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Effects of Future Heat Waves on Pine and Oak Tree Photosynthesis
Ameye, M., Wertin, T.M., Bauweraerts, I., McGuire, M.A., Teskey, R.O. and Steppe, K. 2012. The effect of induced heat waves on Pinus taeda and Quercus rubra seedlings in ambient and elevated CO2 atmospheres. New Phytologist 196: 448-461.

The authors write that in studies where the air's CO2 content has been doubled, "increases in net photosynthesis were reported ranging from 43% to 192% in Pinus taeda (Teskey, 1997; Tissue et al., 1997; Ellsworth, 1999; Wertin et al., 2010; Frenck et al., 2011) and from 30% to 256% in Quercus rubra (Kubiske and Pregitzer, 1996; Anderson and Tomlinson, 1998; Cavender-Bares et al., 2000)." Likewise, they say that "generally, an increase in air temperature also has a positive effect on net photosynthesis and growth," citing Sage and Kubien (2007) and Way and Oren (2010). But how will loblolly pine and northern red oak trees respond to the extreme heat waves that are often predicted to occur in a future CO2-enriched world?

What was done
Working with the most recent fully-developed leaves of well watered and fertilized seedlings grown in 7.6-L pots out-of-doors at Athens, Georgia (USA) within polyethylene chambers maintained at ambient and elevated air temperatures (Tamb and Tamb + 3°C), as well as seven-day heat waves consisting of a biweekly +6°C heat wave or a monthly +12°C heat wave - which treatments were maintained throughout the growing season - Ameye et al. measured rates of net photosynthesis before, during and after the mid-summer heat waves they created.

What was learned
The six scientists report that "an immediate and significant decline in net photosynthesis was observed in seedlings that were subjected to a +12°C heat wave, but not in seedlings subjected to a +6°C heat wave." And they remark that "after the third day of the +12°C heat wave, net photosynthesis values stabilized at positive values and did not show signs of further reduction, indicating that the photosynthetic apparatus did not accrue additional stress or damage as the heat wave continued."

What it means
Ameye et al. conclude that "if soil moisture is adequate, trees will experience negative effects in photosynthetic performance only with the occurrence of extreme heat waves." And in light of the fact that "elevated CO2 diminished these negative effects," they opine that "the future climate may not be as detrimental to plant communities as previously assumed."

Anderson, P.D. and Tomlinson, P.T. 1998. Ontogeny affects response of northern red oak seedlings to elevated CO2 and water stress. I. Carbon assimilation and biomass production. New Phytologist 140: 477-491.

Cavender-Bares, J., Potts, M., Zacharias, E. and Bazzaz, F.A. 2000. Consequences of CO2 and light interactions for leaf phenology, growth, and senescence in Quercus rubra. Global Change Biology 6: 877-887.

Ellsworth, D.S. 1999. CO2 enrichment in a maturing pine forest: are CO2 exchange and water status in the canopy affected? Plant, Cell and Environment 22: 461-472.

Frenck, G., van der Linden, L., Mikkelsen, T.N., Brix, H. and Jorgensen, R.B. 2011. Increased CO2 does not compensate for negative effects on yield caused by higher temperature and O3 in Brassica napus L. European Journal of Agronomy 35: 127-134.

Kubiske, M.E. and Pregitzer, K.S. 1996. Effects of elevated CO2 and light availability on the photosynthetic response of trees of contrasting shade tolerance. Tree Physiology 16: 351-358.

Sage, R.F. and Kubien, D.S. 2007. The temperature response of C3 and C4 photosynthesis. Plant, Cell and Environment 30: 1086-1106.

Teskey, R.O. 1997. Combined effects of elevated CO2 and air temperature on carbon assimilation of Pinus taeda trees. Plant, Cell and Environment 20: 373-380.

Tissue, D.T., Thomas, R.B. and Strain, B.R. 1997. Atmospheric CO2 enrichment increases growth and photosynthesis of Pinus taeda: a 4-year experiment in the field. Plant, Cell and Environment 20: 1123-1134.

Way, D.A. and Oren, R. 2010. Differential responses to changes in growth temperature between trees from different functional groups and biomes: a review and synthesis of data. Tree Physiology 30: 669-688.

Wertin, T.M., McGuire, M.A. and Teskey, R.O. 2010. The influence of elevated temperature, elevated atmospheric CO2 concentration and water stress on net photosynthesis of loblolly pine (Pinus taeda L.) at northern, central and southern sites in its native range. Global Change Biology 16: 2089-2103.

Reviewed 13 March 2013