SUPPLEMENTARY INFORMATION

Transcription

1 doi: /nature06059 SUPPLEMENTARY INFORMATION Plant Ozone Effects The first order effect of chronic ozone exposure is to reduce photosynthetic capacity 5,13,31 (e.g. by enhanced Rubisco degradation 31 ). 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 rootfungi 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 for comparison with data from the Aspen Free-Air Carbon Dioxide Enrichment 1

2 experiment (Aspen-FACE) 6,13 and at a semi-natural grassland site at Le Mouret, Switzerland 14. Four global simulations, repeating years , were conducted for both the High and Low plant-o 3 sensitivity models: Control (C), ambient [O 3 ] and [CO 2 ]; O 3, 1.5 x [O 3 ] and ambient [CO 2 ]; CO 2, ambient [O 3 ] and [CO 2 ]=560 ppmv; O 3 + CO 2, 1.5 x [O 3 ] and [CO 2 ]=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 O 3 alone was simulated to reduce gross productivity by between 5 and 13% relative to the control for the Low and High plant O 3 sensitivity runs respectively. This compares with observed declines in the light-saturated CO 2 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 CO 2, 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 ecosystems 32. The combined effect of O 3 + CO 2 is to increase gross production by between 22 and 27 % for the High and Low plant O 3 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 O 3 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 2

3 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 O 3 sensitivity. Radiative Forcing Calculation We use a very simple approach to estimate the indirect radiative forcing, R FI (O 3 ), associated with O 3 -suppression of the land carbon sink: R FI (O 3 ) = 5.35 ln[1+dco 2 (O 3 )/CO 2 (0)] where CO 2 (0) is the pre-industrial atmospheric [CO 2 ], and dco 2 (O 3 ) represents the change in atmospheric CO 2 content due to ozone effects on land carbon storage. We calculate the latter assuming 50% of the extra CO 2 emitted from the land is sequestered by the oceans

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6 70 % Change in Gross Productivity O3 CO2 O3+CO2 Aspen Birch HIGH LOW

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8 Figure Captions Figure S1. Simulated percentage change in Gross Primary Productivity between 1901 and 2100 due to O 3 effects at fixed pre-industrial atmospheric [CO 2 ] (left column). The O 3 effect (in the presence of changing CO 2 concentrations) is calculated as the percentage change in GPP in year 2100 from simulations with both CO 2 and O 3 changing and simulations with only atmospheric CO 2 concentrations changing. The difference in the two estimates of the O 3 effect on GPP represents the offset in the O 3 suppression of GPP by CO 2 -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 O 3 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 measurements 16,

9 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 [O 3 ], ambient [CO 2 ]); O 3 (1.5 x ambient O 3, ambient [CO 2 ]); CO 2 (ambient [O 3 ], [CO 2 ]=560ppmv (~2 x pre-industrial)); and O 3 +CO 2 (1.5xambient [O 3 ], [CO 2 ]=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, (2005). 32. Norby, R. J. et al. Forest response to elevated CO 2 is conserved across a broad range of productivity. Proc. Natl. Acad. Sci. USA 102, (2005). 9

10 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 F O3crit (nmol m -2 s -1 ) High a (mmol -1 m -2 ) Low a (mmol -1 m -2 )

11 Table S2. Global land-atmosphere CO 2 exchange for the 1980s and 1990s, in units of PgC/yr, and simulated cumulative land uptake for the period in PgC. 1980s 1990s IPCC Residual Land -1.7 (-3.4 to 0.2) -2.6 (-4.3 to -0.9) Sink 22 Model CRU+CO 2 +O 3 : High Plant-O 3 Sensitivity CRU+CO 2 +O 3 : Low Plant- O 3 Sensitivity CRU+CO 2 only: Zero Plant-O 3 Sensitivity 11

12 Table S3. Percentage reduction in simulated carbon fluxes and pools by 2100 due to future O 3 effects at pre-industrial atmospheric CO 2 content, and under increasing future CO 2 content. The difference between these defines the alleviation of the O 3 effect by CO 2. GPP VegC SoilC LandC Model High Plant-O 3 Sensitivity Value in Values in 2100 Δ[CO 2 ], fixed [O 3 ] Fixed [CO 2 ], Δ[O 3 ] Δ[CO 2 ] & Δ[O 3 ] % change due to O 3 at preindustrial CO 2 % change due to O 3 under increasing CO 2 Alleviation of O 3 -damage by CO 2 increase(%) Low Plant- O 3 Sensitivity Value in Values in 2100 Δ[CO 2 ], fixed [O 3 ] Fixed [CO 2 ], Δ[O 3 ] Δ[CO 2 ] & Δ[O 3 ] % change due to O 3 at preindustrial CO 2 % change due to O 3 under increasing CO 2 Alleviation of O 3 -damage by CO 2 increase(%) calculated as 100 x (var_o 3 [2100]-var[2100])/var[2100], where var_o 3 [y] represents the variable (flux or pool) in year, y, from simulation; fixed [CO 2 ], Δ[O 3 ], and var[2100] is the hypothetical value at 2100 from a run with fixed [CO 2 ] and [O 3 ] (var[2100]=var[1901]=initial state). calculated as 100 x (var_o 3 CO 2 [2100]-var_CO 2 [2100])/var_CO 2 [2100], where var_o 3 CO 2 [y] represents the variable (flux or pool) in year, y, from simulation; Δ[CO 2 ] & Δ[O 3 ] and var_co 2 [y] represents the variable (flux or pool) in year, y, from simulation; fixed [O 3 ], Δ[CO 2 ]. 12