Plant Responses to Climatic Warming
Environmental Sciences Division, Oak Ridge National Laboratory


Current Projects
Completed Projects




Temperature and CO2 Interactions in Trees


  • Describe the effects of elevated CO2 and elevated mean air temperature on tree growth and physiological processes, with an emphasis on measures that are relevant to tree growth in a forest and data that are necessary for parameterizing or testing plant and ecosystem process models.
  • Describe the effects of elevated CO2 and air temperature on ecosystem-level processes that control or influence C sequestration in ecosystems.
  • Test specific hypotheses about the differential response of the two species to elevated CO2 and temperature in a way that will lead to a better understanding of community and regional-scale responses to global change. Acer rubrum (red maple) and Acer saccharum (sugar maple) differ in their geographical ranges and ecological amplitude, and we predicted that these characteristics would govern their relative responses to elevated CO2 and temperature.


  • Effects of elevated temperature will be manifested primarily through alteration of developmental rates, phenology, and water relations rather than through direct effects on carbon balance
  • Increased temperature will favor red maple over sugar maple
  • Elevated CO2 will compensate for an adverse response to temperature increase, especially in sugar maple


Site Characteristics
The research was conducted at the Global Change Field Research Site on the Oak Ridge National Environmental Research Park. The soil is classified as Captina silt loam -- fine-silty, siliceous, mesic Typic Fragiudult. The mean annual temperature is 13.9 C, and mean annual precipitation: is 1322 mm; the mean length of the growing season is185 days.

Tree seedlings (Acer saccharum and A. rubrum) were planted in the soil in 1994 within 3-m diameter open-top chambers. The chambers were wrapped in 70% shade cloth to create a more typical forest environment for these species. Air temperature within the chambers was maintained at ambient or ambient + 4 C with a combination of evaporative coolers and resistance heaters. The two temperature levels were crossed with two levels of atmospheric CO2 (ambient and ambient + 300 ppm) with 3 replicates (12 chambers total). The temperature treatments were maintained year-round for 3.5 years (April, 1994 through September, 1997). Soil temperature was 1-2 C higher in the warmer chambers.

Aboveground growth was estimated non-destructively through periodic measurement of tree height and basal diameter, followed by destructive harvest at the end of the experiment. Foliar gas exchange and N content were measured periodically each year. Phenology was tracked through visual observation of bud burst in spring and leaf abscission in fall. Fine root production and turnover were monitored through minirhizotron tubes. Soil respiration was measured in a closed gas exchange system. Changes in soil carbon content were measured in buried mesh bags containing homogenized soil.


  • Temperature elevation increased aboveground growth rates because of its effect on phenology. Buds opened earlier in the spring, and leaves were retained longer in the fall, thereby lengthening the growing season in the warmer chambers.
  • The generally positive effect of elevated temperature on growth did not compensate for the negative effect that occurred during a particularly hot, dry period in the second summer. As the plants were undergoing exponential growth during the course of this experiment, the negative effects of increased temperature on dry matter accumulation that occurred during one period of stress persisted for the remainder of the experiment, and stem mass of was significantly less for plants grown in elevated vs. ambient temperature.
  • Other temperature effects included increased fine root production (especially in elevated CO2) and, in A. saccharum, increased total root respiration. Leaf respiration apparently acclimated to increased temperature and was elevated only in the spring. The negative effects of increased temperature on growth were completely offset by CO2 enrichment.
  • Fine root specific respiration rates were increased with temperature, but only in ambient CO2
  • Leaf respiration was higher in elevated temperature only in the spring, suggesting that acclimation occurred
  • The two species responded similarly to increased temperature despite the more southerly distribution of red maple compared to sugar maple
  • Aboveground growth was enhanced by elevated CO2, which can be associated with the stimulation of photosynthesis. The growth enhancement was sustained in red maple but not in sugar maple
  • High CO2 increased fine root production, total root respiration, and NO3- uptake, and lowered specific root respiration rate and Vmax for NO3- and NH4+
  • Elevated CO2 mitigated against the negative effects of elevated temperature. The effects of CO2 and temperature were mostly additive
  • There were no treatments effects on microbial biomass or respiration, or N mineralization


  • Predictions of forest response to climatic warming must include consideration of the co-occurrence of elevated atmospheric CO2
  • Temperature has variable effects on different plant and soil processes, and the effects vary during the year
  • The net effect of warming will depend on the frequency and timing of events such as drought or freezing temperature
  • Hence, forest response to warming cannot be reduced to a simple function

Project Personnel

Richard J. Norby, principal investigator, aboveground growth
Carla A. Gunderson, foliar gas exchange
Elizabeth G. O'Neill, root growth
Nelson T. Edwards, soil respiration
Stan D. Wullschleger
Chuck Garten, soil nitrogen
Jeff Riggs, instrumentation and control

David Tissue, Texas Tech University
Kurt Pregitzer, Michigan Tech University
Dale Johnson and Weixin Cheng, Desert Research Institute
Gregory Carter, NASA
Raj Bahadur,
Dave Lincoln and Ray Williams, University of South Carolina
Hormoz BassiriRad, University of Illinois-Chicago
Julie Jastrow, Argonne National Laboratory
Kevin Harrison, Duke University (now at Boston University)
Philippe Vivin, INRA

Student participants
Maria Verbrugge
Jennifer Hartz
Rita Irwin
Joanne Ledford
Tammy Long

Project publications

Primary reports

Norby, R. J., N. T. Edwards, J. S. Riggs, C. H. Abner, S. D. Wullschleger, and C. A. Gunderson. 1997. Temperature-controlled open-top chambers for global change research. Global Change Biology 3:259-267.

BassiriRad H., S. A. Prior, R. J. Norby, and H. H. Rogers. 1999. A field method of determining NH4+ and NO3- uptake kinetics in intact roots: Effects of CO2 enrichment on trees and crop species. Plant and Soil 217:195-204.

Edwards, N.T. and R.J. Norby. 1998. Below-ground respiratory responses of sugar maple and red maple saplings to atmospheric CO2 enrichment and elevated air temperature. Plant and Soil 206:85-97. [abstract]

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

Norby R.J., T. M. Long, J. S. Hartz-Rubin, and E. G. O'Neill. 2000. Nitrogen resorption in senescing tree leaves in a warmer, CO2-enriched atmosphere. Plant and Soil 224: 15-29. [abstract at SpringerLink]  |  [full text in pdf]

Williams, R. S., R. J. Norby, and D. E. Lincoln. 2000. Effects of elevated CO2 and temperature-grown red and sugar maple on gypsy moth performance. Global Change Biology 6: 685-695. [abstract]  |  [full text in pdf]

Williams RS, Lincoln DE, Norby RJ. 2003. Development of gypsy moth larvae feeding on red maple saplings at elevated CO2 and temperature. Oecologia 137:114-122. [abstract]  |  [full text in pdf]

Norby RJ, Hartz-Rubin J, Verbrugge MJ. Pheonological responses in maple to experimental atmospheric warming and CO2 enrichment. Global Change Biology 9: 1792-1801. [abstract at Blackwell Synergy]  |  [full text in pdf]

Norby RJ, Luo Y. 2004. Evaluating ecosystem responses to rising atmospheric CO2 and global warming in a multi-factor world. New Phytologist 162: 281-293. [abstract at Blackwell Synergy]  |  [full text in pdf]

Wan S, Norby RJ, Pregitzer KS, Ledford J, O'Neill EG. 2004. CO2 Enrichment and warming of the atmosphere enhance both productivity and mortality of maple tree fine roots. New Phytologist 162: 437-446. [abstract at Blackwell Synergy]  |  [full text in pdf]

Other publications using project results

Norby, R. J., S. D. Wullschleger, and C. A.Gunderson. 1996. Tree responses to elevated CO2 and the implications for forests. pp. 1-21 In: G. W. Koch and H. A. Mooney (eds.), Terrestrial Ecosystem Response to Elevated Carbon Dioxide. Academic Press, San Diego.

Peterson, A.G., Ball, J.T., Luo Y., Field C.B., Reich P.B., Curtis P.S., Griffin K.L., Gunderson C.A., Norby, R.J., Tissue, D.T., Forstreuter M.., Rey A., Vogel C.S. & CMEAL participants. 1999. The photosynthesisóleaf nitrogen relationship at ambient and elevated carbon dioxide: a meta-analysis. Global Change Biology 5:331-346.

Peterson, A.G., Ball, J.T., Luo Y., Field C.B., Curtis P.S., Griffin K.L., Gunderson C.A., Norby, R.J., Tissue, D.T., Forstreuter M., Rey A., Vogel C.S. & CMEAL participants. 1999. Quantifying the response of photosynthesis to changes in leaf nitrogen content and leaf mass per area in plants grown under atmospheric CO2 enrichment. Plant, Cell and Environment 22: 1109-1119.

Abstracts of Presentations

Hartz, J. S. and R. J. Norby. 1995. Effects of elevated temperature and elevated CO2 on foliar senescence of Acer seedlings. Bulletin of the Ecological Society of America (Suppl.) 76:111.

Norby, R. J., C. A. Gunderson, N. T. Edwards, S. D. Wullschleger, and E. G. O'Neill. 1995. TACIT: Temperature and CO2 interactions in trees. Photosynthesis and growth. Bulletin of the Ecological Society of America (Suppl.) 76:197

O'Neill, E. G. and R. J. Norby. 1996. Litter quality and decomposition rates of foliar litter produced under CO2 enrichment. In: G. W. Koch and H. A. Mooney (eds.), Terrestrial Ecosystem Response to Elevated Carbon dioxide. Academic Press, San Diego (in press).

Norby, R. J., M. J. Verbrugge, J. S. Hartz, S. D. Wullschleger, C. A. Gunderson, E. G. O'Neill, and N. T. Edwards. 1998. Increased temperature has both positive and negative influences on tree growth. Abstracts, GCTE-LUCC Open Science Conference on Global Change, Barcelona, Spain, 14-18 March, 1998.

range of red maple red maple range
range of sugar maple sugar maple range

(click to enlarge)

shade cloth OTCs Inside OTC

Respiration collar fall colors canopy access LiCor - upper leaves measuring photosynthesis LiCor OTC at end of experiment final harvest soil sampling soil bag

Final dry mass graph

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Last revised: June 13, 2007