Let's Exploit Available Knowledge on Vegetation Fluorescence

The potential to measure vegetation fluorescence from space (1) and to derive from it direct information on the gross primary productivity (GPP) of terrestrial ecosystems is probably the most thrilling development in remote sensing and global ecology of recent years, as it moves Earth observation techniques from the detection of canopy biophysics (e.g., fraction of absorbed radiation) and biochemistry (chlorophyll and nitrogen content) to the realm of ecosystem function.

The existence of a functional relationship
between fluorescence and photosynthesis
has been elucidated over the last decade
by several laboratories, notably as part of the
preliminary studies of the European Space
Agency Fluorescence Explorer (FLEX) Earth
Explorer Mission.
The empirical observation presented by
Guanter et al. (2) of a linear relationship
between fluorescence radiance and GPP,
however, provides the first experimental confirmation
of the feasibility of the approach—
already thoroughly tested at leaf level—at the
desired scale, despite the confounding effects
associated with the satellite detection of such
a faint signal.
A word of clarification is needed here.
The use of fluorescence as a probe of leaf
photochemistry has been a staple of plant
ecophysiology for decades, rooted in a sound
understanding of photosynthetic energy dissipation.
However, most past studies had to
rely for the interpretation of results on active
(pulse-saturated) techniques, making them
unsuitable for remote-sensing applications.
Over recent years, however, novel process based
models have been developed for the
interpretation of steady-state, solar-induced fluorescence at the leaf to canopy scale (3).
We are therefore in a position to move beyond
the mere empirical observation of an
association between GPP and fluorescence
radiance.
In particular, Guanter et al. (2) base their
analysis on the assumption of a constant ratio
between photosynthetic and fluorescence
light use efficiencies (equation 3 in ref. 2).We
know, however, that the ratio is not constant,
but changes widely in response to light, CO2,
stomatal limitations, and extreme stress (4,
5). What’s more, we can make sense of such
changes, thus extracting valuable information
from the very scatter that is apparent in
their data.
However, this process will require the
availability of more tailored instruments,
such as the one planned for the FLEX
mission. As already stressed by Guanter
et al. (2), the spatial resolution of the
Global Ozone Monitoring Experiment-2
sensor (40 × 80 km) makes it difficult to
compare meaningfully the fluorescence signal
with ground measurements, when only
60–70% of the footprint consists of the desired
land-cover type (table S1 in ref. 2),
suggesting that this could be largely responsible
for the low signals observed in
European grasslands. Moreover, the overpass
time of the MetOp-A satellite (9:30 AM)
implies that fluorescence is generally measured
under light-limiting conditions, when
fluorescence is only marginally affected by
stomatal closure even under stress conditions.
This result could explain the seasonal
mismatch with daily GPP observed
in natural ecosystems in the absence of irrigation
(figure 4 in ref. 2). We hope, therefore, that this welcome
contribution to this fast-advancing field will
help demonstrate the potential of the new
technique, and pave the way for more refined
studies under both a technological and scientific
point of view.