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Figure legends
An apparatus for measurement of photosynthesis and stomatal
conductance of attached leaves. The experiment is being conducted
near the top of an aspen (Populus tremulodes) canopy in Northern
Saskatchewan. The experiments are being conducted to calibrate a
leaf model of photosynthesis and stomatal conductance. Exchange
of CO2, and water vapor by the whole forest are measured using
mirometeorological methods from a nearby tower. The leaf model
is then used as part of a larger system of models to simulate the
whole system measurements. Calibration -- validation experiments of
this type are used to construct models of forest ecosystems that can
be used to simulate the forest - atmosphere interactions on a regional
scale.
 
Click on the image for higher resolution (Warning-116K!)
A schematic diagram of a model used to simulate ecosystem-atmosphere
interaction. Atmospheric boundary conditions include: wind speed,
, temperature, and concentrations of CO2, and water vapor at a
reference level above the canopy, shortwave, and longwave incident
radiation, and precipitation. The exchanges of radiation, momentum,
sensible heat, latent heat (the heat equivalent of evaporated
water) and carbon dioxide between the surface and the atmosphere are
simulated. The model can be run for peroids as long as a year
and keeps track of the leaf and soil tempertures, the soil moisture
content and the stomatal conductance. Transport of materials between
the soil, leaves, canopy air, and the atmosphere is treated as
an analog to an electrical circuit made up of resistors. The model
can be used with local climate measurements or run interactively as
part of a climate model.
Beginning in 1989, I began to devote a major portion of my time
to an interdisciplinary research project sponsored by NASA under its
EOS (Earth Observing System). The goal of this team is to assemble
the scientific basis to model the interactions between the biosphere
and the atmosphere on a global scale. My role in this work has been
to develop models of processes which regulate the conductance of
leaves and canopies to evaporation of water, and control the exchange
of CO2 between terrestrial ecosystems and the atmosphere. This work
has been based in large part on prior work in the 1980's on
photosynthesis modeling (conducted in collaboration with Graham
Farquhar) and analysis of stomatal conductance (with Tim Ball).
Two papers, Collatz etal (1991; 1992) describe coupled models of
photosynthesis and stomatal conductance for C3 and C4
leaves developed and tested in our lab. Sellers etal, (1992)
describes a simple theoretical scheme for integration of these
leaf models to the canopy-scale. Actual implementation of the concepts
described in these papers in a model that could run at a global
scale has taken a great deal of time and effort. This involved; recoding
the models to FORTRAN and integration with an existing land-process
model (SiB); development of new numerical schemes so that the
model would run efficiently when coupled to a global climate model,
and developing calibration data to fit the model to the diverse range
of vegetation types and climates that exist in the world. This
long-term research effort is only now reaching the stage of
publication. Three papers are now in press in { The Journal
of Climate} (Sellers etal, 1996a; 1996b and Randall
etal, 1996).
Another related interest has been to develop the basis for using
to use stable isotopes as tracers of global scale processes. Papers
by Tans etal, (1993) and Ciais etal, (1995) contribute
toward understanding the influence of ocean and terrestrial biosphere
on the C13/C12 ratio, and paper by Yakir etal. (1994) concerns
the O18/O16 ratio, of atmospheric CO2. A paper by Guy etal,
(1993) examines isotope discrimination in the biological cycling of oxygen.
A conference paper (Berry, 1992) provides an overview of this
work.
During this time I have had one Stanford Ph.D. student, Zoe Cardon.
Her work dealt with the mechanisms of stomatal regulation, and
there are five publications from her work. I believe the most significant,
(Cardon and Berry, 1992) uses chlorophyll fluorescence to show
that guard cell chloroplasts respond to CO2 and O2 just like their
mesophyll counterparts. This indicates that guard cells probably
have a normal photosynthetic carbon reduction pathway and
photorespiration, contrary to popular dogma. Zoe is one of the
best students I have ever been associated with.
I also helped to direct the research of another student, Miquel
Ribas-Carbo who received his Ph.D. from the University of Barcelona.
This work was done at Duke University, and concerns the regulation
of the alternative oxidase and its integration into plant mitochondrial
electron transport. Miquel received his degree in 1993, and is
continuing to work on this project as a postdoc at Duke. Other
collaborators in this work include: Prof. Jim Siedow of Duke
University, Larry Giles of Duke, Dan Yakir of the Weismann Institute
in Israel and Sharon Robinson, now at the the Australian National
University. I spent a short sabbatical at Duke in 1991. During
that time we developed the experimental setups used for isotope
discrimination studies, and over the intervening years I have
made many trips back to Duke to participate in this research. This
research has demonstrated the use of oxygen isotope discrimination
as a non-invasive probe of the alternative pathway in plants, and
has made considerable progress toward understanding the kinetic
mechanisms that regulate the partitioning of electrons between
this pathway and the cytochrome pathway. The function of the alternative
pathway, however, still eludes us.
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