An HDR-LVIS at The Nature Conservancy's Hart Prairie Preserve near Flagstaff, Arizona, tracking how forest and meadow plants respond to climate. Dennis Dye, USGS.
Phenology — the timing of seasonal plant growth — links vegetation to changes in climate and environment. Do grasslands and forests "green up" early or late, and grow more or less, in unusual weather?
Why ground-based sensors
Satellite and aircraft images are widely available but infrequent and coarse: one pixel often covers 900 square meters or more, blurring places where plants with different seasons grow side by side — like mountain meadows where grasses and evergreens mix. USGS scientists are building ground-based instruments that measure local phenology and sunlight more accurately; combined with regional imagery, they reduce uncertainty for scientists and land managers.
High dynamic range imaging
HDR imaging, familiar from everyday and artistic photography but new to scientific remote sensing, records a far wider range of brightness than an ordinary camera. It combines several quick exposures of increasing length so that nothing in the scene is washed out or too dark. The USGS built two instruments on it that work together — one measuring sunlight from the sky, the other how well the landscape's plants absorb it for photosynthesis.
HDR-LVIS: watching the vegetation
The High Dynamic Range Land Vegetation Imaging System uses color (red, green, blue) and near-infrared cameras to measure sunlight reflected from leaves, handling bright clouds and deep shadows that defeat ordinary cameras.
- For each pixel, it calculates the fraction of incoming light a leaf reflects to the camera, and from that an index of vegetation density and vigor — overall "greenness."
- Daily image series show how local vegetation changes over seasons and years — for example, how active plants are after rain — and help interpret the broader patterns seen by satellites.
HDR-LVIS images from Hart Prairie, October 2015: natural color, false-color infrared, near-infrared reflectance, and NDVI (vegetation "greenness"). USGS.
HDR-ASIS: measuring the sky
The High Dynamic Range All-Sky Imaging System points up and measures photosynthetically active radiation (PAR) — the sunlight plants use — from every angle of the sky, including the intense light straight from the Sun.
- Uses: its data feed computer simulations of photosynthesis to study how clouds and aerosols (dust, smoke, volcanic gases) change sunlight and, in turn, plant growth.
- Where: in near-continuous operation at the USGS campus in Flagstaff since 2012; second-generation instruments went to study sites in Brazil's Amazon in 2012 and 2015, in studies of tropical forest seasons sponsored by NASA and the Department of Energy.
An HDR-ASIS atop a 45-meter (148-foot) tower above the Amazon forest near Santarém, Brazil; inset, an HDR-LVIS that photographs the canopy daily. Dennis Dye, USGS.
HDR-ASIS views of the sky over the Amazon at midday on consecutive days in September 2012 (top), with how the light plants use is spread across the sky (bottom). USGS.
What's next
The instruments keep collecting sunlight and phenology data at Flagstaff and in the Amazon, and changes will be analyzed as records grow to years and longer. Real-time reporting and a larger network of instruments could improve understanding of climate change at regional and global scales — part of USGS work on the causes and consequences of land-use and climate change.
Sources
Based on Improved Ground-Based Remote-Sensing Systems Help Monitor Plant Response to Climate and Other Changes, USGS Fact Sheet 2016–3013, U.S. Geological Survey (DOI); a work of the United States government in the public domain. The photographs and images come from the fact sheet's PDF.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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