Automated Data Logging
In order to monitor canopy conditions of the shrub species of interest--Betula nana and Salix pulchra--two CR1000 dataloggers (Campbell Scientific, 815 W. 1800, North Logan, UT, USA) were installed at the sites used for the ITEX shrub canopy studies at Toolik Field Station in Alaska the summer of 2012. The sites (see "Group" category) are labeled as "Upland" and "Outlet"--descriptions of the sample site in relation to Toolik Lake. The Upland site is colocated with Block 1 of the LTER shrub vegetation plots; the Outlet site is colocated with Block 2. The dataloggers were adjacent to, rather than within, LTER plots.

Both the Upland and Outlet datalogger had the following sensors which logged every five minutes:
(5) Photosynthetically active radiation (PAR) sensors: LI-190SB Quantum Sensor (Li-Cor Biosciences, 4647 Superior St, Lincoln, NE, USA)
* Sensors were placed as follows: (1) Above all vegetation at a height of 2.2 meters
(1) Mid-way within the B. nana canopies (approximately 40 cm above the soil)
(1) Mid-way within the S. pulchra canopies (approximately 40 cm above the soil)
(1) Below the B. nana canopies (approximately 10 cm above the soil)
(1) Below the S. pulchra canopies (approximately 10 cm above the soil)

(3) Relative humidity (RH) and temperature sensors: HMP50/HMP60 Probe (Campbell Scientific, North Logan, UT, USA)
* Sensors each had either a 6- or 10-plate radiation shield (Campbell Scientific model 41303-5A or 41003-5)
* Sensors were placed as follows: (1) Above all vegetation at a height of 2 meters
(1) Below the B. nana canopies (approximately 10 cm above the soil)
(1) Below the S. pulchra canopies (approximately 10 cm above the soil)

Additionally, the maximum and minimum recorded values as well as the total daily photon flux were recorded each day. These data are available in the file "ShrubCanopy_DailyLogger".

SunScan LAI and PAR Measurements:
As with other measurments collected the summer 2012 for the ITEX project at Toolik Field Station, a Delta-T SunScan wand (Delta-T Devices Ltd, Burwell Cambridge, UK) was use to characterize the shrub canopies with the dataloggers. These data are included in the datasets: "SunScan_LAI" and "SunScan_PAR". The methods used for each are described below.

The methods used to collect PAR at many heights within each canopy were the same used to collect LAI, with the difference being the setting on the SunScan wand. The measurements were taken near the end of the growing season when leaves were already beginning to senesce. Additionally, while these measurements are traditionally made in conjunction with a Beam Fraction sensor (BF3), this sensor was malfunctioning when the datalogger sites were sampled , and thus it was not used. Instead of the BF3 sensor, wand measurements above the canopy were made immediately prior to each data measurement to record the ambiant total irradiance levels.

Readings were taken by inserting the SunScan wand near to the ground as possible--typically starting at 5 centimeters from the ground (the wand rested atop the moss layer)--and then repeated every 15 cm with the last measurement being above the canopy. Measurements were taken with the field technician opposite the direction of the sun at five locations within the canopy at each height. Five replicates were taken at each height in the canopy immediately surrounding the datalogger sensors; this differs from the ITEX samples which had three replicate scans at each height.

The weather conditions during these measurements were almost uniform cloud cover; thus nearly all of the measurements were taken under "diffuse" light conditions (diffuse light fraction greater than 0.7). The measurements taken with "direct" light conditions were taken on partly cloudy days.

The SunScan wand measures PAR along 64 points along a 1-m horizontal profile. When sampling PAR within the shrub canopy, the data are from the raw output from each PAR sensor. When sampling LAI, there is an internal calculation performed though the Delta-T software that compares the reading above the canopy to the reading within the canopy, and takes into account the percent of absorbed PAR (assumed to be 0.85), and the ellipsoidal leaf angle distribution parameter (ELADP*) (assumed to be 1.0).

In addition to the LAI values, the Delta-T SunScan instrument gives an output of other variables including: transmitted light, spread among sensors, beam fraction, and zenith angle. These values are further described here:

Transmitted Light: the fraction of incident light that passes through a given canopy. It can refer to Direct, Diffuse, or Total incident light.

Spread: the standard deviation of the 64-PAR sensors on the SunScan wand.

Beam Fraction: the fraction of the Total incident PAR in the Direct beam.

Zenith Angle: the angle between the centre of the sun and the point directly overhead.
Zenith angles are calculated from latitude, longitude, and local time using standard astronomical equations as given in Practical Astronomy. The location used to determine the local zenith angle was: 68° 37' 39" N, Longitude 149° 35' 51" W.

*"ELADP is a way of characterizing the horizontal or vertical tendency of leaves in a canopy." p 37 of SS1 User Manual v2.0 (SunScan Canopy Analysis System, type SS1. Delta-T Devices Ltd. May 19, 2008)

For Delta-T SunScan wand used to measure PAR and LAI:
SS1 User Manual v2.0. Delta-T Devices Ltd., May 19, 2008.