Normalized Difference Vegetation Index, Biomass, and Nitrogen Content of Goose Forage, Northern Alaska, 2011-2018

On intervals of 6–10 days, we randomly selected and removed a 20 x 20 cm turve (block of vegetation and soil) from each plot to a depth of approximately 5 cm, clipped and preserved all aboveground biomass, and then returned the substrate to the sample location. Turves were spaced 10–20 cm apart to prevent edge effect from previous sample collection and >10 cm away from the plot perimeter to avoid potential effects of the exclosure material on vegetative growth; a 50 x 50 cm sector of each plot was left undisturbed for NDVI sampling.

At the Colville River Delta, we placed a sensor in the center of each exclosure to measure hourly soil temperature at a depth of 6 cm. Sensors were placed in the ground just after thawing in early June each year and recorded soil temperature across all sampling intervals.

Processing and analysis of vegetation samples varied slightly between the two study sites. At the Colville River Delta, vegetation samples were rinsed in fresh water, dried at ambient temperature, and all live C. subspathacea and Puccinellia spp. were separated from other vegetation and frozen. After the field season, samples were dried to constant mass at 50°C and weighed (g). A subsample (0.5-1.0 g) was analyzed for percent nitrogen using a C-N analyzer at the Washington State University Wildlife Habitat and Nutrition Laboratory. At Smith River, samples were collected in the field and frozen. After the field season, samples were sorted and then dried to constant mass and weighed following the same methods as with the Colville River Delta samples.

In association with vegetation sampling, we obtained hyperspectral reflectance profiles at each plot using handheld spectrometers manufactured by Unispec (on Colville River Delta) and Ocean Optics (JAZ-COMBO; at Point Lonely). Controlled trials comparing the two units across a range of halophytic graminoid densities demonstrated that the units measured essentially equal NDVI values. The spectrometers recorded reflectance profiles at a spectral resolution of 3.2–3.4 nanometers (nm) and range of 301.6–1143.4 nm. The sensor optic was mounted on a 1.2 m-long fiber optic cable and had a 25° field of view, providing a ratio of sensor height to reading footprint diameter of approximately 2:1. Prior to each reflectance reading, spectrometers were calibrated against a white standard to normalize for any changes in light conditions between sampling occasions. Reflectance readings were taken from a height of 50 cm centered on the selected turve and the undisturbed sector. We used the turve reading to compare NDVI with biomass and nitrogen while undisturbed sector reading that was made on the same patch of vegetation served as a control. We calculated two separate NDVI values using reflectance readings from handheld spectrometers: one based on band definitions for the 250 x 250 m scale "expedited" Moderate Resolution Imaging Spectroradiometer (eMODIS) imagery from the Terra satellite, and the other based on 1 m2 scale WorldView-2 satellite imagery. The band definitions for eMODIS NDVI were red = 616–674 nm and near infrared (NIR) = 837–880 nm (SR_eMO); for WorldView-2 NDVI, the definitions were red = 630–690 nm and NIR = 770–895 nm (SR_WV2). Reflectance was derived as the mean of all hyperspectral values falling within each of the band ranges, and NDVI was calculated as: NDVI = (NIR – Red)/ (NIR + Red).