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The Thermal Optimality of Entire Ecosystems
Niu, S., Luo, Y., Fei, S., Yuan, W., Schimel, D., Law, B.E., Ammann, C., Arain, M.A., Arneth, A., Aubinet, M., Barr, A., Beringer, J., Bernhofer, C., Black, T.A., Buchmann, N., Cescatti, A., Chen, J., Davis, K.J., Dellwik, E., Desai, A.R., Etzold, S., Francois, L., Gianelle, D., Gielen, B., Goldstein, A., Groenendijk, M., Gu, L., Hanan, N., Helfter, C., Hirano, T., Hollinger, D.Y., Jones, M.B., Kiely, G., Kolb, T.E., Kutsch, W.L., Lafleur, P., Lawrence, D.M., Li, L., Lindroth, A., Litvak, M., Loustau, D., Lund, M., Marek, M., Martin, T.A. Matteucci, G., Migliavacca, M., Montagnani, L., Moors, E., Munger, J.W., Noormets, A., Oechel, W., Olejnik, J., Paw U., K.T., Pilegaard, K., Rambal, S., Raschi, A., Scott, R.L., Seufert, G., Spano, D., Stoy, P., Sutton, M.A., Varlagin, A., Vesala, T., Weng, E., Wohlfahrt, G., Yang, B., Zhang, Z. and Zhou, X. 2012. Thermal optimality of net ecosystem exchange of carbon dioxide and underlying mechanisms. New Phytologist 194: 775-783.

In introducing their huge collaborative study, the sixty-eight authors say "it is well documented that plants (Mooney et al., 1978; Berry and Bjorkman, 1980; Atkin and Tjoelker, 2003), animals (Parmesan, 2006), and microbes (Bradford et al., 2008) acclimate and/or adapt to prevailing environmental conditions in a way that can optimize their functioning under varying temperatures, which is collectively termed optimality (Parker and Maynard Smith, 1990)," and they hypothesize that "the integrated response of an ecosystem, as an assemblage of interacting organisms, might also demonstrate thermal optimality under temperature change," citing Loreau (2010).

What was done
In a study designed to test the merits of their hypothesis, Niu et al. "compiled data from 169 globally distributed sites of eddy covariance and quantified the temperature response functions of net ecosystem exchange (NEE), an ecosystem-level property, to determine whether NEE shows thermal optimality and to explore the underlying mechanisms."

What was learned
The international team of researchers - hailing from Australia, Austria, Belgium, Canada, China, the Czech Republic, Denmark, Finland, France, Germany, Ireland, Italy, Japan, the Netherlands, Russia, Sweden, Switzerland, the United Kingdom and the United States - say they found that "the temperature response of NEE followed a peak curve, with the optimum temperature (corresponding to the maximum magnitude of NEE) being positively correlated with annual mean temperature over years and across sites," and they say that "shifts of the optimum temperature of NEE were mostly a result of temperature acclimation of gross primary productivity (upward shift of optimum temperature) rather than changes in the temperature sensitivity of ecosystem respiration." In addition, however, they indicate that "extended growing seasons, increased nitrogen mineralization, and enhanced root growth (Penuelas and Filella, 2001; Churkina et al., 2005; Luo et al., 2009) may also have contributed to the increased CO2 uptake under higher temperatures, leading to the upward shift in the optimum temperature of gross primary productivity in warmer years."

What it means
If the world begins to warm again, for whatever reason, we can probably expect the bulk of the planet's vast array of ecosystems to become even more productive than they are today.

Atkin, O.K. and Tjoelker, M.G. 2003. Thermal acclimation and the dynamic response of plant respiration to temperature. Trends in Plant Science 8: 343-351.

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

Bradford, M.A., Davies, C.A., Frey, S.D., Maddox, T.R., Melillo, J.M., Mohan, J.E., Reynolds, J.F., Treseder, K.K. and Wallenstein, M.D. 2008. Thermal adaptation of soil microbial respiration to elevated temperature. Ecology Letters 11: 1316-1327.

Churkina, G., Schimel, D., Braswell, B.H. and Xiao, X.M. 2005. Spatial analysis of growing season length control over net ecosystem exchange. Global Change Biology 11: 1777-1787.

Loreau, M. 2010. Evolution of ecosystems and ecosystem properties. In: Loreau, M. (Ed.). From Populations to Ecosystems, Theoretical Foundations for a New Ecological Synthesis. Princeton University Press, Princeton, New Jersey, USA, p. 225-259.

Luo, Y., Sherry, R., Zhou, X. and Wan, S. 2009. Terrestrial carbon-cycle feedback to climate warming: experimental evidence on plant regulation and impacts of biofuel feedstock harvest. Global Change Biology Bioenergy 1: 62-74.

Mooney, H.A., Bjorkman, O. and Collatz, G.J. 1978. Photosynthetic acclimation to temperature in desert shrub, Larrea-divaricata. 1. Carbon-dioxide exchange characteristics of intact leaves. Plant Physiology 61: 406-410.

Parker, G.A. and Maynard Smith, J. 1990. Optimality theory in evolutionary biology. Nature 348: 27-33.

Parmesan, C. 2006. Ecological and evolutionary responses to recent climate change. Annual Review of Ecology, Evolution and Systematics 37: 637-669.

Penuelas, J. and Filella, I. 2001. Phenology - Responses to a warming world. Science 294: 793-795.

Reviewed 19 September 2012