plant ozone effects - nature with data from the aspen free-air carbon ... experiment (aspen-face)...

Plant Ozone Effects

The first order effect of chronic ozone exposure is to reduce photosynthetic

capacity5,13,31 (e.g. by enhanced Rubisco degradation31). Plants are able to cope with

this to a degree by allocating resources to detoxify the leaf tissues and repair damage;

this leads to a critical value of ozone uptake below which plant function is unaffected.

Although ozone exposure may directly affect stomatal conductance, via damage to

guard cells, the impact of ozone on stomatal conductance appears to be primarily due

to a reduction in photosynthetic rate. The leaves produce less, and use more assimilate

for detoxification and repair. Thus there is less carbon to allocate to other organs. LAI

and growth decreases, allocation to roots decrease (less exudates and reduced root-

fungi symbiosis) and soil microbial communities receive litter of reduced quantity and

altered quality.

The cumulative effect of leaf damage and early senescence is implicitly accounted for

in our calibration of "a" (by compensating a shortened growing season by reduced

growing season photosynthesis in the latter case) (Fig S2).

Evaluation against results from Free-Air field manipulation

experiments

We carried out a further set of factorial simulations for the period 1998-2002 for

comparison with data from the Aspen Free-Air Carbon Dioxide Enrichment

SUPPLEMENTARY INFORMATION

doi: 10.1038/nature06059

www.nature.com/nature 1

experiment (Aspen-FACE) 6,13 and at a semi-natural grassland site at Le Mouret,

Switzerland 14. Four global simulations, repeating years 1998-2002, were conducted

for both the ‘High’ and ‘Low’ plant-O3 sensitivity models: Control (C), ambient [O3]

and [CO2]; O3, 1.5 x [O3] and ambient [CO2]; CO2, ambient [O3] and [CO2]=560

ppmv; O3 + CO2, 1.5 x [O3] and [CO2]=560ppmv.

Global simulations broadly agree with the findings of the Aspen-FACE experiment

(Fig. S3) and with other studies (Fig. 7. ref 6). The effect of elevated O3 alone was

simulated to reduce gross productivity by between 5 and 13% relative to the control

for the ‘Low’ and ‘High’ plant O3 sensitivity runs respectively. This compares with

observed declines in the light-saturated CO2 assimilation rates (Amax) of between 0%

and 20% for Betula papyrifera (Birch) and Populus tremuloides (Aspen) from Aspen-

FACE. Simulated production increased by 33-38% under elevated CO2, compared

with observed increases in Amax of 27 – 64% for Aspen and Birch, respectively, from

Aspen-FACE and a median response of 23% across a broad range of forest

ecosystems32. The combined effect of O3 + CO2 is to increase gross production by

between 22 and 27 % for the ‘High’ and ‘Low’ plant O3 sensitivity runs, compared

with a reduction in Amax of 7% and an increase of 36% for Aspen and Birch,

respectively, from Aspen-FACE.

Here we have compared changes in simulated gross primary productivity with

observed changes in Amax. These variables are not identical, however they are

closely related. Although Aspen is known to be very sensitive to O3 and Birch

regarded to be sensitive, the purpose is to evaluate the model against available data

rather than present a comprehensive evaluation of the actual high and low sensitivity

doi: 10.1038/nature06059 SUPPLEMENTARY INFORMATION


range of species. After a 5-year exposure to elevated concentrations of ozone (1.5

times ambient), yield at the semi-natural grassland site in Switzerland had declined by

a quarter. This compares to a simulated global reduction in C3 grass yield by 16-20%

for the low and high plant O3 sensitivity.

Radiative Forcing Calculation

We use a very simple approach to estimate the indirect radiative forcing, RFI(O3),

associated with O3-suppression of the land carbon sink:

RFI(O3) = 5.35 ln[1+dCO2(O3)/CO2(0)]

where CO2(0) is the pre-industrial atmospheric [CO2], and dCO2(O3) represents the

change in atmospheric CO2 content due to ozone effects on land carbon storage. We

calculate the latter assuming 50% of the extra CO2 emitted from the land is

sequestered by the oceans26.



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Figure Captions

Figure S1. Simulated percentage change in Gross Primary Productivity between 1901

and 2100 due to O3 effects at fixed pre-industrial atmospheric [CO2] (left column).

The O3 effect (in the presence of changing CO2 concentrations) is calculated as the

percentage change in GPP in year 2100 from simulations with both CO2 and O3

changing and simulations with only atmospheric CO2 concentrations changing. The

difference in the two estimates of the O3 effect on GPP represents the offset in the O3

suppression of GPP by CO2-induced stomatal closure (right column). Upper and

lower rows are for ‘low’ and ‘high’ ozone plant sensitivity, respectively.

Figure S2. Calibration of MOSES submodel of O3 effects on plants. □ and Δ represent

modelled grid cell values of relative yield against cumulative uptake of ozone above

the PFT specific critical threshold (1.6 and 5 nmol m-2 s-1 for the woody and grass

PFTs, respectively) for the ‘Low’ and ‘High’ plant ozone sensitivity runs, respectively,

and the solid lines show the linear regression through these points. Model values of

cumulative uptake of ozone are calculated from the simulated CUO for top of the

canopy leaves, multiplied by the foliar projective cover of the canopy, for PFT

fractional coverages greater than 0.2 in the gridcell. MOSES differentiates 5 plant

functional types, Broadleaf Tree (BT), Needleleaf Tree (NT), C3 grass (C3), C4 grass

(C4) and shrub (SHRUB).The dashed lines represent the regressions based on field

measurements16,20.



Figure S3. Percentage change in measured Aspen-FACE (Amax) and modelled global

productivity (GPP), for both ‘Low’ and ‘High’ plant ozone sensitivity, relative to the

control. Experiments: Control C (ambient [O3], ambient [CO2]); O3 (1.5 x ambient O3,

ambient [CO2]); CO2 (ambient [O3], [CO2]=560ppmv (~2 x pre-industrial)); and

O3+CO2 (1.5xambient [O3], [CO2]=560ppmv).

Figure S4. The effect of ozone on land carbon stocks by 2100 with the ‘low’ and

‘high’ ozone plant sensitivity runs, calculated as the difference in simulated land

carbon storage between 2100 and 1901 (units kgC m-2) in a) Vegetation Carbon (CV),

b) Soil carbon (CS) and c) Total land carbon storage (CV+CS). Negative values

denote an ozone-induced reduction in land carbon storage.

References

31. Fiscus, E. L., Booker, F. L. & Burkey, K. O. Crop responses to ozone: uptake,

modes of action, carbon assimilation and partitioning. Plant, Cell and Environment 28,

997-1011 (2005).

32. Norby, R. J. et al. Forest response to elevated CO2 is conserved across a broad

range of productivity. Proc. Natl. Acad. Sci. USA 102, 12052-18056 (2005).



Table S1. Ozone exposure parameters: values for Broad-leaved Tree (BT) & Needle-leaved

Tree (NT) calibrated to Karlsson et al. (2004) (ref. 20) table 4. ‘High’ and ‘Low’ plant ozone

sensitivity parameter ‘a’ calibrated against regressions for "Birch, beech" and “Oak”,

respectively. Parameters for C3 and C4 grass (C3, C4) are calibrated against data from Pleijel

et al. (2004) (ref. 16), with ‘High’ and ‘Low’ plant ozone sensitivity parameter ‘a’ calibrated

against regressions for “Spring Wheat” and “Potato”, respectively. Parameters ‘a’ for Shrub

are calibrated as for BT.

BT NT C3 C4 Shrub

FO3crit (nmol m-2 s-1) 1.6 1.6 5.0 5.0 1.6

‘High’ a (mmol-1 m-2) 0.15 0.075 1.40 0.735 0.10

‘Low’ a (mmol-1 m-2) 0.04 0.02 0.25 0.13 0.03



Table S2. Global land-atmosphere CO2 exchange for the 1980s and 1990s, in units of

PgC/yr, and simulated cumulative land uptake for the period 1901-2000 in PgC.

1980s 1990s 1901-2000

IPCC Residual Land

Sink22

-1.7 (-3.4 to 0.2) -2.6 (-4.3 to -0.9)

Model

CRU+CO2+O3:

‘High’ Plant-O3

Sensitivity

-0.67 -1.34 84

CRU+CO2+O3:

‘Low’ Plant- O3

Sensitivity

-1.04 -1.74 126

CRU+CO2 only:

Zero Plant-O3

Sensitivity

-1.51 -2.31 177



Table S3. Percentage reduction in simulated carbon fluxes and pools by 2100 due to

future O3 effects at pre-industrial atmospheric CO2 content, and under increasing

future CO2 content. The difference between these defines the alleviation of the O3

effect by CO2.

GPP VegC SoilC LandC

Model ‘High’ Plant-O3 Sensitivity Value in 1901 112.7 461.8 1110.8 1572.6 Values in 2100 Δ[CO2], fixed [O3] 201.1 696.8 1732.5 2429.3 Fixed [CO2], Δ[O3] 86.3 372.6 937.4 1310.1 Δ[CO2] & Δ[O3] 171.1 646.6 1543.5 2190.0 % change due to O3 at pre-industrial CO2 †

-23.4 -19.3 -15.6 -16.7

% change due to O3 under increasing CO2 ‡

-14.9 -7.2 -10.9 -9.9

Alleviation of O3-damage by CO2 increase(%)

8.5 12.1 4.7 6.8

‘Low’ Plant- O3 Sensitivity Value in 1901 116.6 488.2 1130.2 1618.4 Values in 2100 Δ[CO2], fixed [O3] 203.5 705.8 1748.5 2454.2 Fixed [CO2], Δ[O3] 100.5 456.3 1018.9 1475.2 Δ[CO2] & Δ[O3] 188.0 690.1 1643.9 2334.0 % change due to O3 at pre-industrial CO2

-13.8 -6.5 -9.9 -8.9

% change due to O3 under increasing CO2

-7.7 -2.2 -6.0 -4.9

Alleviation of O3-damage by CO2 increase(%)

6.2 4.3 3.9 4.0

† calculated as 100 x (var_O3[2100]-var[2100])/var[2100], where var_O3[y] represents the variable (flux or pool) in year, y, from simulation; fixed [CO2], Δ[O3], and var[2100] is the hypothetical value at 2100 from a run with fixed [CO2] and [O3] (var[2100]=var[1901]=initial state). ‡ calculated as 100 x (var_O3CO2[2100]-var_CO2[2100])/var_CO2[2100], where var_O3CO2[y] represents the variable (flux or pool) in year, y, from simulation; Δ[CO2] & Δ[O3] and var_CO2[y] represents the variable (flux or pool) in year, y, from simulation; fixed [O3], Δ[CO2].



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