Bringing Photosynthesis to the Atmosphere: a feedback on terrestrial water cycling

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Bringing Photosynthesis to the Atmosphere a
feedback on terrestrial water cycling

• Gabriel Katul1,2 Ram Oren1

1Nicholas School of the Environment and Earth
Sciences, Duke University 2Department of Civil
and Environmental Engineering, Duke University
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TIME in BILLIONS OF YEARS
From D. Christian (2004)
Photosynthetic Apparatus (3.5 Billion years)
Land Plants (350 million years need for
hydraulic apparatus)
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Background

• The evolution of land plants required hydraulic
  adjustments that permitted the display of the
  photosynthetic machinery within a desiccating
  atmosphere.

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Background

• A productive display of this machinery
  necessitates a vertical distribution of
  chlorophyll so that the light-use efficiency of
  the organism is increased relative to a
  concentrated display of chlorophyll.

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Random Porous Media
Crown Clumping
From C. Song
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Background

• This vertical distribution required certain
  engineering solutions to permit water to be
  supplied to leaves enclosed in an envelope that
  permits a controlled exchange of CO2 for water
  vapor

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Question

• Is there some connections between these
  engineering solutions and the photosynthetic
  properties of the plant?
• Contemporary approaches neglect this connection.

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Introduction 1

• There is growing evidence that ecophysiological
  properties are correlated to hydraulic
  properties.
• Brodribb and Feild (2000) reported strong
  correlation between maximum carboxylation
  capacity and stem hydraulic conductivity.

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Background - 1Field Experiments
Data include conifers, vessel-less and vessel
bearing taxa from New Caledonia and Tasmania
(data from Brodribb and Feild, 2000)
Leuning (2002)
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Background 2Field Experiments

• Numerous ecophysiological studies reported
  linkages between canopy photosynthesis and
  conductance.
• Many hydraulic studies reported linkages between
  plant hydraulics and photosynthesis (e.g. Hubbard
  et al., 2001).

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Data from Hubbard et al. (2001)
Relationship between stomatal conductance (gs)
and photosynthesis (A) and plant hydraulic
conductivity (KL).
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Background 4Missing Link

• To date, no analytic framework has been developed
  to
• relate hydraulic and ecophysiological properties.
• This study proposes an equilibrium hypothesis
  for
• coupled CO2 H2O to arrive at analytic
  expressions
• between soil-plant hydraulics and
  ecophysiological properties

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Hypothesis - 1

• The basic hypothesis is that hydraulic and
  eco-physiological properties co-evolve at time
  scales relevant to stand development.

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Hypothesis contd

• The hydraulic and ecophysiological parameters
  evolve so that the maximum biochemical demand for
  carbon uptake is in equilibrium with the maximum
  carbon gain permissible by the soil-root-xylem
  hydraulics.

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Hydraulic Supply Water Transport in Plants
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Hydraulic Supply Water Transport in Plants
Soil Water Characteristic Curve
Soil Hydraulic Conductivity Function and RAI
Soil Water Status
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Root-to-Epidermis Simplest Model
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Maximum Hydraulic Conductivity
Maximum Hydraulic Capacity is achieved when
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From Hacke et al. (2000) Pinus taeda
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Sperry et al. (2000) demonstrated that plant
pressure is close the cavitation threshold
within the root-xylem system.
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Stomatal Closure and Xylem Pressure
Stomatal closure observed during drought is a
result of a decline in plant hydraulic
conductance in the xylem of the roots. This
can explains why stomatal conductance shuts down
at suction levels well below the turgor suction.
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Vascular Plants
New processes associated with the evolution of a
larger and more complex plant body became more
limiting to gas exchange than those occurring
within individual cells (Sperry, 2000 Katul et
al., 2003)
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Gas Exchange - Stomata
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Water Loss from Epidermis to Atmosphere
Setting E Jw (Steady State) and noting that
Hence, the maximum hydraulic supply of CO2 (per
LAI)
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Biochemical Demand for CO2
Farquhar et al. (1980)
Equate maximum hydraulic supply to maximum
biochemical demand and solve for Ci
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Implications to FACE Conductance/Down-regulation
Reference State The nothing changed except
atmospheric CO2 scenario
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Implications to FACE

• Rogers and Ellsworth (2002) report a reduction in
  Vcmax of 25 (for older foliage).
  Photosynthesis-weighted Vcmax
• drops by 17.

• Schafer et al. (2002) bulk canopy conductance
• increased by 25 but no change in LAI.

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? To Match a 17 reduction in Vcmax and a 1.38
increase in bulk stomatal conductance, Ci/Ca
must increase by 1.21 ( 0.67 ? 0.80).
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Experiment SETRES II

• SouthEast Tree Research and Education Site
  (SETRES) 2.
• A large-scale genotype ? nutrition interaction
  experiment designed to quantify the effects of
  fertilization on C- cycling in a managed southern
  pine forest in North Carolina
• (operated by the U.S. Forest Service).

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SETRES I II Fertilization Experiment

• After 6 years of fertilization
• Fertilization Rate (11 g N m-2 y-1)
• Leaf Area Index Doubled (1.65 ? 3.51)
• Maximum Leaf Carboxilation Capacity Increased
• by about 20.
• Respiring Biomass Increased by 48.

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Case Study Duke Forest and SETRES
Mode of soil moisture pdfs during
growing Season. SETRES 0.09 DUKE
FOREST0.2 Stable Isotope Measurements Ci/Ca
0.66
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Fertilization Increases Vcmax
Leaf Nitrogen
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Effect of N-Fertilization on Vcmax (SETRES I
II)
Using LAI (from SETRES II) and sapflux based
measurements (from SETRES I ) of
Gas Exchange Measurements (SETRES II)
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General Remark
Equilibrium model provides some constraints on
how shifts in plant hydraulic and physiological
properties affect photosynthesis and water
uptake. Hydraulic shifts already measured in
the Prototype (Schafer, Oren) and physiological
shifts reported in FACE, the next logical
question is whether N x CO2 effects can be
predicted by the equilibrium model.
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Engineering Solutions Climate /Reproduction?

• Emergence of land plants from aquatic habitat
  also required engineering solutions for
  re-production (mainly - pollen and its
  dispersal).
• Is there some connection between the engineering
  solutions for plant hydraulics and pollen
  dispersal?

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Timely Topic Pollen Dispersal of GM Trees
? Increased use of GM trees is now being
considered and debated within the policy arena.
? The finger-print of this increase is
perhaps evident in the number of permit
applications to the USDA-APHIS, now exceeding 150
types of transgenic trees (Mann and Plummer
2002). ? Quantifying GM pine pollen and seed
dispersal pattern is a necessary first step to
assessing gene flow, landscape change, and other
unforeseen ecological risk especially with
elevated atmospheric CO2 (LaDaux and Clark,
2001).
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Pr(d)
Dispersal Kernel
zVertical Distance (m)
Mean Wind Direction
d
Uplifting
d
yLateral Distance (m)
xLongitudinal Distance (m)
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List
Litu
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POLLEN SIMULATIONS FOR PINE PLANTATIONS
Reproductive (16 years) and Harvesting Ages (25
years) are used in model calculations. Stand
attributes are based on Leaf area measurements
and site index for the stand at Duke Forest.
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Duke Forest, NC
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Dispersal Kernels for the two stand ages
? Kernel calculations excluded pollen that
escaped the atmospheric boundary layer top or
experienced travel times in excess of 1 hour.
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Dispersal Kernels Excluding Pollen that Escaped
the Canopy Volume
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Biocontainment zones for few GM crops

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Results

1. Conifer pollen is NOT likely to be of negligible
   viability at those distances (viability
   diminishes due to excess UV-B, cold air
   temperatures, or dehydration).
2. Given the long dispersal distances reported here,
   a regulatory framework that distinguishes between
   annual (and perennial) crops and forests is only
   logical.

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Broader Conclusions

• The evolution of land plants required hydraulic
  adjustments that permitted the display of the
  photosynthetic machinery within a desiccating
  atmosphere.
• Does this equilibrium between plant hydraulic
  and photosynthetic properties also govern
  reproductive strategies?