depths, soil temperature (averaging soil thermocouple probe TCAV; Campbell Scientific) and moisture (ML2x; Delta‐T Devices, Cambridge, UK) were measured, and incident photosynthetically active radiation (PAR) (BF2H; Delta‐T Devices) was measured above the canopy at 2 m height. Continuous recordings at 0.05 Hz were averaged and half‐hourly values recorded using an automated station (CR10X; Campbell Scientific). Values of soil CO₂ concentration were corrected for temperature and pressure using the ideal gas law according to the manufacturer (Vaisala, Helsinki, Finland). Soil CO₂ efflux at the soil surface was measured using an automated soil respiration system (Li‐8100 and Li‐8150; Li‐Cor, Lincoln, NE, USA) over measurement intervals of 2 min. The chambers were white to minimize heating. Possible pressure changes due to a Venturi effect are largely eliminated by the design of the vent used with the chamber [Xu et al., 2006].

Each site was equipped with a meteorological station. The station was supplied with a shielded air temperature and relative humidity sensor (HMP50, Vaisala, Helsinki, Finland), PAR sensors (Li 190 Campbell Scientific, Logan, UT, USA), pluviometer, and three soil temperature sensors placed at three soil depth (10, 20, and 30 cm). Data were continuously recorded hourly throughout the growing season of 2013 and 2014 (from 152 to 273 day of year) using datalogger (CR10X, Campbell Scientific, Logan, UT, USA) (Figure 2).