Eclipse Science at Hubbard Brook: Solar Radiation and Carbon Dioxide Flux

Fig. 1. Homemade contraptions for safely viewing solar eclipse: colander method, binocular method, cereal box method, big box method.

On August 21, 2017, people across the United States looked towards the sky for a partial or complete solar eclipse, the first such astronomical event since 1918. In a rare moment of unity, people in all corners of the country gathered together to witness a natural wonder.

At the Hubbard Brook Experimental Forest in New Hampshire, staff and scientists shared in the fun with a variety of contraptions, including a colander, cereal box, telescope and binocular make-shift viewers (Figure 1). Hannah Vollmer, Hubbard Brook field site technician, came to her work day prepared with a pin hole camera to view the event from Weather Station 1 at the forest (Figure 2); and Scott Ollinger, Professor of Ecosystem Ecology and Remote Sensing at the University of New Hampshire, got the award for most creative “telescope on forehead” device (Figure 3)!

While these homemade apparatuses helped people experience the solar eclipse, technical instruments at Hubbard Brook and other research sites, including pyranometers, net radiometers, and sophisticated whole forest canopy carbon flux towers, have been monitoring net solar radiation and whole forest atmospheric carbon dioxide exchange for decades. As wildlife biologists, outdoor enthusiasts, and pet owners across the country noted changes in canine, bird, and insect behavior during the eclipse, forest scientists measured the precise drop in incoming solar radiation, and consequent declines in plant photosynthesis and net uptake of carbon by forest canopies.

Fig. 2 Hannah Vollmer using pin hole camera to safely view solar eclipse at Hubbard Brook.

The Hubbard Brook Experimental Forest, nestled in the White Mountains of New Hampshire, experienced an approximate 50 percent obscuration of the sun. Data from the onsite pyranometer (i.e., solar radiation meter) at Hubbard Brook showed a ~33 percent drop in solar radiation during the short period of the eclipse, going from a high of 0.75 megajoules per meter squared before the eclipse began to a low at the peak of the eclipse of 0.25 megajoules per meter squared (Figure 4). As plants experienced the drop in light, real-time monitoring of carbon dioxide at a nearby flux tower at Thomson Farm in Durham, New Hampshire showed that net canopy carbon uptake declined in response to the false twilight of the eclipse (Figure 5).

Fig. 3 Scott Ollinger is part of a viewing apparatus with the telescope-and-forehead method.

Although the solar eclipse lasted for just a few short hours and resulted in a small and transitory decline in solar radiation and net plant photosynthesis at the Hubbard Brook Experimental Forest, the total reduction in solar radiation and photosynthesis across the entire path of the eclipsed sun translates to big numbers. It is also a reminder that plants, as well as animals, are living, dynamic organisms that respond to alterations in their environment, and that scientists, like those at Hubbard Brook, are consistently and methodically monitoring these responses through time and space.

Fig. 4. Solar radiation at Weather Station 1 for 12 hours around the solar eclipse (peak shown with red line) showing drop in solar radiation (data courtesy of Hannah Vollmer).

Fig. 5. Plants responded to the solar eclipse. As the amount of incoming sunlight (solar radiation, shown in yellow) dropped during the solar eclipse (from about 13:00–16:00 or 1:00–4:00 pm, shown in gray) plants at Thompson Farm Forest in Durham, NH responded with reduced carbon uptake (top) and water loss, known as evapotranspiration (middle). There was also a measurable drop in air temperature (bottom). Measurements of plant carbon uptake and water loss come from an eddy covariance tower funded by NH EPSCoR and the UNH Agricultural Experiment Station (data courtesy of Rebecca Sanders-DeMott).