A burst of forest reveals the awakening of hibernation

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The winters of the northern hemisphere are brutal. The harsh conditions cause some species to hibernate; bears reduce their metabolic state to conserve energy until the spring. Forests also endure the winter by saving energy; they block photosynthesis, a process by which a green pigment called chlorophyll captures sunlight and carbon dioxide (CO2) to produce the chemical energy that feeds the plants. The total production of chemical energy resulting from photosynthesis is called Gross Primary Production (BPP). GPPs in evergreen forests tell scientists how much CO2 these vast and remote systems breathe.

As photosynthesis extracts CO2 from the atmosphere, it is essential to understand the activity of the forest to track carbon levels around the world. For decades, scientists have used satellites to monitor changes in the greenness of hardwood forests to monitor BPP. In autumn and winter, deciduous leaves turn brown and fall when they are dormant. In spring and summer, chlorophyll returns as green leaves and photosynthesis accelerates. However, evergreen trees keep their green needles filled with chlorophyll throughout the year, preventing scientists from detecting the onset and decline of large scale photosynthesis.

For the first time, a new study has linked seasonal GPP cycles with a photosynthetic process, but recently followed by some satellites: Sun-induced fluorescence (SIF). Photosynthesis occurs when solar energy excites chlorophyll in a higher energy state. When chlorophyll returns to its normal state, it emits a photon, producing too little light for the naked eye. The resulting "glow" is SIF.

A collaborative team of researchers used a tower-based scanning spectrometer to measure fluorescent "radiation" throughout the season in a Colorado evergreen forest. The team is the first to associate SIF with needle physiology, canopy photosynthesis and satellite-derived fluorescence. They found that daily and seasonal SIF patterns were closely aligned with the timing and magnitude of the GPP. In the spring, conifers activate chlorophyll in their needles, resulting in both fluorescence and photosynthesis, which closely matches the SIF that satellites have recently been able to measure.

One of the ways that plants protect themselves during harsh winters is to deploy photoprotective pigments that act as a "sunscreen." The study revealed that when plants apply this sunscreen, photosynthesis and fluorescence decrease, allowing scientists to rely on the SIF signal as a proxy to monitor the respiration (CO2 uptake) of forests. evergreen leaves.

Scientists can now use satellite fluorescence measurements as an indicator of photosynthetic activity in evergreen forests on an unprecedented scale. By seeing the glow of evergreen forests from space, we can better understand how these forests respond to climate change.

"We are trying to develop techniques to be able to" see "photosynthesis on a large scale, so we know how much CO2 is consumed by the biosphere … to keep the pulse of the biosphere," said Troy Magney, researcher. NASA Jet Propulsion Laboratory and the California Institute of Technology.

Magney and the team collected data from a spectrometer system mounted at the top of a tower between June 2017 and June 2018 in a subalpine coniferous forest in Niwot Ridge, Colorado. They were able to unravel the physiological changes inside conifer needles to better understand why we are seeing seasonal SIF cycles. It turns out that everything is a matter of pigments.

"You and I can get a sunburn, too much ultraviolet light will damage our cells, some people can protect themselves – their skin produces more melanin, a pigment, to adapt to very bright environments," said David Bowling, Professor of Biology at the University of Toronto. Utah and co-author of the study. "Plants have a different, but similar process."

Without photosynthesis to use the sun's energy, plants must protect themselves. The researchers found that conifers produced a large amount of pigments that were part of the xanthophyll cycle, which protects its tissues against excess light. During the season, the fraction of "sunscreen" changes (more in winter, less in summer), decreasing both fluorescence and photosynthesis.

"Ultimately, measuring the low fluorescent radiation from plants will allow us to see exactly the timing and magnitude of carbon uptake by the terrestrial biosphere, which will help us understand how forests respond to climate change and suggest how they might respond to future climate change, "says Magney.