Background
The authors write that "tropical rain forest has been a persistent feature in South America for at least 55 million years," and that "at times in the past, Amazon surface air temperatures have been higher than those today (Feely and Silman, 2010; Hoorn et al., 2010; Jaramilo et al., 2010; Haywood et al., 2011)." In addition, they say that "experiments show that tropical plants can photosynthesize and maintain a positive carbon balance under higher temperatures than those occurring today (Krause et al., 2010; Way and Oren, 2010)." So the question naturally arises: How high can Amazon temperatures go and its trees still survive?

What was done
In broaching this question, Dick et al. hypothesized that "the older the age of a species prior to the Pleistocene, the warmer the climate it has previously survived," noting that Pliocene and late-Miocene air temperatures of 2.6-5 million years ago (Ma) and late-Miocene air temperatures of 8-10 Ma across Amazonia were "similar to AD 2100 temperature projections under low and high carbon emission scenarios, respectively." In fact, they report that "some 56.3 Ma during the Paleocene-Eocene Thermal Maximum (PETM), global mean temperature increased by 5-6°C over a period of <= 20 ka," citing Haywood et al. (2011). And they say that "fossil pollen from the PETM showed an increase in tree diversity in three South American rainforest sites with abundant rainfall (Jaramillo et al., 2010)." Therefore, they used comparative phylogeographic analyses to determine the age of the tropical tree species that are currently found in Amazonia.

What was learned
The four researchers determined that "9 of 12 widespread Amazon tree species have Pliocene or earlier lineages (>2.6 Ma), with seven dating from the Miocene (>5.6 Ma) and three >8 Ma."

What it means
Dick et al. conclude, as they describe it, that "the remarkably old age of these species suggests that Amazon forests passed through warmth similar to [that predicted by climate alarmists for] AD 2100 levels and that, in the absence of other major environmental changes, near-term high temperature-induced mass species extinction is unlikely [italics added]." Thus, they suggest that "direct human impacts (forest clearance, forest fragmentation, fires, and loss of seed dispersing animals), and their interactions may be more important immediate threats to the integrity of Amazon rain forests and therefore should remain a focus of conservation policy [italics added]."

References
Feeley, K.J. and Silman, M.R. 2010. Biotic attrition from tropical forests correcting for truncated temperature niches. Global Change Biology 16: 1830-1836.

Haywood, A.M., Ridgwell, A., Lunt, D.J., Hill, D.J., Pound, M.J., Dowsett, H.J., Dolan, A.M., Francis, J.E., and Williams, M. 2011. Are there pre-Quaternary geological analogues for a future greenhouse warming? Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 369: 933-956.

Hoorn, C., Wesselingh, F.P., ter Steege, H., Bermudez, M.A., Mora, A., Sevink, J., Sanmartin, I., Sanchez-Meseguer, A., Anderson, C.L., Figueiredo, J.P., Jaramillo, C., Riff, D., Negri, F.R., Hooghiemstra, H., Lundberg, J., Stadler, T., Sarkinen, T. and Antonelli, A. 2010. Amazonia through time : Andean uplift, climate change, landscape evolution, and biodiversity. Science 330: 927-931.

Jaramillo, C., Ochoa, D., Contreras, L., Pagani, M., Carvajal-Ortiz, H., Pratt, L.M., Krishnan, S., Cardona, A., Romero, M., Quiroz, L., Rodriguez, G., Rueda, M.J., Parra, F., Moron, S., Green, W., Bayona, G., Montes, C., Quintero, O., Ramirez, R., Mora, G., Schouten, S., Bermudez H., Navarrete, R., Parra, F., Alvarán, M., Osorno, J., Crowley, J., Valencia, V. and Vervoort, J. 2010. Effects of rapid global warming at the Paleocene-Eocene boundary on neotropical vegetation. Science 330: 957-961.

Krause, G.H., Winter, K., Krause, B., Jahns, P., Garcia, M., Aranda, J. and Virgo, A. 2010. High-temperature tolerance of a tropical tree, Ficus insipida: methodological reassessment and climate change considerations. Functional Plant Biology 37: 890-900.

Way, D.A. and Oren, R. 2010. Differential responses to changes in growth temperature between trees from different functional groups and biomes: a review and synthesis of data. Tree Physiology 30: 669-688.