Features
Primary productivity: the link to
global health
A look at how terrestrial ecosystems will respond to changing climate
O
ne of the greatest ecological stead act as a source, compounding
conundrums of our time cen- the problem. The lack of consensus
ters on the question of glo- stems from a lack of crucial data.
bal warming. Scientists have long
Filling the data gaps stood at censpeculated that increased atmo- ter stage during a symposium for
spheric carbon dioxide levels from plant ecologists and physiologists
the burning of fossil fuels would, held last March in Montpellier,
through an enhancement of the France. Participants sought to examGreenhouse Effect, lead to global ine how plant primary productivity
warming. Today, considerable evi- functions in the biosphere, particudence suggests that, indeed, the ob- larly in regard to global change.
served rise in Earth's mean tempera- However, the scientists used the
tures over the past century is a result meeting not only to look ahead at
of increased carbon dioxide and other ways to fill data gaps, but also to
look back to 1962, at another symGreenhouse gas emissions.
Moreover, scientists from a wide posium of plant ecophysiologists held
range of disciplines predict that, if in Montpellier, and to reflect on the
present trends in fossil fuel combus- history of their field over the 35
tion continue, so will global warm- intervening years.
ing, bringing with it profound
changes in the way humankind lives Primary productivity research:
and does business. Climate change a look back
carries the potential to alter crop
growing patterns, to increase the The term plant ecophysiology was
spread of diseases and of insect pests, coined by Frode Eckardt, a Danish
to raise coastal sea levels-the list botanist with the Institut de Biologie
goes on and on. At stake in the im- in Montpellier and the organizer of
mediate future are the vast economic the 1962 meeting. That meeting emimpacts for all nations of attempting phasized methods for studying the
to control global warming by reduc- physiology of vegetation above the
level of the individual plant. As
ing the burning of fossil fuels.
One critical question about glo- Eckardt wrote in the conference probal warming concerns how terres- ceedings, "With advances in microtrial primary productivity-the rate meteorology and data processing,
at which land plants fix carbon research on the physiology of plant
through photosynthesis-will re- communities or whole ecosystems was
spond to climate trends. Although coming within the range of possibilthe scientific literature is replete with ity." The symposium produced the
elements of the answer, scientists are landmark reference Methodology of
still far from a consensus on whether Plant Eco-physiology (UNESCO
the terrestrial biosphere will in the 1965), which included several chapfuture act as a sink for atmospheric ters concerned with the estimation
carbon dioxide, buffering increases of primary productivity.
The first concerted effort at develfrom fossil fuel combustion, or inoping a global database on primary
productivity began two years after
by Deborah Schoen
Eckardt's meeting, when the InterSeptember 1997
national Council of Scientific Unions,
a nongovernmental body funded primarily by its member organizations,
set up the International Biological
Programme (IBP). Citing the need
"for a better understanding of the
environment as the basis for rational
management of natural resources,"
the International Council chose terrestrial, freshwater, and marine primary productivity as the theme for
the IBP.
The first three years of the tenyear program were devoted primarily to planning and establishing national IBP organizations, with each
national program orienting and funding its subsequent research projects
according to its own particular objectives. Many IBP publications mention scientists' concerns with human
population growth and its demands
on world resources. However, IBP
interests lay more in basic scientific
knowledge than in solving human
problems. "Today, we often think in
terms of policy issues driving science," says Hal Mooney, of Stanford
University. "But in the 1960s, [the
motivation] was a feeling that we
needed more basic information about
Earth's biological processes, particularly productive capacity. The natural driving force for the IBP was not
policy but rather global information
gathering. "
IBP research focused on harvesting and measuring plant biomass at
regular intervals to estimate plant
growth, or what is termed net primary productivity. Researchers also
measured gas exchange by enclosing
plant parts or portions of plant communities in chambers and recording
the uptake of carbon dioxide. Such
investigations had been done before,
but IBP put the research on a global
477
Frode Eckardt, the plant physiologist who called the first Montpellier meeting on primary
productivity in 1962, gathers data in a sunflower field at France's Centre d'Ecologie in
1967. The assimilation chamber (left) measured carbon dioxide and water vapor, while
the device in the center measured canopy light interception. Eckardt died in 1990, but last
March plant physiologists again gathered in Montpellier to examine primary productivity and its effect on global warming. Photo: Centre d'Ecologie Fonctionnelle et Evolutive.
scale by seeking to collect data on
representative ecosystems from all
parts of the world.
Thanks to this effort, estimates of
global net primary productivity were
in hand by 1975. Up to three-fourths
of current productivity data come
from the IBP era, says Sam McNaughton, of Syracuse University, adding
that in following years IBP ecologists moved away from productivity
research and toward investigations
of much more detailed processes.
In the 1980s, however, a second
generation of terrestrial productivity research began, arriving through
the back door of weather satellite
programs. Starting in 1983, ecologists began to compile and analyze
data from satellites, such as the National Oceanic and Atmospheric
Administration's Advanced Very
High Resolution Radiometer. The
remote sensing measurements include
solar radiation reflected in both the
478
visible and infrared spectral domains,
which depend on the amount of vegetation covering Earth's surface and
its physiological state.
The resulting "greenness index"
and accompanying data on surface
temperature have helped scientists
to create models aimed at estimating
primary productivity. "But the models have not yet been sufficiently
validated with the estimates derived
from the more traditional methods,
such as those used in the IBP," says
Bernard Lacaze, of the French National Scientific Research Center.
Lacaze points out, however, that one
of the great advantages of remote
sensing methods is that they yield
long-term data.
In addition, because different satellite sensors have different resolutions, satellites provide data at various spatial scales. This variability
allows ecologists to analyze year-toyear changes in productivity and to
explore how observations made at
one spatial scale can be translated to
larger or smaller scales . Most important, the satellite images have documented widespread changes in land
use, such as deforestation and forest
regrowth-critical factors in the global carbon balance.
According to Bernard Saugier, of
the University of Paris-Sud, the need
to validate remote sensing models
renewed ecologists' motivation for
measuring net primary productivity
on the ground. At the same time,
researchers developed rnicrorneteorological methods that measure carbon dioxide flux above plant canopies . These methods led to better
estimates of net ecosystem productivity-equivalent to the rate at which
plants take up carbon dioxide for
photosynthesis minus its rate of release through plant, animal, and
microbial respiration-which, in
turn, can be compared with net primary productivity.
Global productivity and the
missing sink
Ecosystem productivity research continued with little fanfare throughout
the 1980s. But in 1990, following
publication of the second scientific
assessment of the Intergovernmental
Panel on Climate Control (IPCC),
interest in terrestrial productivity
surged among scientists concerned
with global climate change. That in terest remains strong to this day.
The IPCC was set up in 1988 by
the World Meteorological Organization and the United Nations Environment Programme "to assess more
closely the current state of knowledge on climate change." With this
objective in mind, IPCC scientists
carried out a global carbon mass
balance based on data from 1980 to
1989, adding up the two major inputs responsible for increasing atmospheric carbon dioxide-fossil
fuel combustion and deforestationand subtracting the modeled ocean
uptake. The resulting figure would
represent the increase in atmospheric
BioScience Vol. 47 No. 8
carbon dioxide levels for the ten-year
period, presuming that all sources and
sinks were accounted for.
In fact , atmospheric carbon dioxide levels had increased more slowly
than the mass balance equation predicted, leading IPCC researchers to
speculate that the difference was due
to increases in terrestrial primary
productivity. The terrestrial biosphere accordingly acquired the
popular but misleading label of
"missing sink." As symposium participants pointed out, the terrestrial
biosphere's increased uptake of carbon dioxide was "missing" only in
the sense of not having been included
in the original mass bal ance. According to later IPCC assessments,
th e terrestrial biosphere took up the
equivalent of approximately one quarter of the carbon dioxid e reA tower of man y measurements: Infr ared devices on thi s tower measure car bon
leased by fossil fuel combustion in dio x ide and wat er va po r fro m a boreal fo rest in France, wh ile an anemometer
the 1980s. The oceans took up an- records wind veloci ty . Eq uipment such as thi s is used in studies o f pr im ar y
other quarter, and the other half productivit y and may offer clue s ab out how th e terrestrial biosphere w ill affect
remained in the atmosphere.
glo bal warming. Photo: Dermi s Baldocchi.
To distinguish ocean uptakes of
carbon dioxide from terrestrial up takes, scientists have used measure- or limit this uptake. These force s ofLanguedoc-Roussillon, focu sed on
ments of atmospheric carbon-13 gra - range from increasin g atmospheric identifying information needs. Pardients. Using this technique, Philippe carbon dioxide to increasing levels ticipants put these information needs
Ciais, of the Climate and Environ- of nitrogen, from agricultural fertili- in the context of a question unimagined
ment Modeling Laboratory in Gif- zation and pollution to forest re- in 1962-how the terrestrial biosur-Yvette, France, provided addi- growth, changes in water availabil- sphere will respond to and influence
tion al evidence for the existence of a ity, loss of spe cies diversity, and climate change.
significant terrestrial sink. Ciais increases in glob al temperature. "DeThe scientists cited two needs comviews the partitioning of carbon di- pending on the dri ving forces, the mon to many areas of ecological reoxide between the oceans and the biosphere uptake could continue in- search: the need for more informaterrestrial biosphere as more than an definitely, could follow another tra- tion o n year-to-year variation in
academic question. Knowledge of jectory, or it could sto p tomorrow," primar y productivity and the need
these roles will allow us to predict says Chris Field, of the Carnegie In- for better methods of extrapolating
future levels of carbon dioxide and stitution of Washington in Stanford, to large regions the results of experihow we can deal with them. " The California.
ments carried out over a limited area.
magnitude of the maximum atmoMore sur pris ingly, however, th e
spheric carbon dioxide level in the Going underground
ecoph ysiologists admitted that the y
next century depends on the relative
had not been pa ying enough attenimp ortance of these two sinks," he The 1962 M ontpellier symposium tion to what was happening below
says . " Carbon fixed by th e bio sphere discussed methods for measuring ground-th e productivity of root
will re-enter the atmosphere with in primary productivit y and other eco- syste ms and associated nutrient cy20 or 30 years. In the oceans, carbon ph ysiological par am et ers. By con- cling. "Unless we get the belowdio xid e is dissolved and isolated at the tra st, th e 1997 symp osium, orga- gro und estimation in net primar y
deep sea levels for severa l millenni a." nized by the Ce n t re d 'Ecologi e pr oductivity, we will never get it
Future uptake of carbon by the Fonctionnell e et Evolutive of the right ," sai d David Hall, of Kin g' s
terrestrial biosphere cannot, how- French N ational Scientific Research College, London. He highlighted his
ever, be forecast without a better Center and co sponsored by NASA group's work on tropical grasslands,
understanding of the force s th at drive and the French regional government in which they showed that failu re to
September 1997
479
o
Wildlife.
Life in the wild can be pretty tough
these days. Without the necessary
ancient-forest habitat to live in,
some species like the northern
spotted owl of the Pacific Northwest are severely threatened .
At the Sierra Club, we believe that
these owls and the ancient forest
ecosystems they depend on need
our help. The Sierra Club's work
to permanently protect our ancient
forests also helps preserve the
habitat of the northern spotted
owl, giving them the range they
need to help their population grow.
measure root and shoot production
resulted in a threefold to fivefold
underestimate of net primary productivity. The importance of belowground productivity was echoed repeatedly as symposium participants
stressed the importance of root systems to ecosystem water flux, carbon storage, and nutrient cycling.
Participants also acknowledged a
past bias for measuring aboveground
biomass production. Sampling below ground, everyone agreed, can be
difficult and laborious. During the
IBP era, the focus was on primary
productivity as an energy source available to higher trophic levels. This research emphasis, together with the
technical difficulties of sampling roots,
led many scientists to limit their estimates to aboveground increases in
biomass or to only approximating root
productivity. "Yet, roughly one-third
of the net primary productivity of the
world's ecosystems is in fine roots,"
according to Rob Jackson, of the University of Texas in Austin . "In some
plant species, the roots go 30 feet or
more below the surface. We almost
never sample at that depth."
With improved belowground information, additional directly and
remotely sensed data on productivity, and better methods for scaling
up results from physiological experi-
ments, ecologists may, in the near
future, more confidently forecast the
behavior of the terrestrial biosphere
in response to global change. jacques
Roy, of the French National Scientific Research Center, adds, "To be
useful, ecological models need to incorporate information on the control of primary productivity. Among
the [different controls], atmospheric
carbon dioxide and biodiversity are
currently given high research priority. In most cases, trophic interactions, particularly in the soil, are
found to be a key, but insufficiently
understood, factor."
For participants of the 1997 symposium, the time has come to set
ecological research on a better course
for understanding the forces that
control primary productivity of the
terrestrial biosphere. "We're talking
about dramatic changes in the world's
economy as a result of limiting fossil
fuel combustion," says Chris Field. "It
would behoove us to have the clearest
possible idea of what is controlling a
sink that currently accounts for about
one-fourth of fossil fuel carbon dioxide emissions. "
0
Freelance writer Deborah Schoen is a resident of Montreal, Quebec. She covered
the Montpellier meeting while on a 12month visit to France.
To learn more about our work protecting the forest habitats of endangered species such as the
northern spotted owl, please write
us at:
S rra Club, Dept. PB
730 Polk Stre t
San Francisco, CA 94109
(415) TI6-2211
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BioScience Vol. 47 No. 8