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Temperature x CO2 Interaction – Plant Growth Response (Trees)
As the air’s CO2 content continues to rise, most trees will likely exhibit increased rates of photosynthesis and biomass production, which can subsequently increase the amount of timber products available to meet the growing needs of our expanding human population.  However, some individuals have predicted that global warming may negate the growth-promoting effects of atmospheric CO2 enrichment and actually reduce tree growth.  In order to determine if this conjecture has any validity, we turn to the peer-reviewed scientific literature and summarize the results of several CO2-enrichment studies designed to reveal the concurrent effects of elevated CO2 and air temperature on the growth of trees.

Before progressing, however, we point out that the optimum growth temperature for several plants has 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 5°C 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 plants.  One would thus expect plant photosynthetic rates to rise with 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 positive CO2 x temperature interactions have been validated in the recent scientific literature for any tree species.

In the study of Kellomaki and Wang (2001), birch seedlings were grown at atmospheric CO2 concentrations of 350 and 700 ppm in combination with ambient and elevated (ambient plus 3°C) air temperatures.  After five months of treatment, the authors reported that photosynthetic rates of CO2-enriched seedlings were 21 and 28% greater than those displayed by their ambiently-grown counterparts at ambient and elevated air temperatures, respectfully.  In another study, Carter et al. (2000) observed that a 300 ppm increase in the air’s CO2 content allowed leaves of sugar maple seedlings to remain green and non-chlorotic when exposed to air temperatures 3°C above ambient air temperature.  On the other hand, seedlings fumigated with ambient air exhibited severe foliar chlorosis when exposed to the same elevated air temperatures.  These results thus indicate that at elevated air temperatures, rates of photosynthesis are greater and foliar health is typically better in CO2-enriched as opposed to ambiently-grown trees.

Other studies report similar results.  Sheu et al. (1999), for example, grew a sub-tropical tree at day/night temperatures of 25/20 (ambient) and 30/25°C (elevated) for six months and reported that seedlings exposed to 720 ppm CO2 displayed photosynthetic rates that were 20 and 40% higher, respectively, than that of their ambiently-grown controls.  In addition, the CO2-induced increases in total dry weight for this species were 14 and 49%, respectively, at ambient and elevated air temperatures.  Likewise, Maherali et al. (2000) observed that a 5°C increase in ambient air temperature increased the CO2-induced biomass enhancement resulting from a 750 ppm CO2 enrichment of ponderosa pine seedlings from 42 to 62%.  Moreover, Wayne et al. (1998) reported that a 5°C increase in the optimal growth temperature of yellow birch seedlings fumigated with an extra 400 ppm CO2 increased the CO2-induced increase in biomass from 60 to 227%.  Thus, the beneficial effects of elevated CO2 on tree species photosynthesis and growth is often enhanced due to elevated air temperatures, which can also be assessed during natural seasonal temperature changes, as documented by Hymus et al. (1999) for loblolly pine and Roden et al. (1999) for snow gum seedlings.

In some cases, however, there appear to be little interactive effects between elevated CO2 and temperature on photosynthesis and growth in tree species.  When Tjoelker
et al. (1998a)
, for example, grew seedlings of quaking aspen, paper birch, tamarack, black spruce and jack pine at atmospheric CO2 concentrations of 580 ppm, they reported average increases in photosynthetic rates of 28%, regardless of temperature, which varied from 18 to 30°C.  After analyzing the CO2-induced increases in dry mass for these seedlings, Tjoelker et al. (1998b) further reported that dry mass values were about 50 and 20% greater for the deciduous and coniferous species, respectively, regardless of air temperature.

In conclusion, the recent scientific literature continues to indicate that as the air’s CO2 content rises, trees will likely exhibit enhanced rates of photosynthesis and biomass production that will not be negated by any global warming that might occur concurrently.  In fact, if the ambient air temperature rises, 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 future ability of earth’s trees to produce greater amounts of biomass and, hence, more timber products to meet the increasing needs of an expanding human population looks promising indeed, as long as the CO2 content of the air continues to rise.

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

Carter, G.A., Bahadur, R. and Norby, R.J.  2000.  Effects of elevated atmospheric CO2 and temperature on leaf optical properties in Acer saccharumEnvironmental and Experimental Botany 43: 267-273.

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

Hymus, G.J., Ellsworth, D.S., Baker, N.R. and Long, S.P.  1999.  Does free-air carbon dioxide enrichment affect photochemical energy use by evergreen trees in different seasons?  A chlorophyll fluorescence study of mature loblolly pine.  Plant Physiology 120: 1183-1191.

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: 53-203.

Kellomaki, S. and Wang, K.-Y.  2001.  Growth and resource use of birch seedlings under elevated carbon dioxide and temperature.  Annals of Botany 87: 669-682.

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.

Maherali, H. and DeLucia, E.H.  2000.  Interactive effects of elevated CO2 and temperature on water transport in ponderosa pine.  American Journal of Botany 87: 243-249.

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.

Roden, J.S., Egerton, J.J.G. and Ball, M.C.  1999.  Effect of elevated [CO2] on photosynthesis and growth of snow gum (Eucalyptus pauciflora) seedlings during winter and spring.  Australian Journal of Plant Physiology 26: 37-46.

Sheu, B.-H. and Lin, C.-K.  1999.  Photosynthetic response of seedlings of the sub-tropical tree Schima superba with exposure to elevated carbon dioxide and temperature.  Environmental and Experimental Botany 41: 57-65.

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.

Tjoelker, M.G., Oleksyn, J. and Reich, P.B.  1998a.  Seedlings of five boreal tree species differ in acclimation of net photosynthesis to elevated CO2 and temperature.  Tree Physiology 18: 715-726.

Tjoelker, M.G., Oleksyn, J. and Reich, P.B.  1998b.  Temperature and ontogeny mediate growth response to elevated CO2 in seedlings of five boreal tree species.  New Phytologist 140: 197-210.

Wayne, P.M., Reekie, E.G. and Bazzaz, F.A.  1998.  Elevated CO2 ameliorates birch response to high temperature and frost stress: implications for modeling climate-induced geographic range shifts.  Oecologia 114: 335-342.