Tropical rain forest has been a persistent feature in South America for at least 55 million years. The future of the contemporary Amazon forest is uncertain, however, as the region is entering conditions with no past analogue, combining rapidly increasing air temperatures, high atmospheric carbon dioxide concentrations, possible extreme droughts, and extensive removal and modification by humans.
Given the long-term Cenozoic cooling trend, it is unknown whether Amazon forests can tolerate air temperature increases, with suggestions that lowland forests lack warm-adapted taxa, leading to inevitable species losses. In response to this uncertainty, we posit a simple hypothesis: the older the age of a species prior to the Pleistocene, the warmer the climate it has previously survived, with Pliocene (2.6–5 Ma) and late-Miocene (8–10 Ma) air temperature across Amazonia being similar to 2100 temperature projections under low and high carbon emission scenarios, respectively.
Using comparative phylogeographic analyses, we show 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. The remarkably old age of these species suggest that Amazon forests passed through warmth similar to 2100 levels and that, in the absence of other major environmental changes, near-term high temperature-induced mass species extinction is unlikely.
Warmth from anthropogenic greenhouse gas emissions may severely impact Amazon trees because the latitudinal distance species would need to move to maintain a constant temperature is large, implying that many species may be unable to track the future climate and become extinct (Colwell et al. 2008; Jones et al. 2009). More formally, climate envelope modeling arrives at a similar conclusion, but because upper-bound heat-tolerance in these models cannot exceed the observable maximum current surface air temperature, predicted extinctions may be inflated (Feeley and Silman 2010; Corlett 2011). Moreover, 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), thus suggesting that persistence may be possible for many lowland rain forest tree species. A complementary approach to assist in distinguishing the likely thermal tolerance of Amazon tree species is to consider historical evidence, because at times in the past, Amazon surface air temperatures have been higher than those today (Feeley and Silman 2010; Hoorn et al. 2010; Jaramillo et al. 2010; Haywood et al. 2011).
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 (Haywood et al. 2011). Although many marine species became extinct, fossil pollen from the PETM showed an increase in tree diversity in three South American rainforest sites with abundant rainfall (Jaramillo et al. 2010). The mid-Miocene climatic optimum (13–15 Ma) was the warmest time of the Neogene (the ca. 20 Ma Period encompassing the complete Pliocene and Miocene Epochs; (Zachos et al. 2001). More recently, the early Pliocene (3.6–5.3 Ma) and parts of the Late Miocene (8–10 Ma) had surface air temperatures similar to IPCC predictions for the year 2100 under low and high CO2 emissions scenarios, respectively (IPCC 2007; You et al. 2009; Haywood et al.2011). Fossil evidence suggests that megathermal (frost intolerant) forests were globally more extensive during the Neogene (Morley2000) and possibly harbored higher levels of tree diversity than today’s tropical forests (Hoorn et al. 2010). However, the relevance of Neogene warmth to the thermal tolerance of modern tropical trees depends, in part, on establishing the age of the tree species and their prior geographic distributions. If contemporary species extend back to the Pliocene or Late Miocene and did not have access to upper elevation thermal refugia, then they have likely survived air temperatures as high as or higher than those predicted for the coming decades.
Here, we use a phylogeographic approach to determine if 12 contemporary Amazon tree species have endured historical air temperatures that exceed current thermal maxima. Because many of the 12 study species are rain forest specialists, their history provides proxy information about tropical rain forest distributions through time.
Reference: Dick, C. W., Lewis, S. L., Maslin, M. and Bermingham, E. (2012), Neogene origins and implied warmth tolerance of Amazon tree species. Ecology and Evolution. doi: 10.1002/ece3.441
Source: Wiley Online Library, 14/december/2012