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The Primary Effect of Past Increases in the Air's CO2 Content on Two Amazon Tree Species
Reference
Bonal, D., Ponton, S., Le Thiec, D., Richard, B., Ningre, N., Herault, B., Ogee, J., Gonzalez, S., Pignal, M., Sabatier, D. and Guehl, J.-M. 2011. Leaf functional response to increasing atmospheric CO2 concentrations over the last century in two northern Amazonian tree species: a historical δ13C and δ18O approach using herbarium samples. Plant, Cell and Environment 34: 1332-1344.

Background
The authors write that "the impact of global change during the last century on the biology of tropical rainforest trees is largely unknown," but they say that "an increase in tree radial growth increment over recent decades in Amazonian tropical rainforests has been observed, leading to increased above-ground biomass at most study sites," citing the studies of Phillips et al. (1998, 2009) and Malhi et al. (2004) in this regard, while subsequently noting that "the stimulating impact on photosynthesis of increased CO2 concentrations in the air (Ca) could explain these growth patterns (Lloyd and Farquhar, 2008)."

What was done
In further investigating this phenomenon, Bonal et al. assessed the impacts of historical environmental changes on leaf morphological (stomatal density, stomatal surface, leaf mass per unit area) and physiological traits (carbon isotope composition, δ13Cleaf, and discrimination, Δ13Cleaf, oxygen isotope composition, δ18Oleaf) of two tropical rainforest species (Dicorynia guianensis; Humiria balsamifera) that are abundant in the Guiana shield (Northern Amazonia)," working with leaf samples from different international herbariums that covered a 200-year time period (AD 1790-2004).

What was learned
The eleven researchers say their results revealed "a clear response of leaf physiological characteristics to increasing Ca for both species," which were consistent with previous studies "from different ecosystems (Penuelas and Azcon-Bieto, 1992; Beerling et al., 1993; Van de Water et al., 1994; Pedicino et al., 2002; Penuelas et al., 2008), and with data from tree rings in Europe (Bert et al., 1997; Duquesnay et al., 1998; Saurer et al., 2004), Africa (Gebrekirstos et al., 2009) and in tropical rainforests (Hietz et al., 2005; Silva et al., 2009; Nock et al., 2011)." More specifically, they say their results pointed to "an increase in water-use efficiency over recent decades of about 23.1 and 26.6% for Humiria and Dicorynia, respectively," driven mostly by increases in leaf photosynthesis. And they indicate that "the range of change in water-use efficiency for these two species was consistent with many results observed not only in tropical forests (Hietz et al., 2005; Nock et al., 2011), but in boreal (Saurer et al., 2004) and temperate forests (Francey and Farquhar, 1982; Penuelas and Azcon-Bieto, 1992; Bert et al., 1997; Duquesnay et al., 1998)."

What it means
Bonal et al. state that the responses of the two tree species they studied to increasing Ca appear to be "simply related to the availability of CO2 in the air (fertilization effect)," and they say that "this trend seems to be consistent with recent tree growth patterns in the Amazonian region," and indeed it is, as the CO2-induced greening of the earth phenomenon continues to quietly transform the surface of the planet by increasing its productivity.

References
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Bert, D., Leavitt, S.W. and Dupouey, J.L. 1997. Variations of wood δ13C and water-use efficiency of Abies alba during the last century. Ecology 78: 1588-1596.

Duquesnay, A., Breda, N., Stievenard, M. and Dupouey, J.L. 1998. Changes of tree-ring δ13C and water-use efficiency of beech (Fagus sylvatica L.) in north-eastern France during the past century. Plant, Cell and Environment 21: 565-572.

Francey, R.J. and Farquhar, G.D. 1982. An explanation of 13C/12C variations in tree rings. Nature 297: 28-31.

Gebrekirstos, A., Worbes, M., Teketay, D., Fetene, M. and Mitlohner, R. 2009. Stable carbon isotope ratios in tree rings of co-occurring species from semi-arid tropics in Africa: patterns and climatic signals. Global and Planetary Change 66: 253-260.

Hietz, P., Wanek, W. and Dunisch, O. 2005. Long-term trends in cellulose δ13C and water-use efficiency of tropical Dedrela and Swietenia from Brazil. Tree Physiology 25: 745-752.

Lloyd, J. and Farquhar, G.D. 2008. Effects of rising temperatures and [CO2] on the physiology of tropical forest trees. Philosophical Transactions of the Royal Society B 363: 1811-1817.

Malhi, Y., Baker, T.R., Phillips, O.L., Almeida, S., Alvarez, E., Arroyo, L., Chave, J., Czimczik, C.I., Di Fiore, A., Higuchi, N., Killeen, T.J., Laurance, S.G., Lauranch, W.F., Lewis, S.L., Montoya, L.M.M., Agudo, A., Neill, D.A., Vargas, P.N., Patino, S., Pitman, N.C.A., Quesadah, C.A., Salomao, R., Silva, J.N.M., Lezama, A.T., Martinez, R.V., Terborgh, J., Vinceti, B. and Lloyd, J. 2004. The above-ground coarse wood productivity of 104 Neotropical forest plots. Global Change Biology 10: 563-591.

Nock, C.A., Baker, P.J., Wanek, W., Albrecht, L., Grabner, M., Bunyavejchewin, S. and Hietz, P. 2011. Long-term increases in intrinsic water-use efficiency do not lead to increased stem growth in a tropical monsoon forest in western Thailand. Global Change Biology 17: 1049-1063.

Pedicino, L., Leavitt, S.W., Betancourt, J.L. and Van De Water, P.K. 2002. Historical variations in δ13C leaf of herbarium specimens on the Southwestern U.S. Western North American Naturalist 62: 348-359.

Penulas, J. and Azcon-Bieto, J. 1992. Changes in leaf δ13C of herbarium plant species during the last 3 centuries of CO2 increase. Plant, Cell and Environment 15: 485-489.

Penuelas, J., Hunt, J.M., Ogaya, R. and Jump, A.S. 2008. Twentieth century changes of tree-ring δ13C at the southern range-edge of Fagus sylvatica: increasing water-use efficiency does not avoid the growth decline induced by warming at low altitudes. Global Change Biology 14: 1076-1088.

Phillips, O.L., Aragao, L., Lewis, S.L., Fisher, J.B., Lloyd, J., Lopez-Gonzalez, G., Malhi, Y., Monteagudo, A., Peacock, J., Quesada, C.A., van der Heijden, G., Almeida, S., Amaral, I., Arroyo, L., Aymard, G., Baker, T.R., Banki, O., Blanc, L., Bonal, D., Brando, P., Chave, J., de Oliveira, A.C.A., Cardozo, N.D., Czimczik, C.I, Feldpausch, T.R., Freitas, M.A., Gloor, E., Higuchi, N., Jimenez, E., Lloyd, G., Meir, P., Mendoza, C., Morel, A., Neill, D.A., Nepstad, D., Patino, S., Penuela, M.C., Prieto, A., Ramirez, F., Schwarz, M., Silva, J., Silveira, M., Thomas, A.S., ter Steege, H., Stropp, J., Vasquez, R., Zelazowski, P., Davila, E.A., Andelman, S., Andrade, A., Chao, K.-J., Erwin, T., Di Fiore, A., Honorio C., E., Keeling, H., Killeen, T.J., Laurance, W.F., Cruz, A.P., Pitman, N.C.A., Vargas, P.N., Ramirez-Angulo, H., Rudas, A., Salamao, R., Silva, N., Terborgh, J. and Torres-Lezama, A. 2009. Drought sensitivity of the Amazon rainforest. Science 323: 1344-1347.

Phillips, O.L., Malhi, Y., Higuchi, N., Laurance, W.F., Nunez, P.V., Vasquez, R.M., Laurance, S.G., Ferreira, L.V., Stern, M., Brown, S. and Grace, J. 1998. Changes in the carbon balance of tropical forests: Evidence from long-term plots. Science 282: 439-442.

Saurer, M., Siegwolf, R.T.W. and Schweingruber, F.H. 2004. Carbon isotope discrimination indicates improving water-use efficiency of trees in northern Eurasia over the last 100 years. Global Change Biology 10: 2109-2120.

Silva, L.C.R., Anand, M., Oliveira, J.M. and Pillar, V.D. 2009. Past century changes in Araucaria angustifolia (Bertol.) Kuntze water use efficiency and growth in forest and grassland ecosystems of southern Brazil: implications for forest expansion. Global Change Biology 15: 2109-2120.

Van de Water, P.K., Leavitt, S.W. and Betancourt, J.L. 1994. Trends in stomatal density and 13C/12C ratios of Pinus flexilis needles during last glacial-interglacial cycle. Science 264: 239-243.

Reviewed 28 September 2011