This file contains irradiance (PAR) and diffuse light data logged from a beam fraction (BF3) sensor near Toolik Lake, Alaska during the summer of 2012. The data comes from a compilation of automated datalogger readings as well as measurements taken during the field season in conjunction with the Delta-T SunScan wand to measure PAR in tall shrub canopies dominated by Betula nana or Salix pulchra species. The sensor was leveled and mounted to a 2m tripod in each location, and programmed to record instantaneous readings in 30 second to 5 minute intervals.
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This dataset is a compilation of data taken with the beam fraction sensor (called the BF3 sensor, manufactured by Delta-T Devices Ltd, Burwell Cambridge, UK) during the summer of 2012 at the Toolik Field Station in Alaska. While the goal was for the BF3 sensor to monitor conditions continuously throughout the growing season, the sensor experienced frequent malfunctions, causing there to be significant gaps in the data record.
As this is a compilation of time when the BF3 sensor was logging, the interval between measurements and the device regulating the data collection vary. Before describing the methods used in each kind of collection, it is useful to describe the BF3 set-up that was common to all collection times:
* The BF3 sensor was mounted and leveled on the tripod sent with the instrument from the manufacturer (at height approximately 2 m) every time it was used. Care was taken to ensure that the BF3 was not near any structures that might influence the PAR readings, and that it would not be located in a highly trafficked area.
* The BF3 sensor consists of seven PAR sensors arranged in a mathematically formulated way and shaded with a uniquely shaped "shield" that enables the device to calculate the fraction of diffuse light regardless of the sun angle or time of day. This "shield" has a strip through the center that is open through the diameter of the hemisphere where the PAR sensors are located. As recommended by the manufacturer, this open strip was always arranged such that it was oriented in an East - West direction. [This supposedly ensures one PAR sensor will always be shaded; an important component to the BF3 calculations. See the BF3 Instruction manual for further details.]
BF3 Data Logging with a CR800
Whenever possible, the beam fraction sensor was programmed to log total and diffuse irradiance every five minutes on a CR800 datalogger (Campbell Scientific, 815 W. 1800, North Logan, UT, USA).
The site initially chosen for this automated logging was behind a laboratory near Toolik Lake; this site was called "Lab2". While at least 10 meters from the nearest structure, it was thought that the proximity of the lab buildings might influence or even shade the BF3 sensor at low sun angles. Thus a second site was chosen--"Toolik Gate". This site is located at the entrance to the gravel pad of Toolik Field Station and had no nearby structures that could interfere with the BF3 readings.
The positions and elevations of these sites were estimated from GoogleEarth maps.
Autologging during Chamber Flux or Point Frame Measurements
For the duration of fielld days when chamber flux (CH) or point frame (PF) measurements were being made, the BF3 sensor was set to autolog with the Trimble PDA and SunScan wand (also Delta-T) designed for use with the BF3 sensor. The log interval was set to take an instantaneous reading either every sixty or thirty seconds. The location of the sensor was within five meters of the latitude and longitude associated with the "Plot" listed on the data page, though the exact position of the BF3 sensor was not recorded. The point frame (PF) and chamber flux (CH) measurements GPS coordinates were measured with a handheld Garmin Etrex GPS unit while tracking at least five satellites; elevation for all sites was estimated from GoogleEarth.
In some cases, the SunScan wand (which logs 64-PAR readings along the 1-m wand in synch with the BF3 sensor) was placed beneath a plot previously sampled. In these intances, the Plot ID of the canopy associated with the SunScan wand is listed in the "Notes" section. The raw output of these PAR readings are listed in PAR 1... PAR 64(tip).
SunScan PAR Measurements:
The SunScan wand was used to characterize PAR at many heights within each canopy; as the BF3 sensor was also used for these measurements, those data have been included here as ambient BF3 and total irradiance data. The PAR data for all plots are summarized in the file, "SunScan_PAR" , and the collection methods are summarized here:
The methods used to collect PAR at many heights within each canopy were the same used to collect LAI (see "SunScan_LAI_Data"), the primary difference being the settings in the Delta-T software. We measured PAR at 64 points within a 1-m horizontal profile using a DeltaT SunScan wand in conjuction with the BF3 sensor . The BF3 sensor recorded total irradiance and incident diffuse light simultaneously with the SunScan wand's 64-PAR readings evenly spaced along the 1m long wand.
Readings were taken by inserting the SunScan wand as near to the ground as possible--typically ~5cm from the ground as the wand rested on top of moss--and then measured vertically every 15 cm with the last measurement being above the canopy. Measurements were taken from the side of the chamber or point frame opposite the sun at three locations for each height under both direct (ambient) and diffuse light conditions. In many cases the replicates at each height were differentiated by row (1-3, or occasionaly 3-8 which correspond to the point frame pins). Typically diffuse light conditions were achieved by shading only the shrub canopy with photographic diffuser panels (the BF3 sensor was not shaded). On occasion, measurements were taken during cloudy light conditions where the diffuse light fraction was greater than 0.7 and no diffuser panel was needed; on these occasions the direct and diffuse light estimates may have been taken in slightly different locations as they were taken at different times and the precise position of the SunScan wand could not be replicated exactly.
For Delta-T SunScan wand used to measure PAR and LAI:
SS1 User Manual v2.0. Delta-T Devices Ltd., May 19, 2008.
User Manual for the Sunshine Sensor type BF3 v1.0. Delta-T ltd., May 2002.
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SunScan WAND AVG
BF3 TOT IRRAD
DIFFUSE LIGHT FRACTION
HEIGHT FROM GROUND
NOTES and ROW#