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Temperature x CO2 Interaction Plant Growth Response (Other Species)
As the air's CO2 content continues to rise, most plants will exhibit increased rates of photosynthesis and biomass production, including those of various natural ecosystems.  This increase in productivity could possibly reduce the land area occupied by bare soil in certain places and thus increase the amount of biological carbon sequestration occurring within terrestrial environments.  However, some individuals have predicted that global warming may negate the growth-promoting effects of atmospheric CO2 enrichment and stimulate respiratory carbon losses.  In this summary, we review the results of recent scientific articles that have studied plant responses to elevated CO2 and temperature to better understand how various ecosystems' native plants will respond to possible changes in these atmospheric parameters.

Before beginning our mini-review of the pertinent literature in this field, it is important to note that the optimum growth temperatures of several plants have already been shown to rise substantially with increasing levels of atmospheric CO2 (McMurtrie and Wang, 1993; McMurtrie et al., 1992; Stuhlfauth and Fock, 1990; Berry and Bjorkman, 1980).  This phenomenon was predicted by Long (1991), who calculated from well-established plant physiological principles that most C3 plants should increase their optimum growth temperature by approximately 5C for a 300 ppm increase in the air's CO2 content.  Subsequently, Cowling and Sykes (1999) produced a literature review demonstrating that this was indeed the case for a number of different species.  One would thus expect plant photosynthetic rates to rise in response to concomitant increases in the air's CO2 concentration and temperature, as previously documented by Idso and Idso (1994).  Hence, we here proceed to see if these previously-determined positive CO2 x temperature interactions are continuing to appear in the scientific literature, particularly among the native plants of various natural ecosystems.

In a mechanistic model study of Mediterranean shrub vegetation, Osborne et al. (2000) reported that increased warming and reduced precipitation would likely decrease net primary production.  However, when the same model was run at twice the ambient atmospheric CO2 concentration, it predicted a 25% increase in vegetative productivity, in spite of the increased warming and reduced precipitation.  Although we tend to not review studies based on mechanistic models, it is also interesting to note that
Bunce (2000) demonstrated that field-grown Taraxacum officinale plants exposed to 525 ppm CO2 and low air temperatures (between 15 and 25C) displayed photosynthetic rates that were 10 to 30% greater than what was predicted by state-of-the-art biochemical models of photosynthesis for this range of temperatures.  Thus, at both high and low air temperatures, elevated CO2 appears to be capable of significantly increasing the photosynthetic prowess of certain of the native plants of earth's natural ecosystems.

In the real world, Stirling et al. (1998) grew five fast-growing native species at various atmospheric CO2 concentrations and air temperatures, finding that twice-ambient levels of atmospheric CO2 increased photosynthetic rates by 18-36% for all species regardless of air temperature, which was up to 3C higher than ambient air temperature.  In addition, atmospheric CO2 enrichment increased average plant biomass by 25%, also regardless of air temperature.  Likewise, in a study of vascular plants from Antarctica, Xiong et al. (2000) reported that a 13C rise in air temperature increased plant biomass by 2- to 3-fold.  We can only imagine what the added benefit of atmospheric CO2 enrichment would do for these species!

On another note, Hamerlynck et al. (2000) demonstrated that the desert perennial shrub Larrea tridentata maintained more favorable midday leaf water potentials during a nine-day high-temperature treatment when fumigated with 700 ppm CO2, as compared to 350 ppm, which would likely favor enhanced rates of photosynthesis as well.

In conclusion, the recent scientific literature continues to indicate that as the air's CO2 content rises, the native plants of earth's natural ecosystems will likely exhibit enhanced rates of photosynthesis and biomass production that will not be diminished by any realistic increase in air temperature.  In fact, if the ambient air temperature does rise somewhat, the growth-promoting effects of atmospheric CO2 enrichment will likely rise right along with it, in agreement with the experimental observations reviewed by Idso and Idso (1994).  Thus, the productivity of earth's natural ecosystems will likely continue its upward trend, even in the face of modest global warming, as long as the air's CO2 content is in an ascending mode as well.

References
Berry, J. and Bjorkman, O.  1980.  Photosynthetic response and adaptation to temperature in higher plants.  Annual Review of Plant Physiology 31: 491-543.

Bunce, J.A.  2000.  Acclimation to temperature of the response of photosynthesis to increased carbon dioxide concentration in Taraxacum officinalePhotosynthesis Research 64: 89-94.

Cowling, S.A. and Sykes, M.T.  1999.  Physiological significance of low atmospheric CO2 for plant-climate interactions.  Quaternary Research 52: 237-242.

Hamerlynck, E.P., Huxman, T.E., Loik, M.E. and Smith, S.D.  2000.  Effects of extreme high temperature, drought and elevated CO2 on photosynthesis of the Mojave Desert evergreen shrub, Larrea tridentataPlant Ecology 148: 183-193.

Idso, K.E. and Idso, S.B.  1994.  Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: A review of the past 10 years' research.  Agricultural and Forest Meteorology 69: 153-203.

Long, S.P.  1991.  Modification of the response of photosynthetic productivity to rising temperature by atmospheric CO2 concentrations: Has its importance been underestimated?  Plant, Cell and Environment 14: 729-739.

McMurtrie, R.E. and Wang, Y.-P.  1993.  Mathematical models of the photosynthetic response of tree stands to rising CO2 concentrations and temperatures.  Plant, Cell and Environment 16: 1-13.

McMurtrie, R.E., Comins, H.N., Kirschbaum, M.U.F. and Wang, Y.-P.  1992.  Modifying existing forest growth models to take account of effects of elevated CO2Australian Journal of Botany 40: 657-677.

Osborne, C.P., Mitchell, P.L., Sheehy, J.E. and Woodward, F.I.  2000.  Modeling the recent historical impacts of atmospheric CO2 and climate change on Mediterranean vegetation.  Global Change Biology 6: 445-458.

Stirling, C.M., Heddell-Cowie, M., Jones, M.L., Ashenden, T.W. and Sparks, T.H.  1998.  Effects of elevated CO2 and temperature on growth and allometry of five native fast-growing annual species.  New Phytologist 140: 343-354.

Stuhlfauth, T. and Fock, H.P.  1990.  Effect of whole season CO2 enrichment on the cultivation of a medicinal plant, Digitalis lanataJournal of Agronomy and Crop Science 164: 168-173.

Xiong, F.S., Meuller, E.C. and Day, T.A.  2000.  Photosynthetic and respiratory acclimation and growth response of Antarctic vascular plants to contrasting temperature regimes.  American Journal of Botany 87: 700-710.