Late Quaternary Vegetation and Climate History from the Lake Junín Plateau
Schiferl, Jake Daniel
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Whether the Andean ecosystem is resilient against future climate change remains unclear. A 2018 IPCC special report identified that warming more than 1.5ºC above pre-industrial temperatures by the year 2100 would be a major disrupter to natural systems, by causing more frequent extreme heatwaves, prolonged droughts, extreme precipitation events, and biome shifts. The tropical Andes will likely exceed this 1.5ºC threshold within the next century, threatening the diverse and endemic biota found in one of the world’s largest biodiversity hot spots. Yet few studies have examined the response of the Andean system through past interglacial periods when the temperatures were similar to those predicted for 2100. Vegetation responses to past periods of extreme climates can be examined using analogues. Prior interglacial periods, Marine Isotope Stage (MIS) 11c, MIS 9e, and MIS 5e, are arguably the closest analogues to our modern climate and to future climate trajectories, making these periods prime candidates for investigation. In this study, I use paleoecological data from a high-Andean lake, Lake Junín, in the Peruvian puna grasslands to investigate interglacial periods from the past 500 thousand years ago (ka), that are thought to exceed modern temperatures. I performed a paleoecological analysis of the Junín Plateau by reconstructing the history of fire, vegetation, megafauna, and the limnological history to assess ecosystem responses to past interglacial climate variability. By investigating interglacial periods of the past, I ask these key questions: (1) What were the Andean vegetation responses to past periods of enhanced warming? (2) Were Andean communities resilient to more extreme climates than today? At Lake Junín, interglacial intensity varied considerably. MIS 5e, MIS 9, and MIS 11 were more intense interglacial periods than the Holocene, respectively. MIS 5e was a particularly intense interglacial, when temperatures of at least 2ºC above modern temperatures were evident, with prolonged negative evaporitic conditions. Past interglacial periods had overlapping vegetation assemblages at Lake Junín and were characterized by productive puna grasslands with extensive Polylepis cover. Polylepis, however, never formed a dominant and widespread high-elevation forest. Polylepis woodlands were most likely present as interspersed islands or grew within shrubby grasslands, regulated by microclimatic conditions, rather than megafaunal herbivory or fire. Pollen concentration marked glacial-interglacial transitions more so than changes in pollen composition alone. Each interglacial had unique vegetational assemblages in response to specific interglacial climate variability. Prior to c. 130 ka, natural fires signatures were rare in high-Andean grasslands of the Junín Plateau. Natural fires increased in frequency after temperatures exceeded 2ºC during MIS 5e and remained more frequent when temperatures cooled, after the interglacial periods, suggesting a shift to a more fire prone system. Arrival of humans to the plateau, c. 12–13 ka, marked dramatic alterations to the natural state of the high Andes, modifying vegetation assemblages, creating a defined treeline, and killing off megafaunal populations already decimated by unfavorable warming climates. Vegetation communities from the Peruvian Andes migrated up and down the Andean slope in response to warming and cooling climates over the last 470 ka. When climates exceeded 2ºC above modern temperatures, Podocarpus forests flourished on the plateau, well above its modern elevational limit. These vegetational responses suggested that in the absence of human activities, Andean communities would likely be well suited to survive future climate change, however, anthropogenic pressures threaten the historical resilience of Andean ecosystems. Future warming would require Andean forests to infiltrate the puna grasslands, which are maintained today by burning and cattle herding. As warming exceeds 2ºC above modern temperatures, the local puna vegetation will become even more flammable making it unlikely that arboreal species will be able to migrate past modern treeline.