Korean Journal of Ecology and Environment (생태와환경)

The Korean Society of Limnology (한국수생태학회)
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Effects of Elevated Atmospheric $CO_2$ on Wetland Plants: A Review

대기중 이산화탄소 농도 증가가 습지 식물에 미치는 영향

  • Kim, Seon-Young (Department of Environmental Science and Engineering, Ewha Womans University) ;
  • Kang, Ho-Jeong (Department of Environmental Science and Engineering, Ewha Womans University)
  • Published : 2003.12.31
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Abstract

Last 20 years have witnessed many studies dealing with effects of elevated $CO_2$ on terrestrial ecosystems. However, fewer efforts have been made to elucidate effects on wetland ecosystems, although they play a key role in global biogeochemical cycles. This review synthesizes published data to reveal effects of elevated $CO_2$ on wetland plants. In particular, we focused on the changes in primary production, community structures, evapotranspiration, and nutrients in plants. Many studies have reported increases in primary production in individual plants, but we could not conclude that this will lead to increases in carbon sequestration in wetland ecosystems. The reasons include transport of photosynthates into belowground parts, speciesspecific responses, interaction among different species, and limitation of other nutrients. However, elevated $CO_2$ increased transpiration rates in many wetland plants, suggesting substantial influences on water budgets of wetlands. In addition, similar to terrestrial ecosystems, elevated $CO_2$ increased C/N ratio of many plants, which may impede organic matter decomposition in the long term. However, further information on dynamics of belowground carbon supplied from wetland plants is warranted to assess effects of elevated $CO_2$ on wetland carbon cycle accurately.

지난 20여년간 대기 중 이산화탄소 농도의 증가가 육상 생태계에 미칠 영향에 대한 많은 연구가 진행되었다. 그러나 전지구적 물질 순환에 중요한 역할을 담당하는 습지 생태계에서 일어나는 반응에 대한 연구는 미흡하다. 본 종설에서는 대기 중 이산화탄소 농도가 증가했을 때 습지의 식생들이 어떠한 반응을 보일 것인지에 대해 알아보고자, 이와 연관하여 발표된 논문들의 결과를 모아 정리하였다. 특히, 습지 식생의 일차생산성, 군집 구조, 증발산량, 식물체의 영양소 등에 미치는 영향을 살펴 보았다. 이산화탄소 증가가 개개 식물의 광합성량을 증가 시키는 것은 많이 관찰 되었으나,이러한 현상이 바로 습지식생의 탄소보유를 증가시키는 것으로 결론 내릴 수 없었다. 그 이유는 고정된 탄소의 지하부로의 전달, 개개 종의 상이한 반응, 종간의 상호작용, 영양소의 부족 등 다른 요인들의 작용 때문이다. 그러나 이산화탄소 농도의 증가는 전반적으로 습지 식물의 증발산량을 감소 시키는 경향을 보였다. 한편, 육상 식물의 반응과 유사하게 많은 습지에서 이산화탄소의 증가가식생의 C/N비를 증가 시키는 것이 일부 종에서 관찰 되었으며,이러한 종에서는 장기적인 유기물분해의 속도가 감소될 수 있음을 암시한다. 그러나 지하부로 유입되는 새로운 광합성 산물들의 동태에 대한 더 많은 정보가 모아져야 정확한 예측이 가능할 것이다.

Keywords

References

  1. Adams, J.M. and H. Faure. 1998. A new estimate ofchanging carbon storage on land since the lastglacial maximum, based on global land ecosystemreconstruction. Global Plane. Change 16-17: 3-241.
  2. Arp, W.J. 1991. Effects of source-sink relations onphotosynthetic acclimation to elevated CO2. PlantCell Environ. 14: 869-875.
  3. Arp, W.J. and B.G. Drake. 1991. Increased photosyntheticcapacity of Scirpus olneyi after 4 years ofexposure to elevated CO2. Plant Cell Environ. 14:1003-1006.
  4. Arp, W.J., J.E.M. Van Mierlo, F. Berendse and W.Snijders. 1998. Interaction between elevated CO2concentration, nitrogen and water: effects ongrowth and water use of six perennial plant species.Plant Cell Environ. 21: 1-11.
  5. Azcon-Bieto, J., M.A. Gonzalez-Meler, W. Dohertyand B.G. Drake. 1994. Acclimation of respiratoryO2 uptake in green tissues of field-grown nativespecies after long-term exposure to elevatedatmospheric CO2. Plant Physiol. 106: 1163-1168.
  6. Ball, A.S. and B.G. Drake. 1998. Stimulation of soilrespiration by carbon dioxide enrichment of marshvegetation. Soil Biol. Biochem. p. 1203-1205.
  7. Berendse, F., H. Rydin, N. van Breemen, A. Buttler,S. Saarnio, H. Vasander and B. Wallen. 2001.Raised atmospheric CO2 levels and increased Ndeposition cause shifts in plant species compositionthat affects C sequestration in Sphagnumbogs. Global Change Biol. 7: 591-598.
  8. Bettarini, I., F.P. Vaccari and F. Miglietta. 1998. ElevatedCO2 concentrations and stomatal density:observations from 17 plant species growing in aCO2 spring in central Italy. Global Change Biol.4: 17-22.
  9. Chu, C.C., J.S. Coleman and H.A. Mooney. 1992.Controls of biomass partitioning between rootsand shoots: atmospheric CO2 enrichment and theacquisition and allocation of carbon and nitrogenin wild radish. Oecologia 89: 580-587.
  10. Cicerone, R.J. and R.S. Oremland. 1988. BiogeochemicalAspects of Atmospheric Methane. Global Biogeochem.Cycles 2: 299-327.
  11. Cotrufo, M.F., B. Berg and W. Kratz. 1998. Increasedatmospheric CO2 and litter quality. Environ. Rev.6: 1-12.
  12. Cotrufo, M.F. and A. Gorissen. 1997. Elevated CO2enhances belowground C allocation in three perennial grass species at different levels of N availability.New Phytol. 137: 421-431.
  13. Cotrufo, M.F. and P. Ineson. 1995. Effects of enhancedatmospheric CO2 and nutrient supply on the qualityand subsequent decomposition of the fine rootsof Betula pendula Roth. and Picea sitchensis(Bong.) Carr. Plant Soil 170: 267-277.
  14. Cotrufo, M.F., P. Ineson and A.P. Rowland. 1994.Decomposition of tree leaf litters grown underelevated CO2: effect of litter quality. Plant Soil163: 121-130.
  15. Curtis, P.S. and X. Wang. 1998. A meta-analysis ofelevated CO2 effects on woody plant mass, form,and physiology. Oecologia 113: 299-313.
  16. Curtis, P.S., B.G. Drake and D.R. Whigham. 1989.Nitrogen and carbon dynamics in C3 and C4 marshplants grown under elevated CO2 in stiu. Oecologia78: 297-301.
  17. Curtis, P.S., L.M. Balduman, B.G. Drake and D.F.Whigham. 1990. Elevated atmospheric CO2 effectson belowground processes in C3 and C4 estuarinemarsh communities. Ecology 71: 2001-2006.
  18. Dacey, V.W.H., B.G. Drake and M.J. Klug. 1994. Stimulationof methane emission by carbon dioxideenrichment of marsh vegetation. Nature 370: 47-49.
  19. Dakora, F.D. and B.G. Drake. 2000. Elevated CO2stimulates associative N2 fixation in a C3 plant ofthe Chesapeake Bay wetland. Plant Cell Environ.23: 943-953.
  20. Delucia, E., J. Hamilton, S. Naidu, R. Thomas, J.Andrews, A. Finzi, M. Lavine, R. Matamala, J.Mohan, G. Hendry and W. Schlesinger. 1999. NetPrimary production of a forest ecosystem withexperimental CO2 enrichment. Science 284: 1177-1179.
  21. Den Hertog, J., I. Stulen, F. Fonseca and P. Delea.1996. Modulation of carbon and nitrogen allocationin Urtica dioica and Plantago major by elevatedCO2: impact of accumulation of non-structuralcarbohydrates and ontogenetic drift. Physiol.Plant. 97: 77-88.
  22. Drake, B.G. 1992. A field study of the effects of elevatedCO2 on ecosystem processes in a ChesapeakeBay wetland. Aust. J. Bot. 40: 579-595.
  23. Drake, B.G., M.A. Gonzalez-Meler and S.T. Long.1997. More efficient plants: a consequence of risingatmospheric CO2. Annu. Rev. Plant Physiol.48: 609-639.
  24. Drake, B.G., M.S. Muehe, G. Peresta, M.A. Gonzalez-Meler and R. Matamala. 1996. Acclimation ofphotosynthesis, respiration and ecosystem carbonflux of a wetland on Chesapeake Bay, Marylandto elevated atmospheric CO2 concentration. PlantSoil 187: 111-118.
  25. Edwards, N.T. and R.J. Norby. 1999. Below-groundrespiratory response of sugar maple and red maplesaplings to atmospheric CO2 enrichment and elevated air temperature. Plant Soil 206: 85-97.
  26. Field, C.B., C.P. Lund, N.R. Chiariello and B.E. Mortimer.1997. CO2 effects on the water budget ofgrassland microcosm communities. Global ChangeBiol. 3: 197-206.
  27. Freeman, C., Lock M.A. and B. Reynolds. 1993. Fluxesof CO2, CH4, and N2O from a Welsh peatland followingsimulation of water table draw-down:potential feedback to climatic change. Biogeochemistry19: 31-60.
  28. Gorham, E. 1991. Northern peatlands: role in thecarbon cycle and probable responses to climaticwarming. Ecol. Appl. 1: 182-195.
  29. Griffin, K.L., D.T. Tissue, M.H. Turnbull, W. Schusterand D. Whitehead. 2001. Leaf dark respiration asa function of canopy position in Nothofagus fuscatrees grown at ambient and elevated CO2 partialpressures for 5 years. Funct. Ecol. 15: 497-505.
  30. Grulke, N.E., G.H. Riechers, W.C. Oechel, U. Hjelm,and C. Jaeger. 1990. Carbon balance in tussocktundra under ambient and elevated atmosphericCO2 Oecologia 83: 485-494.
  31. Hamilton, J.G., R.B. Thomas and E.H. Delucia. 2001.Direct and indirect effects of elevated CO2 on leafrespiration in a forest ecosystem. Plant Cell Environ.24: 975-982.
  32. Hayward, P.M. and R.S. Clymo. 1982. Profiles ofwater content and pore size in Sphagnum andpeat, and their relation to peat bog ecology. Proc.R. Soc. Lond. Series B 215: 299-325.
  33. Heijmans, M.M.P.D., F. Berendse, W.J. Arp, A.K.Masselink, H. Klees, W. de Visser, and N. VanBreemen. 2001a. Effects of elevated carbon dioxideand increased nitrogen deposition on bog vegetationin the Netherlands. Ecology 89: 268-279.
  34. Heijmans, M.M.P.D., H. Klees, and F. Berendse.2002. Competition between Sphagnum magellanicumand Eriophorum angustifolium as affectedby raised CO2 and increased N deposition. Oikos97: 415-425.
  35. Heijmans, M.M.P.D., W.J. Arp and F. Berendse.2001b. Effects of elevated CO2 and vascular plantson evapotranspiration in bog vegetation. GlobalChange Biol. 7: 817-827.
  36. Helal, H.M. and D.R. Sauerbeck. 1984. Influence ofplant roots on C and P metabolism in soil. PlantSoil 76: 175-182.
  37. Hobbie, E.A., D.M. Olszyk, P.T. Rygiewicz, D.T.Tingey and M.G. Johnson. 2001. Foliar nitrogenconcentrations and natural abundance of 15N suggestnitrogen allocation patterns of Douglas Firand mycorrhizal fungi during development in elevatedcarbon dioxide concentration and temperature.Tree Physiol. 21: 1113-1122.
  38. Hoosbeek, M.R., N. van Breemen, F. Berendse, P.Grosvernier and H. Vasander. 2001. Limited effectof increased atmospheric CO2 concentration onombrotrophic bog vegetation. New Phytol. 150:459-463.
  39. Hymns, G.J., N.R. Baker and S.P. Long. 2001. Growthin elevated CO2 can both increase and decreasephotochemistry and photoinhibition of photosynthesisin a predictable manner. Dactylis glomeratagrown in two levels of nitrogen nutrition. PlantPhysiol. 127: 1204-1211.
  40. IPCC. 2001. Climate Change 2001: The ScientificBasis, Cambridge University Press, Cambridge.
  41. Jacob, J., C. Greitner and B.G. Drake. 1995. Acclimationof photosynthesis in relation to Rubisco andnon-structural carbohydrate contents and in situcarboxylase activity in Scirpus olneyi grown atelevated CO2 in the field. Plant Cell Environ. 18:875-884.
  42. Jauhiainen, J. and J. Silvola. 1996. The effect of elevatedCO2 concentration on photosynthesis ofSphagnum fuscum. p. 11-14. In: Laiho, R., Laine,J. and Vasander, H. (eds), Northern Peatlands inGlobal Climatic Change, Publications of the Academyof Finland 1/96.
  43. Jauhiainen, J., H. Vasander and J. Matero. 1996.The effect of elevated CO2 and N-input on Sphagnawith different trophy. p. 15-17. In: Laiho, R.,Laine, J. and Vasander, H. (eds), Northern Peatlandsin Global Climatic Change, Publications ofthe Academy of Finland 1/96.
  44. Jauhiainen, J., H. Vasander and J. Silvola. 1993. Differencesin response of two Sphagnum species toelevated CO2 and Nitrogen input. Suo 43(4-5):211-215.
  45. Jauhiainen, J., H. Vasander and J. Silvola. 1994.Response of Sphagnum fuscum to N depositionand increased CO2. J. Bryology 18: 83-95.
  46. Jauhiainen, J., H. Vasander and J. Silvola. 1998bNutrient concentration in Sphagnum at increasedN-deposition rates and raised atmospheric CO2concentrations. Plant Ecol. 138: 149-160.
  47. Jauhiainen, J., J. Silvola and H. Vasander. 1998a.The effects of increased nitrogen deposition andCO2 on Sphagnum angustifolium and S. warnstorfii.Ann. Bot. Fenn. 35: 247-256.
  48. Jauhiainen, J., J. Silvola, K. Tolonen and H. Vasander.1997. Response of Sphagnum fuscum to waterlevels and CO2 concentration. J. Bryology 19: 391-400.
  49. Joabsson, A., T.R. Christensen and B. Wallén. 1999.Vascular plant controls on methane emission fromnorthern peatforming wetlands. Trends Ecol. Evol.14: 385-388
  50. Kang, H.J., C. Freeman and T.W. Ashendon. 2001.Effects of elevated CO2 on fen peat biogeochemistry.Sci. Total Environ. 279: 45-50.
  51. King, J.S., R.B. Thomas and B.R. Strain. 1997. Morphologyand tissue quality of seedling root systemsof Pinus taeda and Pinus ponderosa as affectedby varying CO2, temperature, and nitrogen.Plant Soil 195: 107-119.
  52. Koch, G.W. and H.A. Mooney. 1996. Response of terrestrialecosystems to elevated CO2: a synthesisand summary. In: Koch, G.W. and Mooney, H.A.(eds) Carbon dioxide and Terrestrial Ecosystems,Academic Press, San Diego, CA., p. 415-429.
  53. Körner, C. 1996. The response of complex multispeciessystems to elevated CO2. In: Walker, B. and Steffen,W. (eds) Global Change and Terrestrial Ecosystems.Cambridge University Press, Cambridge,UK. p. 20-42.
  54. Kuehny, J.S., M.M. Peet, P.V. Nelson and D.H. Willis.1991. Nutrient dilution by starch in CO2-enrichedChrysanthemum. J. Exp. Bot. 42: 711-716.
  55. Lambers, H. 1987. Growth, respiration, exudationand symbiotic associations: the fate of carbontranslocated to the roots, root Development andFunction. Soc. Exp. Biol. Sem. Ser. 30: 125-145.
  56. Leadley, P.W., P.A. Niklaus, R. Stocker and C. Korner.1999. A field study of the effects of elevatedCO2 on plant biomass and community structure ina calcareous grassland. Oecologia 118: 39-49.
  57. Levis, S., J.A. Foley and D. Pollard. 2000. Large scalevegetation feedbacks on a doubled CO2 climate. J.Clim. 13: 1313-1325.
  58. Long, S.P. and B.G. Drake, 1992. Photosynthetic CO2assimilation and rising atmospheric CO2 concentrations.In: Crop photosynthesis: Spatial andTemporal Determinations. (eds) Baker, N.R. andThomas., H. p. 69-103. Elsevier Science PublisersB.V. Amsterdam, The Netherlands.
  59. Matamala, R. and B.G. Drake. 1999. The influence ofatmospheric CO2 enrichment on plant-soil nitrogeninteractions in a wetland plant community onthe Chesapeake Bay. Plant Soil 210: 93-101.
  60. Megonigal, J.P. and W.H. Schlesinger. 1997. EnhancedCH4 emissions from a wetland soil exposed toElevated CO2. Biogeochemistry 37: 77-88.
  61. Mitchell, E.A.D., A. Buttler, P. Grosvernier, H. Rydin,A. Siegenthaler and J-M. Gobat. 2002. Contrastedeffects of increased N and CO2 supply on two keystonespecies in peatland restoration and implicationsfor global change. Ecology 90: 529-533.
  62. Mooney, H.A., J. Canadell, F.S. Chapin, J.R. Ehleringer,C. Körner, R.E. McMurtrie, W.J. Parton, L.F.Pitelka and E-D. Schulze. 1999. Ecosystemphysiology responses to global change. In: WalkerB, Steffen W, Canadell J, Ingram J. (eds), The terrestrialbiosphere and global change, CambridgeUniversity Press, Cambridge, UK, p. 141-189.
  63. Niklaus, P.A., M. Wohlfender, R. Slegwolf and C.Korner. 2001. Effects of six years of atmosphericCO2 enrichment on plant, soil and soil microbial Cof a calcareous grassland. Plant Soil 233: 189-202.
  64. Norby, R.J., S.D. Wullschleger, C.A. Gunderson,D.W. Johnson and R. Ceuemans. 1999. Tree responsesto rising CO2 in field experiments: implicationsfor the future forest. Plant Cell Environ 22:683-714.
  65. Oechal, W.C. and G.L. Vourlitis. 1996. Direct effectsof elevated CO2 on arctic plant and ecosystemfunction. In: Koch, G.W. and Mooney, H.A. (eds),Carbon dioxide and Terrestrial Ecosystems, AcademicPress, San Diego, CA., p. 163-176.
  66. Oechel, W.C., S. Cowles, N. Grulke, S.J. Hastings, B.Lawrence, T. Prudhomme, G. Riechers, B. Strain,D. Tissue and G. Vourlitis. 1994. Transient natureof CO2 fertilization in Arctic tundra. Nature 371:500-503.
  67. Owensby, C.E., J.M. Ham, A.K. Knapp, D. Bremerand L.M. Auen. 1997. Water vapour fluxes andtheir impact under elevated CO2 in a C4-tallgrassprairie. Global Change Biol. 3: 189-195.
  68. Poorter, H. 1993. Interspecific variation in the growthresponse of plants to an elevated ambient CO2concentration. Vegetatio 104/105: 77-97.
  69. Schrope, M.K., J.P. Chanton, L.H. Allen and J.T.Baker. 1999. Effect of CO2 enrichment and elevatedtemperature on methane emissions from rice,Oryza sativa. Global Change Biol. 5: 587-599.
  70. Silvola, J. 1990. Combined effects of varying watercontent and CO2 concentration on photosynthesisin Sphagnum fuscum. Hol. Ecol. 13: 224-228.
  71. Silvola, J. and U. Ahlholm. 1993. Effects of CO2 concentrationand nutrient status on growth, growthrhythm and biomass partitioning in a willow, salixphylicifolia. Oikos 67: 227-234.
  72. Smolders, A.J.P., H.B.M. Tomassen, H.W. Pijnappel,L.P.M. Lamers and J.G.M. Roelofs. 2001. Substrate-derived CO2 is important in the developmentof Sphagnum spp. New Phytol. 152: 325-332.
  73. Stulen, I., J. Den Hertog, F. Drelon and J. Roy. 1994.An integrated approach to the influence of CO2 onplant growth using data for three herbaceous species.In: A Whole Plant Perspective on Carbon-Nitrogen Interactions. (eds) Roy, J. and E. Garnier,p. 229-245. SPB Academic Publishing BV,The Hague.
  74. Tissue, D.T. and W.C. Oechel. 1987. Response of Eriophorumvaginatum to elevated CO2 and temperaturein the Alaskan tussock tundra. Ecology 68:401-410.
  75. Tissue, D.T., K.L. Griffin, M.H. Turnbull and D. Whitehead.2001. Canopy position and needle ageaffect photosynthetic response in field grown Pinusradiata after five years exposure to elevated carbondioxide partial pressure. Tree Physiol. 21: 915-923.
  76. Tissue, D.T., R.B. Thomas and B.R. Strain. 1993.Long-term effects of elevated CO2 and nutrientson photosynthesis and Rubisco in loblolly pine.Plant Cell Environ. 16: 859-865.
  77. Van Breemen, N. 1995. How Sphagnum bogs downother plants. Trends Ecol. Evol. 10: 270-275.
  78. Van der Heijden, E., J. Jauhiainen, J. Matero and H. Vasander. 1996. The effects of elevated CO2 andN-input on Sphagnum physiology, p. 57-58. In:Schedule and abstracts of Second InternationalSymposium on the biology of Sphagnum, UniversitéLaval, Québec City, Canada, July 12th-13th.
  79. Van der Heijden, E., J. Jauhiainen, J. Silvola, H.Vasander and P.J.C. Kuiper. 2000b. Effects ofelevated atmospheric CO2 concentration and increasednitrogen deposition on growth and chemicalcomposition of ombrotrophic Sphagnum balticumand oligo-mesotrophic Sphagnum papillosum.J. Bryology 22: 175-182.
  80. Van der Heijden, E., S.K. Verbeek and P.J.C. Kuiper.2000a. Elevated atmospheric CO2 and increasednitrogen deposition: effects on C and N metabolismand growth of the peat moss Sphagnum recurvumP. Beauv. Var. mucronatum (Russ.) Warnst. GlobalChange Biol. 6: 201-212.
  81. Van der Kooij, T.A.W. and L.J. De Kok. 1996. Impactof elevated CO2 on growth and development ofArabidopsis thaliana L. Phyton 36(2): 173-184
  82. Van Ginkel, J.H., A. Gorissen and J.A. van Veen.1997. Carbon and nitrogen allocation in Loliumperenne in response to elevated atmospheric CO2with emphasis on soil carbon dynamics. Plant Soil188: 299-308.
  83. Van Oosteen, J.J. and R.T. Besford. 1995. Some relationshipsbetween the gas exchange, biochemistryand molecular biology of photosynthesis duringleaf development of tomato plants after transferto different carbon dioxide concentrations. PlantCell Environ. 18: 1253-1266.
  84. Vann, C.D. and J.P. Megonigal. 2002. Productivityresponses of Acer rubrum and Taxodium distichumseedlings to elevated CO2 and flooding. Environ.Pollut. 116: S31-S36.
  85. Ven der Heijden, E., J. Jauhiainen, J. Matero, M.Eekhof and E. Mitchell. 1998. Effects of elevatedCO2 and nitrogen deposition on Sphagnum species.In: de Kok, L.J., and Stulen, I. (eds), Responsesof Plant Metabolism to Air Pollution, Backhuys,Leiden, p. 475-478.
  86. Walch-Liu, P., G. Neumann and C. Engels. 2001.Elevated atmospheric CO2 concentration favorsnitrogen partitioning into roots of tobacco plantsunder nitrogen deficiency by decreasing nitrogendemand of the shoot. J. Plant Nutr. 24: 835-854.
  87. Wand, S.E.J., G.F. Midgley, M.H. Jones, and P.S.Curtis. 1999. Responses of wild C4 and C3 grass(Poaceae) species to elevated atmospheric CO2concentration: a test of current theories and perceptions.Global Change Biol. 5: 723-741.
  88. Warwick, K.R., G. Taylor and H. Blum. 1998. Biomassand compositional changes occur in chalkgrassland turves exposed to elevated CO2 for twoseasons in FACE. Global Change Biol. 4: 375-385.
  89. Whipps, J.M. and J.M. Lynch. 1983. Substrate flowand utilization in the rhizosphere of cereals. NewPhytol. 95: 605-623.
  90. Zak, D.R., K.S. Pregitzer, J.S. King and W.E. Holmes.2000. Elevated atmospheric CO2, fine roots andthe response of soil microorganisms: a review andhypothesis. Nesw Phytol. 147: 201-222.
  91. Ziska, L.H., K.P. Hogan, A.P. Smith and B.G. Drake.1991. Growth and photosynthetic response of ninetropical species with long-term exposure to elevatedcarbon dioxide. Oecologia 86: 383-389.