Where the plants are – new global fluorescence map

First-of-its-kind fluorescence map offers a new view of the world’s land plants

A first-of-a-kind global map of land 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. Credit: NASA's Earth Observatory

Scientists from NASA’s Goddard Space Flight Center in Greenbelt, Md., have produced groundbreaking global maps of land plant fluorescence, a difficult-to-detect reddish glow that leaves emit as a byproduct of photosynthesis. While researchers have previously mapped how ocean-dwelling phytoplankton fluoresce, the new maps are the first to focus on land vegetation and to cover 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. 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 — even weeks — before changes in greenness are apparent to satellites.

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 Elizabeth Middleton, a NASA Goddard-based biologist and a member of the team that created the maps.

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 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.

While additional research is required to sort out the subtleties of the fluorescence signal, the new maps are significant as 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 could also 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 proven the concept,” said Joanna Joiner, the deputy project scientist for NASA’s Aura mission and the leader of the Goddard team that created the maps. “The specific applications will come later.”

Glowing Plants?

The same mechanism that makes plants fluoresce causes a range of everyday objects — ground-up plant leaves, white shirts, jellyfish, and even blood and urine — to glow intensely under black light.

However, plants fluoresce in specific parts of the blue, green, red, and far-red spectrum. Chlorophyll fluorescence from green foliage, for example, is produced at the red and far-red wavelengths.

“In plants, fluorescence is not something that you can see with your naked eye because background light overwhelms it,” explained Joiner, the lead author of the paper. When sunlight strikes a leaf, disc-like green structures called chloroplasts absorb most of the light and convert it into carbohydrates through photosynthesis.

Chloroplasts re-emit about two percent of incoming light at longer, redder wavelengths. This re-emitted light — fluorescent light — is what the Goddard scientists measured to create their map. Fluorescence is different than bioluminescence, the chemically-driven mechanism lightning bugs and many marine species use to glow without exposure to light.

For decades, scientists have measured fluorescence in plants by exposing leaves to laser beams that, like black light, make fluorescence more apparent. Such experiments have revealed much about how certain types of plants fluoresce, but researchers have not been able to use lasers to measure the phenomenon across broad swaths of the Earth’s surface.

To create their global fluorescence map, Joiner and her colleagues used a different technique. They analyzed an unusually dark section of the infrared portion of the solar spectrum embedded within a feature called a “Fraunhofer line.” There is little background light at the line they focused on — at about 770 nanometers — which made it possible to distinguish the faint fluorescence signal.

The Future of Fluorescence

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 creation of the maps also bolsters the argument that an experimental mission being developed by the European Space Agency (ESA) — the Fluorescence Explorer (FLEX) mission — would make significant breakthroughs. The ESA is currently in the midst of feasibility studies and has not yet set a launch date for FLEX.

The findings also suggest that NASA’s Orbiting Carbon Observatory-2 (OCO-2), a mission that is designed to measure carbon dioxide levels much like GOSAT, should be able to make useful fluorescence measurements on a global scale. OCO 2 will launch no earlier than February of 2013 from Vandenberg Air Force Base in California.

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 needs to be done, for example, to understand how plant fluorescence varies depending on light conditions. In strong afternoon light, the conditions that 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 in the process of honing similar fluorescence-monitoring methods.

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26 thoughts on “Where the plants are – new global fluorescence map

  1. Good to see proof that plants grow better in summer but don’t grow during the cold winter. I kind of knew that.

  2. Environmental sciences contain far too much ‘choose your assumption and adjust the data to prove it’.

    The above report says;
    “In strong afternoon light, the conditions that 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.”

    Oh dear!
    I have a srong suspicion as to how they will “continue refining the mathematical methods they used to calculate fluorescence” and what the effect of that “refining” will indicate.

    I sincerely hope my suspicion is wrong.

    Richard

  3. Omnologous: See this item here
    The Pindan Woodland and Vine forest near Broome, http://pindanpost.com/2011/04/18/the-pindan-woo…ts-near-broome/
    Many of the plants in sub-tropical Broome, like in this column, slow their growth during winter when it gets cold in July, then about August, flowering and fruiting takes place on a massive scale until October.
    The hottest month of November finds a large majority of species leafless, while other species that dropped leaves earlier receive a new flush of plant growth.
    This all happens during the period of April to January, whilst no rain usually falls.
    Desert Heat is another column I wrote describing a visit at the hottest time of year, http://pindanpost.com/2011/04/19/desert-heat/

  4. Without the captions, I would have assumed that the winter map showed the extent of glaciation in the last Ice Age. It’s awfully close!

    In other words, the winter maps are partly or mostly showing snow cover instead of photosynthesis.

  5. Richard S Courtney says:
    June 7, 2011 at 3:37 am

    Woe betide anyone finding conclusions opposite to the mantra of “it’s worse than we thought!”

    Enjoy the interglacial, everyone :-)

  6. So, essentially, the satellite can’t detect any ongoing photosynthesis under a blanket of snow during winter in the northern hemisphere, but detects that the Sahara desert is covered by photosynthetic biological machinery all year around.

    The dispersion of greenness is kind of different looking from below though. :p

  7. It would be rather nice if someone could match the levels of photosynthesis, land and ocean, over the course of a year and mate it to the saw-tooth of the Keeling curve.
    I would really like to know if the annual decrease tracks biota or ocean cooling.

  8. I dunno. The images show Australia as distinctly more purply/mauve than either India or the British Isles. If that really means that dry Oz has more vegetation per unit of land, then that seems decidedly counter intuitive.

  9. Is it the Fraunhofer “A” line, at 759.4 – 762.1 nm (O2) that they’re looking at?

  10. You wouldn’t think so but Australia is actually very lush in parts. I flew from Sydney to Darwin a few days ago – while the red centre was very reminiscent of Mars, flying over the south east and over Arnhem Land was very green. The Cape York peninsula is also very green. The Indian subcontinent has a great deal of desert in the North West as does much of Afghanistan/Iran and neighbouring central Asian states (I remember being struck by the sheer barrenness of the landscape overflying it in the mid 1990′s). Australia has also recently had record rainfalls with rejuvenation of much parched soil. Maybe not so counterintuitive after all.

  11. 1DandyTroll says:
    June 7, 2011 at 4:20 am

    So, essentially, the satellite can’t detect any ongoing photosynthesis under a blanket of snow during winter in the northern hemisphere, but detects that the Sahara desert is covered by photosynthetic biological machinery all year around.

    The dispersion of greenness is kind of different looking from below though. :p

    A seriously good point!

  12. Robert Morris says:
    June 7, 2011 at 4:37 am

    I dunno. The images show Australia as distinctly more purply/mauve than either India or the British Isles. If that really means that dry Oz has more vegetation per unit of land, then that seems decidedly counter intuitive.

    Well, most of Oz has some kind of vegetation all year round. There is the desert, or course, named with particularly Scottish pragmatism, the ‘Great Sandy Desert’ (as opposed to the ‘Snowy Mountains’ – want to guess what happens to them in the winter?).

    However, as 1 Dandy Troll pointed out earlier, the Sahara is similarly painted with good photosynthesis, so I agree that it is probably BS. Smell test fail, IMO.

  13. Although the SH certainly lacks the extensive upper latitude non polar landmass of the NH, I am surprised that places like Patagonia do not incur the mid winter die off seen in North America and Eurasia.

  14. Qualitatively, those maps are very interesting, and surprising, too.

    They will peruse the hell out of what the images show, but here are some of what I see:

    1. The eastern US in July seems to be about the deepest blue.
    2. In July, Africa north of the Equator seems to be at its bluest. I would have thought it the other way around – hottest and driest in July.
    3. The Arabian peninsula seems to have plant growth in both July and December. Both seem to disagree with Google Earth and other satellite images, which show most of Arabia being abject desert.

    Due to 2. and 3., I wonder if they are getting false positives.

  15. Our initial results showed positive values of fluorescence
    over the Sahara where none was expected. The root cause
    was found to be unexplained systematic spectral structure in
    the core of the K I line that we believe is due in part to un-
    dersampling and the effects of rotational-Raman scattering.

  16. In the north of Australia (Figs. 12–13, panel 7), EVI (and
    NDVI) generally peaks in the austral autumn owing to the
    summer monsoonal rains. In contrast, we see a more broad
    peak in F starting earlier. Higher spatial scale studies show
    significant variability in NDVI in this region as well as in-
    terannual variability (e.g. Martin et al., 2009). A different
    pattern is seen in the EVI in the south western region of Aus-
    tralia with a distinct peak in the spring (Fig. 12, panel 9).
    This pattern is not seen in the derived fluorescence. These
    regions warrant further study and investigation into poten-
    tial effects of sub-pixel variability and sources of error in the
    satellite-derived products.

  17. Basically, guys read the paper first before coming to conclusions. It’s an Open access paper. From a brief skim it looks a like a 2 degree grid map.

  18. I believe the fluorescence is from the effect of ultraviolet light on the magnesium in the chlorophyll molecule. There is very little UV light in the far north in winter but there is a lot of green so I suspect they are probably underestimating the amount of photosynthesis from pines and other evergreens. I would think they need to calibrate it using the UV index maps. They might already be doing that but it just seems that the large white spot in North America and Asia is a serious exaggeration. I suppose that might be true on a very cold day.

  19. To Steven Mosher – The map presented at the top of the article is a big problem. Those of us who have been in the desert, or even seen pictures of the desert, can say with certainty there should be no fluorescence from photosynthetic plants to the degree shown on the map in places like the Sahara and the Saudi Arabian peninsula. Those locations should be as white, or almost as white, as the polar regions. Therefore the whole thing becomes suspect.

  20. Tom Harley: “The hottest month of November finds a large majority of species leafless, while other species that dropped leaves earlier receive a new flush of plant growth.
    This all happens during the period of April to January, whilst no rain usually falls.”
    —————————–
    No rain April through January??? I suspect that’s why the the plants are leafless by November, not the heat. Lack of water is a far greater stressor than heat. Think of deserts and oasis.

  21. Jay Davis says:
    June 7, 2011 at 7:31 pm

    To Steven Mosher – The map presented at the top of the article is a big problem. Those of us who have been in the desert, or even seen pictures of the desert, can say with certainty there should be no fluorescence from photosynthetic plants to the degree shown on the map in places like the Sahara and the Saudi Arabian peninsula. Those locations should be as white, or almost as white, as the polar regions. Therefore the whole thing becomes suspect.

    Really, did you read the paper? So for example, when they compare this map with the normal maps that show “green” and they not the areas of agreement and the areas that still need more work, how does that make the WHOLE THING suspect. They are looking at a very specific fraunhaufer line. Now, to pull that line out with clouds and other effects is a process that will take more research. And when they themselves note issues in australia and the deserts, why do you think that you are pointing something out that the paper misses.
    This is the first of its kind map. Nobody expects it to be perfect and nobody claims it is perfect.

  22. The usual concept of fluorescence involves irradiation in the UV and emission in the visible, with a low conversion efficiency. It is also possible that visible light is being used here to excite fluorescence – I cannot see what wavelength is used – and it is known that chlorophyll-type compounds can emit in the red and infra-red.
    If the excitation is via UV, the big problem is getting the UV light onto the surface. Leaves coated with waxy compounds are not uncommon and even a thin water film will absorb low wavelength UV light. BTW, the selected Fraunhofer line is barely outside of visible red into the near IR. So, it has problems getting out of the atmosphere as well.

    Even if there is excitation from near UV or visible light being used, there is a problem in getting ground truth measurements of incoming flux because of clouds, shadowing, sun angle and a host of other variables. Therefore, the Visible or IR signal that goes to the satellite will be difficult to interpret because of the unmeasured nature of some important inputs. IIRC, the excitation physics tro produce fluorescence like this does not go into the visible; and the greater the wavelength gap between excitation wavelength and emission wavelength, the less productive the transition.

    An Australian complication is that, IIRC, there are only about 4 truly deciduous trees, most rare in extent, including Toona ciliata, Melia azedarach, Adansonia gibbosa and Nothofagus gunnii, the latter the most like NH Autumn trees that colour up in the Fall, but restricted to a few spots in the highlands of Tasmania.
    A further Australian complication is shown in the image here:

    The concept, however, is a useful one and I’m not knocking it. There is always room for experimentation with different parts of the EM spectrum – that’s how advances arise. This one just seems a bit behind the 8 ball from the beginning, but I lack a full description of the operating conditions and am happy to be shown wrong. From the maps above, I cannot guess at any obvious cause of the reported fluorescence in Australia.

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