Comparison of landsat and land-based phenology camera normalized difference vegetation index (NDVI) for dominant plant communities in the Great Basin

Authors: Snyder, Keirith; HUNTINGTON, JUSTIN - DESERT RESEARCH INSTITUTE; WEHAN, BRYCE - UNIVERSITY OF NEVADA; MORTON, CHARLES - DESERT RESEARCH INSTITUTE; STRINGHAM, TAMZEN - UNIVERSITY OF NEVADA

Submitted to: Sensors
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/27/2019
Publication Date: 3/6/2019
Citation: Snyder, K.A., Huntington, J.L., Wehan, B., Morton, C., Stringham, T.K. 2019. Comparison of landsat and land-based phenology camera normalized difference vegetation index (NDVI) for dominant plant communities in the Great Basin. Sensors. 19(5):1139. https://doi.org/10.3390/s19051139.
DOI: https://doi.org/10.3390/s19051139

Interpretive Summary: Studies and observations of plant phenology have been critical to ecology, agricultural production, and understanding multi-trophic interactions. Land managers and producers have been assessing phenophases through field-based surveys for centuries to determine relationships with weather, climate, and proper management strategies. In the last several decades, remotely sensed data has provided various indices of plant vigor at large spatial scales that have provided invaluable information on vegetation cover. This study looks at using simple land-based cameras programmed to take multiple images in one day of common community types in rangelands of the Great Basin. We compared camera-based indices of plant vigor with those obtained from satellite-based imagery. We found these the land-based cameras can assess phenological changes in sagebrush steppe, pinyon and juniper woodlands, and meadows. Indices of vigor were comparable between land-based cameras and satellite-based imagery, except in the pinyon and juniper woodlands. These land-based cameras, commonly referred to as phenocams, can fill spatial and temporal gaps in satellite imagery and field-based measurements, and provide a new tool for land management decisions and monitoring.

Technical Abstract: Phenology of plants is important for ecological interactions. The timing and development of green leaves, plant maturity, and senescence affects biophysical interactions of plants with the environment. In this study we explored the agreement between land-based camera and satellite-based phenology metrics to quantify plant phenology and phenophases dates in five plant community types characteristic of the semi-arid cold desert region of the Great Basin. Three years of data were analyzed. We calculated the Normalized Difference Vegetation Index (NDVI) for both land-based cameras (i.e., phenocams) and Landsat imagery. NDVI from camera images was calculated by taking a standard RGB (red, green, and blue) image and then a near infrared (NIR) plus RGB image. Phenocam NDVI was calculated by extracting the red digital number (DN) and the NIR DN from images taken a few seconds apart. Landsat has a spatial resolution of 30 m2, while phenocam spatial resolution can be analyzed at the single pixel level at the scale of cm2 or area averaged regions can be analyzed with scales up to 1 km2. For this study, phenocam regions of interest were used that approximated the scale of at least one Landsat pixel. In the tall-statured pinyon and juniper woodland sites, there was a lack of agreement in NDVI between phenocam and Landsat NDVI, even after using National Agricultural Imagery Program (NAIP) imagery to account for fractional coverage of pinyon and juniper versus interspace in the phenocam data. Landsat NDVI appeared to be dominated by the signal from the interspace and was insensitive to subtle changes in the pinyon and juniper tree canopy. However, for short-statured sagebrush shrub and meadow communities, there was good agreement between the phenocam and Landsat NDVI as reflected in high Pearson’s correlation coefficients (r > 0.75). Due to greater temporal resolution of the phenocams with images taken daily, versus the 16-day return interval of Landsat, phenocam data provided more utility in determining important phenophase dates: start of season, peak of season, and end of season. More specific species-level information can be obtained with the high temporal resolution of phenocams, but only for a limited number of sites, while Landsat can provide the multi-decadal history and spatial coverage that is unmatched by other platforms. The agreement between Landsat and phenocam NDVI for short-statured plant communities of the Great Basin, shows promise for monitoring landscape and regional-level plant phenology across large areas and time periods, with phenocams providing a more comprehensive understanding of plant phenology at finer spatial scales, and Landsat extending the historical record of observations.