The United States of America is witnessing a decline in its forest cover. Rising forest areas on the country’s east coast and west coast are rising sea levels and wildfires, respectively. The effects of loss and decaying trees on atmospheric carbon dioxide concentrations and climate change are difficult to assess from the ground. To better assess forest health for upcoming Earth-observing missions, NASA research scientist John Ranson is trying to develop new technology to examine trees from the atmosphere.
“Trees do a huge service to the planet, sequestering carbon dioxide, taking it out of the atmosphere and putting it into wood,” Ranson said. But trees are very sensitive to our changing climate. We are trying to see the changes in the forest ecosystem. If you detect things early enough, you may be able to do something about it,’ Ranson said in a statement.
The wavelengths of light reflected by plants can be observed from airplanes and satellites to allow scientists to measure the rate of photosynthesis and, consequently, the amount of atmospheric carbon dioxide that plants absorb and store. The average of the two broadest regions of the infrared spectrum, known as the NDVI, or Normalized Differential Vegetation Index, is the current benchmark for vegetation research. NASA’s 40-year history of NDVI data provides a low-resolution but accurate picture of forest conditions.
‘Trees do a huge service for the planet,’ says NASA scientist Ranson
According to Ranson, while NDVI is useful for measuring plant abundance and vigor, a technology known as hyperspectral imaging that breaks infrared and visible light into many more wavelengths can shed light on changes in plant water content, chlorophyll levels and even health.
‘Plants have this broad spectrum of properties,’ said Ian Adams, Earth Sciences Division technologist at NASA’s Goddard Space Flight Center. ‘With hyperspectral imaging, you get many different measurements at small, close frequencies. There’s a lot more information if we can get better spectral resolution.’
Ranson’s work supports the priorities of the recent Earth Science Decadal Survey, which establishes the field’s long-term priorities by significantly increasing the number of frequencies available for investigation. “Surface topography and vegetation” is included in the survey’s list as an important area of study requiring more sophisticated techniques. Adams added, “Strategically for NASA, and more broadly for the remote sensing community, hyperspectral is an area that we see as the future.”
Future orbital missions will use hyperspectral imaging, but the first data analysis methods must be tested close to the ground. A visible and infrared (VIS/IR) hyperspectral camera and lidar technology was installed by Ranson’s team on a Skyfish drone (UAV) from partner University Virginia Tech. At a place called Mountain Lake near Blacksburg, Virginia, they flew the imaging equipment over the forest.
Ranson said in the project ‘Are our forests healthy’ and if not, why not’
Ranson’s team improved estimates of how much carbon was removed from the atmosphere by comparing their UAV observations with actual CO2 levels measured by sensors on nearby National Ecological Observatory Network towers.
They were able to improve their methods for analyzing hyperspectral data through this comparison. For example, his sensors can detect situations in which plants under stress from too much sunlight release pigments to protect their chloroplasts. Plants may develop leaves with a larger surface area if they receive too much shade, causing the sensor to overestimate plant productivity. To more accurately determine reflectance from leaves and other plant material and eliminate a potential source of inaccuracy, Ranson plans to include short-wave infrared sensors.
Ranson wants to study and examine forests around the world from space. ‘We’re trying to get the whole picture,’ Ranson said. “So that when we go into space, we know how to find out the answer to the question, ‘Are our forests healthy’ and if not, why not?”
A mission concept called the Concurrent Artificially Intelligent Spectrometry and Adaptive LIDAR System, or CASALS, was developed by Goddard scientists and engineers. If selected, CASALS will launch a satellite equipped with lidar and hyperspectral cameras. A constellation of such satellites could take the regular observations needed to quantify changes in forest productivity over time using models refined by Ranson’s UAV missions.
“Forests take up as much carbon as the ocean, yet forests cover only 9 percent of the Earth’s surface,” Ranson said. ‘So, if something goes wrong with our forests, it’s a dramatic thing. We’ve got this great, great resource in the forest, and we have to take care of them so they take care of us.’