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Tropical Tree Seedlings in a Warming World
Cheesman, A.W. and Winter, K. 2013b. Growth response and acclimation of CO2 exchange characteristics to elevated temperatures in tropical tree seedlings. Journal of Experimental Botany 64: 3817-3828.

The authors say "it is hypothesized that tropical trees have adapted to operate within a narrow range of temperatures with only a limited potential to acclimate," citing Janzen (1967) and Ghalambor et al. (2006). And they also note that "climate envelope or species distribution models often highlight the narrow range of temperatures to which tropical lowland forests are restricted, emphasizing their vulnerability to increasing temperatures," citing Colwell et al. (2008), Wright et al. (2009), Laurance et al. (2011) and Zelazowski et al. (2011).

What was done
To further explore this subject, Cheesman and Winter conducted experiments related to the thermal niches of seedlings of ten neo-tropical tree species under favorable conditions (i.e. medium light levels, high water and nutrient availability) and a range of temperature regimes within plant growth chambers. And they followed this set of experiments with more detailed studies of the thermal acclimation - or not - of three species at temperature regimes known to be sub- and super-optimal for growth.

What was learned
In their first set of experiments, the two Smithsonian Tropical Research Institute scientists found that "under well-watered conditions, all species showed optimal growth at temperatures above those currently found in their native ranges," with the exception of two species that did not survive under the highest temperature treatment. And they thus report that "clear and novel evidence" was provided that "certain tropical trees do have an ability to maintain growth as seedlings at temperatures above those of their current home range, with a fundamental thermal niche generally broader than that realized in the field," noting that "clear evidence is also provided that thermal acclimation of leaf-level processes may play an important role in this response." And they buttress the validity of these conclusions by noting that "in the small number of empirical studies that have sought to elucidate fundamental thermal niches of tropical tree species, maximum growth has often been found under the highest temperature regime tested," citing Herwitz (1993), Cunningham and Read (2003), Allen and Vu (2009), Esmail and Oelbermann (2012), and Cheesman and Winter (2013a)," adding that "in these cases, the thermal optimum for growth was found to be higher than that predicted by either climatic home range or photosynthetic thermal optimum," citing Cunningham and Read (2002).

What it means
Tropical trees are likely much more able to successfully deal with rising temperatures than many people have long supposed.

Allen, L.H. and Vu, J.C.V. 2009. Carbon dioxide and high temperature effects on growth of young orange trees in a humid, subtropical environment. Agricultural and Forest Meteorology 149: 820-830.

Cheesman, A.W. and Winter, K. 2013a. Elevated night-time temperatures increase growth in seedlings of two tropical pioneer tree species. New Phytologist 197: 1185-1192.

Colwell, R.K., Brehm, G., Cardelus, C.L., Gilman, A.C. and Longino, J.T. 2008. Global warming, elevational range shifts, and lowland biotic attrition in the wet tropics. Science 322: 258-261.

Cunningham, S.C. and Read, J. 2002. Comparison of temperate and tropical rainforest tree species: photosynthetic responses to growth temperature. Oecologia 133: 112-119.

Cunningham, S.C. and Read, J. 2003. Comparison of temperate and tropical rainforest tree species: growth responses to temperature. Journal of Biogeography 30: 143-153.

Esmail, S. and Oelbermann, M. 2011. The impact of climate change on the growth of tropical agroforestry tree seedlings. Agroforestry Systems 83: 235-244.

Ghalambor, C.K., Huey, R.B., Martin, P.R., Tewksbury, J.J. and Wang, G. 2006. Are mountain passes higher in the tropics? Janzen's hypothesis revisited. Integrative and Comparative Biology 46: 5-17.

Herwitz, S.R. 1993. Growth rates of selected Australian tropical rain-forest tree species under controlled conditions. Oecologia 96: 232-238.

Janzen, D.H. 1967. Why mountain passes are higher in the tropics. American Naturalist 101: 233-249.

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Wright, S.J., Muller-Landau, H.C. and Schipper, J. 2009. The future of tropical species on a warmer planet. Conservation Biology 23: 1418-1426.

Zelazowski, P., Malhi, Y., Huntingford, C., Sitch, S. and Fisher, J.B. 2011. Changes in the potential distribution of humid tropical forests on a warmer planet. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 369: 137-160.

Reviewed 11 December 2013