Paper Reviewed
Reef, R. and Lovelock, C.E. 2014. Historical analysis of mangrove leaf traits throughout the 19th and 20th centuries reveals differential responses to increases in atmospheric CO2. Global Ecology and Biogeography 23: 1209-1214.

Introducing their intriguing study, Reef and Lovelock (2014) write that specific leaf area or SLA - which is the ratio of leaf area to weight - is "a key indicator of CO2-induced changes in photosynthetic activity and growth," citing Cornelissen et al. (1999), Medlyn et al. (1999), Yin (2002) and Poorter and Navas (2003), due to the fact that "species that respond more strongly to elevated CO2 (e.g. grow more rapidly or assimilate more carbon) also show steeper declines in SLA as CO2 increases," citing Cornelissen et al. (1999). And they also note, in this regard, that "tree species with initially higher SLA values respond more strongly, in terms of photosynthesis and/or growth to elevated CO2 than tree species with initially lower SLA," once again citing the study of Cornelissen et al. (1999) as well as the newer work of Ali et al. (2013).

With these facts as background, the two Australian researchers determined historical changes in the SLA of preserved mangrove specimens maintained at the Queensland Herbarium in Brisbane and the National Herbarium of New South Wales in Sydney, focusing on two common species - Avicennia marina and Rhizophora stylosa - which are currently distributed throughout the Indo-Pacific region. More specifically, they gathered pertinent data from a total of 262 specimens of A. marina that were collected between 1842 and 2009 and 150 specimens of R. stylosa that were collected between 1860 and 2009; and they compared the SLA values they derived from them with atmospheric CO2 concentrations that had previously been derived from the ice core record of Friedli et al. (1986), supplemented by more current atmospheric CO2 measurements made at the Mauna Loa laboratory.

This work revealed - in the words of Reef and Lovelock - that "the SLA of R. stylosa was less variable than that of A. marina and did not show a significant decline in response to the rise in CO2 concentrations over time." On the other hand, they indicate that "the SLA of A. marina has decreased substantially (23%) over the past 165 years," which decline they attribute to "the rise in atmospheric CO2." And they further note in this regard that "due to the linear relationship between SLA and CO2, reductions were slower during the first decades of the period in this study and have increased in magnitude as CO2 levels have increased more rapidly, with no indication of reaching saturation." Thus, their findings suggest that (1) "substantial increases in productivity and carbon assimilation of mangrove forests have occurred in the recent past, particularly in Avicennia-dominated forests such as those of southern Australia, New Zealand (Morrisey et al., 2010) and the United States," and that (2) "further increases may be anticipated with future increases in atmospheric CO2 concentration."

References
Ali, A.A., Medlyn, B.E., Crous, K.Y. and Reich, P.B. 2013. A trait-based ecosystem model suggests that long-term responsiveness to rising atmospheric CO2 concentration is greater in slow-growing than fast-growing plants. Functional Ecology 27: 1011-1022.

Cornelissen, J.H.C., Carnelli, A.L. and Callaghan, T.V. 1999. Generalities in the growth, allocation and leaf quality responses to elevated CO2 in eight woody species. New Phytologist 141: 401-409.

Friedli, J.H., Lotscher, H., Oeschger, H., Siegenthaler, U. and Stauffer, B. 1986. Ice core record of the 13C/12C ratio of atmospheric CO2 in the past two centuries. Nature 324: 237-238.

Medlyn, B.E., Badeck, F.W., De Pury, D.G.G., Barton, C.V.M., Broadmeadow, M., Ceulemans, R., de Angelis, P., Forstreuter, M., Jach, M.E., Kellomäki, S., Laitat, E., Marek, M., Philippot, S., Rey, A., Strassemeyer, J., Laitinen, K., Liozon, R., Portier, B., Roberntz, P., Wang, K. and Jarvis, P.G. 1999. Effects of elevated [CO2] on photosynthesis in European forest species: a meta-analysis of model parameters. Plant, Cell and Environment 22: 1475-1495.

Morrisey, D.J., Swales, A., Dittmann, S., Morrison, M.A., Lovelock, C.E. and Beard, C.M. 2010. The ecology and management of temperate mangroves. In: Gibson, R.N., Atkinson, R.J.A. and Gordon, J.D.M. (Eds.). Oceanography and Marine Biology: An Annual Review, CRC Press, Boca Raton, Florida, USA, pp. 43-160.

Poorter, H. and Navas, M.-L. 2003. Plant growth and competition at elevated CO2: on winners, losers and functional groups. New Phytologist 157: 175-198.

Yin, X. 2002. Responses of leaf nitrogen concentration and specific leaf area to atmospheric CO2 enrichment: a retrospective synthesis across 62 species. Global Change Biology 8: 631-642.