Standing up to the Heat

A dry seasonal heat wave, known as a hamsin in Israel, usually sends residents indoors to the air conditioning. But a group of researchers from Austria, Germany and Italy made a special trip to Israel’s Negev desert early last summer during the height of a hamsin. The semi-arid pine tree forest there, surrounding the Yatir Research Station operated by the Weizmann Institute of Science for the past 18 years, provided the perfect testing ground for their innovative approach to checking how well plants and trees in a given area remove carbon from the atmosphere under different conditions.

Plants take up carbon and convert it to biomass during photosynthesis, so tracking photosynthesis on the level of a forest or ecosystem is vital to assessing how much carbon the living plants and trees can remove from the atmosphere. But current methods rely on indirect observation. In particular, they often employ satellite data that measure the reflection of solar light, using this as a proxy for photosynthesis. The problem is that from space evergreen forests like the Yatir pine forest look the same over the short periods in which heat waves pass through, or as long as there is no significant change in the leaves’ storage of chlorophyll (the pigment required for photosynthesis). That is, the amount of pigment may remain the same, even as photosynthesis levels go up or down.

The SIF method may eventually be used locally, as it was in the Yatir forest, or the equipment might be mounted on drones or planes, and on satellites

Some scientists had suggested a proxy – one that would give a closer estimate of photosynthesis: testing the light that is reemitted in the process. While most of the light plants take in is converted to chemical energy for photosynthesis, very small amounts are either dissipated as heat or reemitted as light. This re-emitted light, technically known as fluorescence, is limited to specific wavelengths, so measuring just those wavelengths would reveal the amount of photosynthesis taking place. But until recently, no one had managed to create a device to accurately measure the fluorescence from plants in the presence of sunlight, when actual photosynthesis occurs.

Rising above the forest canopy

Yatir Research Station, which is headed by Prof. Dan Yakir of the Weizmann Institute of Science’s Earth and Planetary Sciences Department, has been a magnet for research teams from Europe and the US wanting to investigate the flux of carbon and its compounds – including the major greenhouse gas, carbon dioxide – through the soil, biome and atmosphere. Both in the Yatir forest itself and a lab-in-a-truck known as the Biosphere-Atmosphere Research Mobile Lab, also operated by Yakir and his group, tall towers or masts enable equipment to rest above the forest canopy and conduct various experiments on plant-atmosphere interactions.

The scientists – Dr. Georg Wohlfahrt, Katharina Gerdel, Dr. Albin Hammerle and Felix Spielmann of the University of Innsbruck, Austria; Mirco Migliavacca of the Max Planck Institute for Biogeochemistry in Jenna, Germany; Tommaso Julitta of the University of Milano-Bicocca, Italy; and members of Yakir’s team: Dr. Eyal Rotenberg, Dr. Fedor Tatarinov and Jonathan Müller – set out to test a new fluorescence-tracking system developed in Italy and Germany.

“When we heard about the prototype of a new device to measure “sun induced fluorescence” (SIF), we invited the teams working on it to bring the equipment to Yatir to try it out,” says Yakir. “One of the attractions here in Israel is the seasonal hamsin events (extreme heat waves lasting three to five days) that provide an excellent opportunity for experimentation, as the only environmental variables that change during a hamsin are temperature and humidity, but these change drastically. This means that we could directly measure the effects of the heatwave on the photosynthetic level.” Indeed, the European team arrived with the equipment and hooked it up to the research station just at the start of a hamsin season. A sophisticated camera-like device hoisted to the top of the tower was aimed at the foliage and connected through optical sieves to a very high resolution spectrometer. Further computer analysis translated the fluorescence signal of the trees to an indicator of photosynthetic activity. The inventors hope that the SIF method may eventually be used locally, as it was in the Yatir forest, or the equipment might be mounted on drones or planes, and on satellites.

A dramatic drop

The findings of the experiment – a dramatic drop in photosynthesis right at the peak of the heatwave – showed the advantage of using sun-induced fluorescence:  “Photosynthesis can slow and ultimately stop for two reasons,” says Yakir. “Either the plants tightly close the pores in their leaves, called stomata, limiting evaporation but also carbon dioxide uptake; or else there is direct damage to the photosynthetic mechanisms in the leaves.” Combining SIF with other methods developed in Yakir’s lab, the team showed that during the hamsin, the first effect – the stomata closing – developed along with the rising temperatures; at the height of the hamsin, however, the internal damage to photosynthesis occurred and this was reflected in the sharp dip in the trees’ fluorescence.

The levels of photosynthesis returned to normal within 24 hours after the hamsin had passed. “It’s a sign that even after suffering extreme heat – a phenomenon that is becoming increasingly frequent – the forest is, for now, resilient,” says Yakir.

The findings provide evidence that the SIF method, because it tracks light that is emitted from the internal structures of the leaves, rather than that reflecting off them, is indeed a powerful new and better way of assessing photosynthesis than existing methods. This knowledge is crucial for understanding the role that forests play now and will play in the future in removing carbon from the atmosphere.

Prof. Dan Yakir's research is supported by the Sussman Family Center for the Study of Environmental Sciences; the Cathy Wills and Robert Lewis Program in Environmental Science; and Dana and Yossie Hollander.