Past Rapid Warming Event Increased Plant Diversity and Promoted Speciation
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., de la Parra, F., Morón, 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.L., Valencia, V. and Vervoort, J. 2010. Effects of rapid global warming at the Paleocene-Eocene boundary on neotropical vegetation. Science 330: 957-961.
The PETM was clearly identifiable in terms of temperature, where it was between 31 and 34 °C during the peak of global warmth. Using pollen and other organic materials, the authors were also able to identify various plant species that existed during this period. In spite of the high temperatures, and possibly less moist conditions, it was found that drought tolerant plants did not increase. Neither were moisture-requiring plants shown to decrease during the PETM, leading the authors to speculate that either rainfall increased and/or water use efficiency increased due to higher levels of atmospheric carbon dioxide, which would have compensated for the higher temperatures.
It was also noted that the PETM saw a radical increase in diversity, measured in both absolute terms (number of species/taxa) and in terms of evenness (relatively more even species percentages). This was achieved by addition of new taxa. The origination rate for new taxa during the PETM was 2 to 6 times higher than during the periods before and after, but extinction rates were not unusual. The species/taxa that originated, and the overall diversity, continued after the PETM. Thus the gains in plant species richness during the PETM were relatively persistent rather than transient. So what are some possible implications of these findings?
It is believed that tropical forests of today are currently growing in climates that are near the maximum temperature that the plants can tolerate (~27.5 °C), and that future global warming in tropical zones may stress plants and cause a reduction in forest growth, perhaps leading some species to extinction. However, as shown by Jaramillo et al., temperatures of 31 to 34 °C seem to not only have been tolerated, but to have caused a burst of speciation in the PETM, including major taxa that persist today. Indeed, the high temperatures of the PETM were not detrimental to the tropical moist forests examined in this study and the same combination of factors will likely benefit tropical vegetation under any plausible warming scenario dreamed up by the alarmists in the future.