Groundbreaking maps of land-based plant fluorescence are revealing new information for the first time regarding vegetation spanning the entire globe. To date, most satellite-derived information related to the health of vegetation has come from “greenness” indicators based on reflected rather than fluorescent light, a difficult-to-detect reddish glow that plant leaves emit as a byproduct of photosynthesis. Greenness typically decreases in the wake of droughts, frosts or other events that limit photosynthesis and cause green leaves to die and change color. However, there is a lag between what happens on the ground and what satellites can detect. It can take days or weeks before changes in greenness are apparent to satellites. A first-of-a-kind global map of land-based plant fluorescence shows stronger photosynthetic activity in the Northern Hemisphere in July, when light and temperature conditions were most conducive to plant growth, and the reverse in December. The maps are based on data from a spectrometer aboard the Japanese satellite GOSAT. (Images: NASA Earth Observatory) Chlorophyll fluorescence offers a more direct window into the inner workings of the photosynthetic machinery of plants from space. “With chlorophyll fluorescence, we should be able to tell immediately if plants are under environmental stress — before outward signs of browning or yellowing of leaves become visible,” said biologist Elizabeth Middleton, a member of the map-making team, which is based at NASA’s Goddard Space Flight Center. The new maps, based on data collected in 2009 from a spectrometer aboard a Japanese satellite called the Greenhouse Gases Observing Satellite (GOSAT), show sharp contrasts in plant fluorescence between seasons. In the Northern Hemisphere, for example, fluorescence production peaked during July, while in the Southern Hemisphere, it did so in December. The new findings help confirm previous lab and field experiments that suggest chlorophyll fluorescence should taper off in the fall as the abundance of green foliage declines and stress increases as a result of lower temperatures and less favorable light conditions. Although additional research is required to sort out the subtleties of the fluorescence signal, the new maps are significant because they demonstrate the feasibility of measuring fluorescence from space. In the future, the Goddard team expects that fluorescence measurements will complement existing measures of “greenness” in a variety of ways. They could help farmers respond to extreme weather or make it easier for aid workers to detect and respond to famines. Fluorescence also could lead to breakthroughs in scientists’ understanding of how carbon cycles through ecosystems — one of the key areas of uncertainty in climate science. “What’s exciting about this is that we’ve proved the concept,” said Joanna Joiner, the leader of the Goddard team. “The specific applications will come later.” The new findings have implications for both current and upcoming satellite missions. In the near term, awareness of the fluorescence signal should help atmospheric scientists refine measurements of carbon dioxide and methane from the GOSAT mission. The maps, published online in the journal Biogeosciences, represent just a first attempt to detect terrestrial fluorescence on a broad scale and will be enhanced and expanded over time, the scientists involved in the project emphasized. More work must be done, for example, to understand how plant fluorescence varies depending on light conditions. In strong afternoon light, the conditions under which GOSAT made its observations, unstressed plants produce a stronger fluorescence signal than stressed plants. However, complicating matters, the reverse is true in the morning or evening, when light is less intense. To disentangle the two opposing effects, the Goddard-based group plans to continue refining the mathematical methods they used to calculate fluorescence. Meanwhile, groups of scientists at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. — as well as Japanese and European research groups — are honing similar fluorescence-monitoring methods. For more information, visit: www.nasa.gov