Increasing atmospheric CO2 concentration

1. Tropical model ecosystems under elevated CO2 (1990-1995) and later tropical CO2 works (1996-2009)

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.

- Körner C, Arnone J (1992) Responses to elevated carbon dioxide in artificial tropical ecosystems. Science 257:1672-1675
- Würth MKR, Winter K, Körner C (1998) In situ responses to elevated CO2 in tropical forest understorey plants. Funct Ecol 12:886-895
- Würth MKR, Winter K, Körner C (1998) Leaf carbohydrate responses to CO2 enrichment at the top of a tropical forest. Oecologia 116:18-25
- Granados J, Körner C (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. CO2-enrichment in alpine grasslands (1991-2011)

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

- Körner C, Diemer M, Schäppi B, Niklaus P, Arnone J (1997) The responses of alpine grassland to four seasons of CO2-enrichment: a synthesis. Acta Oecologica 18:165-175

b. 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 part a.

- Inauen N, Körner C, Hiltbrunner E (2012) No growth stimulation by CO2 enrichment in alpine glacier forefield plants. Glob Change Biol 18:985-999

c. Hydrological consequences of in situ CO2-enrichment of mature alpine vegetation (2008-2011)
This FACE (see part b) 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!

- Obojes N, Bahn M, Tasser E, Walde J, Inauen N, Hiltbrunner E, Saccone P, Lochet J, Clement JC, Lavorel S, Tappeiner U, Körner C (2015) Vegetation effects on the water balance of mountain grasslands depend on climatic conditions. Ecohydrol 8:552-569

For more information on the surroundings of the Furka pass and the scientific facilities see ALPFOR – Alpine Forschungs- und Ausbildungsstation Furka



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

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.

- Hättenschwiler S, Körner C (1995) Responses to recent climate waming of Pinus sylvestris and Pinus cembra within their montane transition zone in the Swiss Alps. J Veg Sci 6:357-368
- Hättenschwiler S, Körner C (1996) Effects of elevated CO2 and increased nitrogen deposition on photosynthesis and growth of understorey plants in spruce model ecosystems. Oecologia 106:172-180
- Hättenschwiler S, Körner C (1996) System-level adjustments to elevated CO2 in model spruce ecosystems. Global Change Biol 2:377-387
- Hättenschwiler S, Schweingruber FH, Körner C (1996) Tree ring responses to elevated CO2 and increased N deposition in Picea abies. Plant Cell Environ 19:1369-1378




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

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.

- Würth MKR, Winter K, Körner C (1998) In situ responses to elevated CO2 in tropical forest understorey plants. Funct Ecol 12:886-895



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

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. 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.

- Lauber W, Körner C (1997) In situ stomatal response to long-term CO2 enrichment in calcareous grassland plants. Acta Oecol 18:221-229
- Leadley PW, Niklaus P, Stocker R, Körner C (1997) Screen-aided CO2 control (SACC): a middle ground between FACE and open-top chambers. Acta Oecol 18:207-219
- Rötzel C, Leadley PW, Körner C (1997) Non-destructive assessment of the effects of elevated CO2 on plant community structure in a calcareous grassland. Acta Oecol 18:231-239
- Stocker R, Leadley PW, Körner C (1997) Carbon and water fluxes in a calcareous grassland under elevated CO2. Funct Ecol 11:222-230
- Stöcklin J, Leadley PW, Körner C (1997) Community and species level responses to elevated CO2 in designed calcareous grassland communities. Acta Oecol 18:241-248
- Zaller H, Arnone J (1997) Activity of surface-casting earthworms in a calcareous grassland under elevated atmospheric CO2. Oecologia 111:249-254
- Niklaus PA, Leadley PW, Stöcklin J, Körner C (1998) Nutrient relations in calcareous grassland under elevated CO2. Oecologia 116:67-75
- Hector A, Schmid B, Beierkuhnlein C, Caldeira MC, Diemer M, Dimitrakopoulos PG, Finn JA, Freitas H, Giller PS, Good J, Harris R, Högberg P, Huss-Danell K, Joshi J, Jumpponen A, Körner C, Leadley PW, Loreau M, Minns A, Mulder CPH, O'Donovan G, Otway SJ, Pereira JS, Prinz A, Read DJ, Scherer-Lorenzen M, Schulze E-D, Siamantziouras A-SD, Spehn EM, Terry AC, Troumbis AY, Woodward FI, Yachi S, Lawton JH (1999) Plant diversity and productivity experiments in European grasslands. Science 286:1123-1127
- Leadley PW, Niklaus PA, Stocker R, Körner C (1999) A field study of the effects of elevated CO2 on plant biomass and community structure in a calcareous grassland. Oecologia 118:39-49
- Stocker R, Körner C, Schmid B, Niklaus PA, Leadley PW (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
- Niklaus PA, Stocker R, Körner C, Leadley PW (2000) CO2 flux estimates tend to overestimate ecosystem C sequestration at elevated CO2. Funct Ecol 14:546-559
- Niklaus PA, Kandeler E, Leadley PW, Schmid B, Tscherko D, Körner C (2001) A link between plant diversity, elevated CO2 and soil nitrate. Oecologia 127:540-548
- Niklaus PA, Leadley PW, Schmid B, Körner C (2001) A long-term field study on biodiversity x elevated CO2 interactions in grassland. Ecol Monographs 71:341-356



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

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. Major results were 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.

- Egli P, Körner C (1997) Growth responses to elevated CO2 and soil quality in beech-spruce model ecosystems. Acta Oecol 18:343-349
- Egli P, Maurer S, Günthardt-Goerg MS, Körner C (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
- Egli P, Maurer S, Spinnler D, Landolt W, Günthardt-Goerg MS, Körner C (2001) Downward adjustment of carbon fluxes at the biochemical, leaf, and ecosystem scale in beech-spruce model communities exposed to long-term atmospheric CO2 enrichment. Oikos 92:279-290
- Spinnler D, Egli P, Körner C (2002) Four-year growth dynamics of beech-spruch model ecosystems under CO2 enrichment on two different forest soils. Trees 16:423-436
- Spinnler D, Egli Ph, Körner C (2003) Provenance effects and allometry in beech and spruce under elevated CO2 and nitrogen on two different forest soils. Basic Appl Ecol 4:467-478




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

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 4 and publication by Würth et al.).This research was conducted by Julian Granados.

- Granados J, Körner C (2002) In deep shade, elevated CO2 increases the vigor of tropical climbing plants. Global Change Biol 8:1109-1117
- Körner C (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)

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).

- Hättenschwiler S, Körner C (2000) Tree seedling responses to in situ CO2-enrichment differ among species and depend on understorey light availability. Global Change Biology 6:213-226
- Hättenschwiler S, Körner C (2003) Does elevated CO2 facilitate naturalization of the non-indigenous Prunus laurocerasus in Swiss temperate forests? Funct Ecol 17:778-785
- Zotz G, Cueni N, Körner C (2006) In situ growth stimulation of a temperate zone liana (Hedera helix) in elevated CO2. Funct Ecol 20:763-769



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

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).

- Hättenschwiler S, Miglietta F, Raschi A, Körner C (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)

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).

- Grünzweig JM, Körner C (2000) Growth and reproductive responses to elevated CO2 in wild cereals of the northern Negev of Israel. Global Change Biology 6:631-638
- Grünzweig JM, Körner C (2001) Growth, water and nitrogen relations in grassland model ecosystems of the semi-arid Negev of Israel exposed to elevated CO2. Oecologia 128:251-262
- Grünzweig JM, Körner C (2003) Differential phosphorus and nitrogen effects drive species and community responses to elevated CO2 in semi-arid grassland. Funct Ecol 17:766-777
- Morgan JA, Pataki DE, Körner C, Clark H, Del Grosso SJ, Grünzweig JM, Knapp AK, Mosier AR, Newton PCD, Niklaus PA, Nippert JB, Nowak RS, Parton WJ, Polley HW, Shaw MR (2004) Water relations in grassland and desert ecosystems exposed to elevated atmospheric CO2. Oecologia 140:11-25



11. Mature forest canopy CO2-enrichment (2000-2015)

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 after 8 years and CO2 enrichment of spruce commenced on the hemisphere under the crane.

- Bader MFK, Leuzinger S, Keel SG, Siegwolf RTW, Hagedorn F, Schleppi P, Körner C (2013) Central European hardwood trees in a high‐CO2 future: synthesis of an 8‐year forest canopy  CO2 enrichment project. J Ecol 101:1509-1519
- Körner C (2006) Plant CO2 responses: an issue of definition, time and resource supply. New Phytol 172:393-411
- Körner C (2017) Carbon sequestration: a matter of tree longevity. Science 355:130-131
- Körner C, Asshoff R, Bignucolo O, Hättenschwiler S, Keel SG, Peláez-Riedl S, Pepin S, Siegwolf RTW, Zotz G (2005) Carbon flux and growth in mature deciduous forest trees exposed to elevated CO2. Science 309:1360-1362
- 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
- Palacio S, Hoch GU, Sala A, Körner C, Millard P (2014) Does carbon storage limit tree growth? New Phytol 201:1096-1100

For more information see the separate SCC module in the project master list.