1. A glowing way to monitor forest health
A
faint glow emitted from plants dur-
ing photosynthesis could be key to
measuring the health of large areas
of forests and croplands in real time. The
glow can be detected by spectrometers
aboard orbiting satellites. Currently,
the standard technique for measuring
photosynthesis, called eddy covariance,
relies on ground-based, tower-mounted
detectors that monitor only smaller tracts
of vegetation.
Through a ground-based study, a
research team has now helped to confirm
that this subtle glow, known as solar-in-
duced chlorophyll fluorescence (SIF), can
serve as a strong proxy for photosynthetic
activity in a deciduous forest. The glow is
invisible to the naked eye.
The team was led by geoscientists from
Brown University in Providence, R.I., and
from the Marine Biological Laboratory in
Woods Hole, Mass. Its findings have been
published in the journal Geophysical Re-
search Letters (doi: 10.1002/2015gl063201).
“We found that solar-induced chloro-
phyll fluorescence is highly correlated
with canopy photosynthesis at diurnal and
seasonal scales,” said Xi Yang, a postdoc-
toral researcher at Brown and the study’s
lead author.
“This is the first time anyone has
linked fluorescence to photosynthesis over
a long time scale in a deciduous forest and
validated orbital measurements of fluores-
cence with ground-based measurements.”
“Photosynthesis plays an important role
in the global carbon cycle and climate
change studies, but we cannot estimate
it accurately at global scale.” Yang
indicated that he was inspired by works
in Geophysical Research Letters (doi:
10.1029/2011gl048738) and in Biogeosci-
ences (doi: 10.5194/bg-8-637-2011), each
of which independently estimated global,
solar-induced chlorophyll fluorescence
using satellite data.
“However, no ground validations of
these satellite estimates of SIF have
been done, and the relationship between
SIF and photosynthesis at different time
scales [is] not entirely clear. This is an
important research question, and I like
to design field instruments, so we started
this work.”
In the photosynthetic process, chlo-
rophyll molecules in the leaves of plants
absorb photons from sunlight. The plant
converts this light energy into sugar and
other carbohydrates using carbon dioxide
absorbed from the atmosphere. The faint
glow is the result of a small percentage of
photons not absorbed by the chlorophyll.
They are re-emitted as lower-energy
photons.
During the summer of 2013 in Massa-
chusetts’ Harvard Forest, the researchers
compared photosynthesis measurements
from the forest’s eddy covariance tower
with fluorescence data taken with their
tower-mounted spectrometer system.
Yang said he designed the system,
FluoSpec, to measure chlorophyll fluo-
rescence. “We used one spectrometer
to measure both solar and vegetation
spectra, with the help of a mechanical
switch – it switches between the incoming
lights from sun and vegetation, so that we
can automatically measure both spectra
in a short time period.” Two long fiber
optics – one pointed at the sky and the
other at the vegetation – were connected
to the switch.
The readings from the ground-based
spectrometer were compared to readings
from a spectrometer aboard the European
Space Agency’s Global Ozone Monitor-
ing Experiment-2 (GOME-2) satellite.
The researchers found that fluorescence
measurements from the ground-based
spectrometer and the satellite were tightly
correlated to photosynthesis as measured
by eddy covariance.
The study also revealed that the fluores-
cence measurements appeared to capture
day-to-day fluctuations in photosynthe-
sis, as well as fluctuations over time.
Fluorescence could be a much better way
of getting real-time data compared with
other remote-sensing methods now in use,
Yang said.
Yang said that there are proposals to
launch an orbital system to track photo-
synthesis but none have yet been funded.
He is optimistic, though. “Photosynthesis
is an important process and has enor-
mous applications in both basic research
and people’s daily life. As technology
progresses, we will see satellites for this
purpose.”
Caren B. Les
caren.les@photonics.com
POSTSCRIPTS
Chlorophyll fluorescence, an invisible glow produced by plants as a byproduct of photosynthesis, could provide a way for orbiting satellites to measure global
photosynthetic activity. Scientists have conducted a study to help confirm that the fluorescence is a good proxy for photosynthesis in a forest canopy. Courtesy of
Marc Mayes/Brown University.
Reprinted from the July/August 2015 issue of BioPhotonics © Laurin Publishing Co. Inc.