# Measurement of Diurnal Variation in Needle PRI and Shoot Photosynthesis in a Boreal Forest
## Materials and Methods  
### Needle Optical Measurements  
Two spectral measurement campaigns were carried out in a boreal coniferous forest at the Station for Measuring Ecosystem Atmosphere Relationships (SMEAR II) in Hyytiala, Southern Finland (61\({}^{\circ}\)51'N, 24\({}^{\circ}\)18'E) in 2014 and 2015. The growing season in this area, determined as the period for which mean temperature is above 5 \({}^{\circ}\)C, lasts between late April and October [26]. The site is located on mineral soils and dominated by Scots pine (_Pinus sylvestris_ L.) and Norway spruce (_Picea abies_ (L.) [PERSON]), with forest understory consisting of common vascular plant species [27]. The campaigns were carried out during the peak growing season, on 24-25 July 2014, and 1-5 July 2015. During the short July 2014 campaign all measurements were carried out within 24 h. Illumination conditions were clear with cumulus clouds only very occasionally interrupting the measurements in the afternoon. During the 2015 campaign, we experienced both complete and broken cloud cover, and also a complete day with no clouds (3 July). The data recorded during the 2015 campaign were pooled to derive a daily cycle of the PRI of sunlit and shaded needles for the campaign period. The number of measurement campaigns was limited by weather: in this region, sunny days are infrequent--it is possible that only a few days during the peak growing period are suitable for daily PRI studies. One mature (ca. 55 year-old) Scots pine (_Pinus sylvestris_) tree was sampled in 2014 and two trees in 2015. The measurements were performed in the upper crowns accessed using two walk-up scaffolding towers (18 m in height). We selected eight shoots in each tree at the beginning of each campaign. Four shoots were marked in the southern (sunlit) side of the crown, and four shoots in the northern (shaded) crown. While all shoots were located in the upper canopy layers visible to a remote sensing sensor, the shaded shoots were not exposed to direct sunlight for most of the day, especially before and around noon. Due to the high latitude, the shaded shoots received some direct sunlight late in the evening when the sun was in the northern part of the sky. The selection of the shoots was based on visual observations of the shoot location. Illumination conditions were validated using hemispherical photographs taken with a 180\({}^{\circ}\) fisheye lens in the location of the shoot. Camera orientation was determined using a compass and a bubble level in the flash shoe. We used the WSL Hemisfer (Birmensdorf, Switzerland) software to calculate the light regime for each shoot. We only included fully-expanded needles from the previous year in the study. To measure the PRI of an individual needle, it was partly inserted into a PP Systems (Amesbury, MA, USA) UNI501 Mini Leaf Clip for a few seconds. A small (diameter approx. 1.5 mm) spot was illuminated on the sample by light arriving from an Ocean Optics (Dunedin, FL, USA) HL-2000 tungsten halogen light source (power 5 W). Light reflected backward was analysed with an Ocean Optics USB4000-VIS-NIR portable modular spectrometer controlled by a notebook computer running the OceanView (Ocean Optics, Dunedin, FL, USA) spectroscopy application. Low OH bifurcated optical cables (PP Systems item UNI410) were used to connect the light source, needle clip, and spectroradiometer. The spectral resolution of the spectrometer was 1.5-2.3 nm (full width half maximum, FWHM) inside the useable wavelength range of the system 400-900 nm. As a reference, a white Spectralon disc was measured after each of three (in 2014) or five (in 2015) needle spectra measurements. The recorded spectra were converted to reflectance factors and smoothed with a Savitzky-Golay filter with parameters chosen not to reduce the spectral resolution determined by the spectroradiometer. The photosynthetic photon flux density (PPFD) received by the target was determined using the Spectralon measurements performed in the field.