A spectrophotometer was used to scan the canopy vegetation of ITEX flux plots. The resulting reflectance spectra were used to calculate several vegetation indices of interest (NDVI, EVI, EVI2, PRI, WBI, Chlorophyll Index). Average values of these vegetation indices for each ITEX flux plot are presented here.
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METHODS: For each ITEX flux plot, a spectrophotometer (Unispec SC, PP Systems, Amesbury, Massachusetts, USA) was used to scan the canopy vegetation and produce reflectance spectra. Scans with the spectrophotometer have a circular field of view with a diameter of approximately 20cm and were performed in a regular grid across the plot. For most plots, 9 replicate scans were performed. The Unispec SC instrument generates reflectance spectra by measuring incoming irradiance and radiance reflected by the canopy vegetation. A foreoptic cable (UNI-684) extending from the machine is equipped with a ferrule over which a 100mm hypotube (UNI-688) is placed. This produces a field of view that extends at an angle of 20 degrees from the end of the hypotube. The hypotube is held vertically over the target vegetation during a data scan. The foreoptic cable is connected to a miniature photodiode array detector in the intrument that produces a signals ranging from zero to 65,000 A/D counts for 256 wavebands. These wavebands represent 3.3 nm wide portions of the visible and near infrared spectrum from 310 to 1100 nm. A scan is performed over a period of milliseconds with the exact integration time determined by the user based on current light conditions. At the time of data collection, a reference scan is performed by positioning the foreoptic cable over a white standard (UNI-420). This reference scan represents the incoming irradiance due to the highly reflective nature of the white standard. A dark scan is also performed by covering the foreoptic with a dark cloth. The raw signal from the dark scan is used by the machine to correct for background noise. Canopy reflectance is calculated for each waveband as follows using the program Multispec (available at http://specnet.info/specnet_toolkit.htm).: Reflectance= (Icanopy / Ireference) Icanopy= signal from foreoptic during data scan (radiance reflected from target vegetation) Ireference = signal from the foreoptic during reflectance scan (incoming irradiance) In most cases, data was processed and immediately interpolated from 3.3nm wavebands to single nanometer intervals in the Multispec program. For some of the data presented here, the reflectance spectra was originally produced for only for 3.3nm wavebands. This data was later linearly interpolated to produce spectra with 1 nm intervals to allow for compilation into this database. Vegetation indices were calculated for a ll replicate reflectance spectra and were then averaged per plot to produces the values (and standard deviations) presented here. Reflectance values equal to or greater than one were replaced with "NaN" before vegetation indices were calculated. Reflectance values equal to or less than one were also replaced with "NaN" before vegetation indices were calculated. VEGETATION INDEX CALCULATIONS: For MODIS indices, Spectral bands are defined as follows: NIR=avg(841nm-876nm) RED=avg(620nm-670nm) Blue=avg(459nm-479nm) Normalized Difference Vegetation Index NDVI (MODIS) = (NIR-Red)/(NIR+Red) Enhanced Vegetation Index EVI (MODIS) =2.5*(NIR-Red)/(NIR+6*Red-7.5*Blue+1) Enhanced Vegetation Index 2 EVI2 (MODIS) =2.5*((NIR-Red)/(NIR+2.4*Red+1)) Photochemical Reflective Index PRI (550 Reference) =(550nm-531nm)/(550nm+531nm) Photochemical Reflective Index PRI (570 Ref) = (570nm -531nm)/(570nm+531nm) Water Band Index WBI= 900nm/970nm Chlorophyll Index CHL=(750nm-705nm)/(750nm+705nm) RESEARCH LOCATIONS: Imnavait Creek, Alaska 2004: Measurements were made on 15 plots along a topographic sequence of the west facing slope of Imnavait Creek catchment. A light response curve was measured on each plot once in late June/early July and once in August. Toolik Lake, Alaska 2004. Measurements were made on a total of 20 flux plots in treatment (N +P) and control blocks of the LTER fertilization experiment at Toolik Lake, including the moist acidic tussock, non-acidic tussock, non acidic non-tussock, inlet wet sedge and heath sites. Light curves were measured on each plot once in late June/early July and once in August. Abisko, Sweden 2004. Measurements were made on 11 plots at the "Stepps" site, each measured once in July and once in August. Measurements were also made on 13 plots at Paddus and 12 plots at Latnjajaure. Abisko, Sweden 2005. 15 plots were located at the "Stepps" site, measurements were made on 3 occasions through June/ early July and once in mid August. Svalbard 2005. Measurements were made on 40 plots at various sites in Adventdalen. Zackenberg 2006. Data were collected on 35 1 x 1m plots and 25 0.3 x 0.3m plots across a range of vegetation types. 13 of the 0.3 x 0.3m plots were located within 1m x 1m plots (identified by plot name with suffix "b1, b2 or b3"), the remainder were independent of the 1 x 1m plots (B14-25). Toolik, Alaska 2009: Data were collected 9 plots, three in each vegetation type. Vegetation types included moist acidic tundra (MAT), dry heath (HTH) and wet sedge (WSG). Each plot was measured at approximately 2 weeks intervals from mid-June to mid-August 2009. Barrow, Alaska 2009: Data was collected on 13 plots along a moisture gradient of wet coastal tundra. FOR MORE INFORMATION CONTACT: Gus Shaver, The Ecosystems Center, Woods Hole, MA, 02543, USA OTHER DATA FILES TO REFERENCE: 2004-2009gsITEXfluxspectra.csv(full reflectance spectra for ITEX flux plots) 2004-2009gsITEXharvestspectra.csv (full reflectance spectra for all ITEX harvest plots) 2004-2009gsITEXharvestindices.csv(average vegetation indices calculated for all ITEX harvest plots) 2003-2009gsfluxleafN.csv (ITEX flux plot leaf N content) 2003-2009gscurveparameters.csv (ITEX flux plot light response curves fit parameters) 2003-2009gsflux.csv (ITEX flux plot light response curves) 2003-2009gsGPSandveg.csv (ITEX flux plot GPS coordinates and vegetation classifications) 2004-2009gscoverft.csv (ITEX flux plot percent cover by functional type) 2004-2009gscoversp.csv (ITEX flux plot percent cover by plant species) FORMAT OF DATA FILE: ASCII See Methods
Updated Metadata sheet Version 2: Updated to newer metadata with site sheet. CH March 2013. Version 3: Corrected eml excel file name wrong extension. JimL 16May13 Version 4: Corrected Distrubution URL. It had xlsfiles in the path. Jim L 19Jun14 Version 5: Changed Distrubution URL since the LTER network DAS system is being discontinued. JimL 9Apr2015