Effects of elevated CO₂-concentration on plants and ecosystems

Our group is heavily engaged in research on the consequences of CO₂-enrichment for plants under as natural as possible growth conditions. Recent projects include examples from various different climatic zones (sorted by years, beginning with earliest projects):

1. Tropical model ecosystems under elevated CO₂ (1990-1995) and later tropical CO₂ works (1996-)

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We built four large (6 m2, 17 m3) stands of a mixed humid tropical community of 15 different species including genera like Cecropia, Ficus and Piper, and grew them under ambient and elevated CO₂ until leaf area index reached a steady state at 7 m2m-2. Results indicate a strong stimulation of carbon turnover and a massive alteration of tissue composition, but surprisingly little growth responses and no effect on LAI.
Ch. Körner and J. Arnone (1992) Responses to elevated carbon dioxide in artificial tropical ecosystems. Science 257:1672-1675
Würth M, Winter K, Körner Ch 1998: In situ responses to elevated CO₂ in tropical forest understorey plants. Funct Ecol 12:886-895
Würth MKR, Winter K, Körner Ch 1998: Leaf carbohydrate responses to CO₂ enrichment at the top of a tropical forest. Oecologia 116:18-25
Granados J, Körner Ch 2002: In deep shade, elevated CO₂ increases the vigor of tropical climbing plants. Global Change Biol 8:1109-1117
Monteiro JAF, Zotz G, Körner Ch 2009: Tropical epiphytes in a CO₂-rich world. Acta Oecologica 35:60-68

2. Four years CO₂-enrichment in alpine grassland (1991-1995)

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Alpine heathland at 2500 m elevation, i.e. 300 m above the treeline in the Swiss Central Alps received a double ambient CO₂-treatment for 4 consecutive seasons. This remote operation caused enormous logistic problems (helicopter transport, solar power only), but represents the first and so far only assessment of CO₂-effects on natural alpine vegetation. The plant biomass was not stimulated by elevated CO₂, irrespectively of whether plots received additional nutrients (fertilization) or not. Firtilizer alone (50 kg N/ha a) doubled biomass. Tissue quality and herbivory were affected. There were no effects on plant phenology or overall carbon balance.
Ch. Körner et al. (1997) The responses of alpine grassland to four seasons of CO₂-enrichment: a synthesis. Acta Oecologica 18:165-175

3. The response of young montane spruce communities to CO₂-enrichment (1991-1996)

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Large containers filled with natural forest soil (400 kg each) were planted with a dense community of Picea abies and a natural understory vegetation. Over three vegetation periods trees grew under three CO₂-concentrations and a simulated montane climate in a phytotron. We found no growth stimulation of trees, but CO₂-enrichment induced serious nitrogen shortage in plants (which was visible) and reduced leaf area index. Photosynthesis of trees was stimulated, but the additional carbon taken up was recycled largely through soil respiration. A partner group (see the physiology division of our institute) studied root processes and mycorrhiza. Understory plants were stimulated by CO₂-enrichment depending on micro-habitat, light conditions underneath these very dense canopies.
S. Hättenschwiler and Ch. Körner (1996) System-level adjustments to elevated CO₂ in model spruce ecosystems. Global Change Biol 2:377-387

4. Responses to CO₂-enrichment in the understory of a tropical forest (1995-1997)

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In cooperation with the Smithsonian Tropical Research Institute, Panama (K. Winter) we exposed seedlings of tropical trees and shrubs in situ to elevated CO₂, using plastic tunnels. Despite very deep shade under the old growth forest of Barro Colorado Island (median 10 (mol photons m-2 s-1, i.e. 0,5 % of midday sun) we found significant stimulation of growth in all species tested, suggesting potential enhancement of forest dynamics by ongoing atmospheric CO₂-enrichment.
M. Würth et al. (1998) In situ responses to elevated CO₂ in tropical forest understory plants. Funct Ecol 12:886-895

5. CO₂-enrichment of lowland calcareous grassland (1995-2000)

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As part of the Swiss Priority Program of the Environment, this six-year project included cooperative work with a number of other Swiss research teams, largely from the University of Basel (see MCO). We manipulated CO₂ (ambient and 600 ppm) and plant biodiversity (5, 12 and 31 species). The whole experiment was also replicated on undisturbed natural grassland. We used 32 plots, half of which were CO₂-enriched by screen-aided CO₂-enrichment (SACC). The analysis concentrated on species responses as well as overall ecosystem responses such as biomass distribution within and among species, growth dynamics, competitive performance of species (demography), reproduction, and ecophysiological responses such as photosynthesis (at leaf and stand level), respiration, carbohydrate budgets, responses of nitrogen and phosphorous concentrations in plants and soils. Major findings are clear CO₂-effects on soil moisture, with consequences for microbes, earthworms and species abundance, moisture-dependent biomass stimulation, reduced tissue quality and frost resistance. Remarkably, legumes (five species of Trifolium) took absolutely no advantage of CO₂-enrichment, which could be explained by phosphate limitation under these natural growth conditions. Loss of plant species always lead to higher concentrations of free nitrate in the soil solution. CO₂-enrichment counteracted this trend.
R. Stocker et al. (1999) A field study of the effects of elevated CO₂ and plant species diversity on ecosystem-level gas exchange in a planted calcareous grassland. Global Change Biol 5:95-105
P. Niklaus et al. (2000) CO₂ flux estimates tend to overestimate ecosystem C sequestration at elevated CO₂. Funct Ecol 14:546-559
H. Zaller, J. Arnone (1997) Activity of surface-casting earthworms in a calcareous grassland under elevated atmospheric CO₂. Oecologia 111:249-254

6. Forest re-growth under elevated CO₂ and enhanced nitrogen deposition (1995-1999)

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Our group participated in a major experiment at the Swiss Federal Research Institute for Forest, Snow and Landscape (WSL) under the European COST 614 program. Using beech-spruce model ecosystems in large lysimeters (1.2 m deep, natural substrate) we studied effects of CO₂-enrichment, N-deposition and substrate type. The four-year project involved more than 500 young trees and was finished by the end of 1998. Presently the project is in the publication phase. Major results are that all responses tested (from biochemical level to ecosystem responses) depended on substrate type (calcareous sandy soil versus acidic loamy soil). For instance, both tree species were stimulated by CO₂-enrichment on the calcareous substrate, but on the acidic substrate beech showed even a negative response to CO₂-enrichment. Growth responses were determined very early during the growth when canopies were still open. Final leaf area index, when trees reached two meter in size and formed very dense thickets, were not affected by CO₂-enrichment. All responses, including those of tissue and wood quality, were opposite for CO₂ and for nitrogen deposition. It is concluded that CO₂-responses of forest re-growth are depending on co-ocurring nitrogen deposition, substrate type and species. Substrate was the single most influential component.
P. Egli, S. Maurer, M.S. Günthardt-Goerg, Ch. Körner (1998) Effects of elevated CO₂ and soil quality on leaf gas exchange and above-ground growth in beech-spruce model ecosystems. New Phytol 140:185-196

7. The response of tropical lianas to CO₂-enrichment (1998-2001)

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The vigor of climbers co-determines the dynamics of tropical forests. It had been suggested that atmospheric CO₂-enrichment could stimulate climbers and thereby cause faster tree turnover, which eventually may even reduce the carbon sink in tropical forests. We tested the possibility that tropical climbers respond to CO₂ in deep shade. Seeds and 1.5 tons of natural soil from the Yucatan Peninsula in Mexico were transfered to the Basel phytotron, and lianas were grown under two light regimes and four CO₂-concentrations including pre-industrial. The work is now in the publication stage. Major findings are that all species responded to CO₂-enrichment. The absolute responses were greater in high light, but the relative responses were greater in low light. The responses were non-linear with increasing CO₂ (more pronounced responses to current ranges of CO₂-enrichment compared to future ones), and different lianas showed different growth responses. The data seem to support the possibility that current CO₂-enrichment is particularly advantageous for tropical climbers. In principle, these results support the findings of in situ CO₂-enrichment measurements in Panama (see the above-mentioned project and publication by Würth et al.).This research is conducted by Julian Granados.
J. Granados, Ch. Körner (2002) In deep shade, elevated CO₂ increases the vigor of tropical climbing plants. Global Change Biol 8:1109-1117
Körner Ch. (2004) Through enhanced tree dynamics carbon dioxide enrichment may cause tropical forests to lose carbon. Phil Trans R Soc Lond 359:493-498

8. In situ regeneration of temperate mixed forest trees under CO₂-enrichment (1996-2000)

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Across a light gradient under a closed mature beech-oak forest we studied the effects of CO₂-enrichment on forest tree seedling growth. Under these natural growth conditions, all tree species tested responded to CO₂-enrichment by enhanced growth, however, the responsiveness of species varied enormously, and this variation was driven by micro-habitat light availability. These results confirm our phytotron studies with understory plants (see project 3 above).
S. Hättenschwiler and Ch. Körner (2000) Tree seedling responses to in situ CO₂-enrichment differ among species and depend on understorey light availability. Global Change Biology 6:213-226

9. The response of natural forest trees to CO₂-enrichment around geological CO₂ springs (1996-1997)

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In cooperation with F. Miglietta and A. Raschi (Firenze) we studied the response of Quercus ilex around two different CO₂ springs in Toscany. Tree ring analysis over the 30-year life span of these oaks revealed a stimulation during early life, which disappeared by the time trees reached an age of ca. 25 years. However, the positive initial responses may still translate into some compound interest effects for some additional years. Overall, these responses suggest enhanced forest turnover, not necessarily enhanced forest carbon storage. We also found a number of qualitative changes in plant tissues and tree morphology (reduced leaf area/branch ratio, enhanced total non-structural carbohydrates).
S. Hättenschwiler, F. Miglietta, A. Raschi, Ch. Körner (1997) Thirty years of in situ tree growth under elevated CO₂: a model for future forest responses? Global Change Biology 3:436-471

10. The response of semi-arid grassland to CO₂-enrichment (1998-2000)

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We used model ecosystems, 400 kg each, with original Negev soil (Isreal) in a series of large growth cabinets in which we simulated the Negev winter climate. Species-rich assemblages of winter annual grasses and herbs showed very little biomass response but significant changes in tissue quality and species dominance. However, these latter effects were due solely to the response of a single legume species. Had we not included this vigorous species, overall responses would have been minute. This highlights the significance of species identity rather than functional group identity, because other legumes did not show such a dramatic response. Communities included wild cereals which also were not stimulated in terms of growth but underwent changes in tissue quality. Communities were found to transpire less moisture (regular weighing of the large experimental units on a freight balance) which enhanced runoff during simulated rainfall, but prolonged moisture into drought periods. However, plants took little advantage of enhanced moisture, because phenology changed quite autonomously towards the end of the season (programmed scenescence).
J. Grünzweig and Ch. Körner (2000) Growth and reproductive responses to elevated CO₂ in wild cereals of the northern Negev of Israel. Global Change Biology 6:631-638

11. Mature forest canopy CO₂-enrichment (2000-)

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Beginning in late 2000, our group started the first-ever CO₂-enrichment experiment in a mature, species-rich natural forest. Using the Swiss Canopy Crane, we developed and installed a CO₂-release system within the crowns of 35-m tall trees of the genera Fagus, Quercus, Carpinus, Prunus, Acer and Tilia. In 2005, treated and control trees are all vigorously growing. After four years of treatment, leaf photosynthesis is still enhanced at increased CO₂ levels of 530 ppm, but tree growth has not been stimulated. Apparently, carbon is channelled through the system at increased rates and is lost via the roots. CO₂-induced changes of tissue quality are mostly small, but species responses differ. Many other physiological responses are also species-specific, highlighting the need to include biodiversity aspects in global change studies. This on-going project is largely funded by the Swiss Science Foundation. In 2009 the work on the deciduous forest was finalized ater 8 years and CO₂ enrichment of spruce commenced on the hemisphere under the crane.
Palacio S, Hoch GU, Sala A, Körner C, Millard P (2014) Does carbon storage limit tree growth? New Phytol 201:1096-1100
Leuzinger S, Fatichi S, Cusens J, Körner C, Niklaus PA (2015) The 'island effect' in terrestrial global change experiments: a problem with no solution? AoB PLANTS 7:plv092, doi: 10.1093/aobpla/plv092
Körner C (2017) Carbon sequestration: a matter of tree longevity. Science 355:130-131
Körner C et al. (2006) Plant CO₂ responses: an issue of definition, time and resource supply. New Phytol 172:393-411
Körner C et al. (2005) Carbon flux and growth in mature deciduous forest trees exposed to elevated CO₂. Science 309:1360-1362

12. Glacier forefield plants exposed to elevated CO₂ (2006-2008)

Using Free air CO₂ enrichment (FACE) at 2460 m elevation near Furka pass in the Swiss Central Alps, we exposed a diverse community of typical high elevation pioneer plant species to c. 600 ppm CO₂ for three seasons. The Miglietta-type mini-FACE worked perfectly in this gusty alpine environment. Surprisingly, plants did not exhibit any stimulation by elevated CO₂, also not, if we added soil nutrients. In fact, in several species there was a significantly negative effect of CO₂-enrichment on biomass production after three seasons. Perhaps the apparent CO₂-saturation of these plants is related to the combination of cold and wet life conditions, with the 25% reduced partial pressure of CO₂ at this elevation, making the system not more sensitive, similar to what we saw in project 2.

13. Hydrological consequences of in situ CO₂-enrichment of mature alpine vegetation (2008-)

This FACE (see project 12) experiment explores the effect of increased CO₂ on the water balance of intact monoliths of alpine vegetation growing in lysimeters. The CO₂ treatment is combined with a land use treatment (simulated grazing). Both treatments do affect evapotranspiration significantly!
For more information on the surroundings of the Furka pass and the scientific facilities see ALPFOR – Alpine Forschungs- und Ausbildungsstation Furka

General Inquiries on all projects Prof. Dr. Christian Körner