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The Primary Effect of Past Increases in the Air's CO2 Content on Two Amazon Tree Species
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.

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.

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Reviewed 28 September 2011