Background/Question/Methods   Cultural collapse in response to climate change has been postulated to explain the demise of past Andean cultures (Abbott et al 1997, Dillehay & Kolata 2004). We present data from three lakes in the Peruvian Andes, separated by c. 150 km, of two major land use changes at c. 2800 and 1300 years ago. Fossil pollen and sedimentary records from Lakes Huaypo (new data), Marcacocha (Chepstow-Lusty et al 2003) and Pacucha (Valencia et al in press) consistently point to reliance on crops in the genus Chenopodium prior to c. 2,800 – 2,700 cal yr BP. Scanning Electron Microscopy suggests that the grains belong to Chenopodium quinoa (Quinoa) and Amaranthus caudatus (Amaranto or kiwicha).

Results/Conclusions   At c. 2,800 cal yr BP in all three sites, Chenopodium pollen decline markedly in abundance and maize (Zea mays) becomes a regular component of the pollen record. A fourth fossil pollen and sedimentary record from Lake Sauce, Peru, reveals a millennial-scale climate change to drier conditions at c. 2800 BP. A second consistent change in the pollen record at c. 1300 yr BP is a noticeable decrease in the abundance of Poaceae pollen as Alnus and Acalypha pollen types increased in abundance. Coincident with this expansion of woody elements, sedimentary charcoal abundance tapers off. Around the same time, a general shift to drier conditions, though with increased variability, occurs in the Quelccaya ice core (Thompson et al 1985). At Sauce, this time period coincides with a weakening of strong El Niño and La Niña activity. Alnus is unlikely to have grown naturally at the elevations of Huaypo (3500 m), strongly suggesting that it was planted. A chronic shortage of wood for fuel and building prompted the Inca to legislate protection of forest patches. Our data suggest that even more active management of the land, via agroforestry, was practiced not only by the Inca but also by pre-Incan societies. The age of the alder pollen in these samples indicates that the Wari, regionally dominant between c. 1400 and 1000 yr BP, may have been planting trees. Overall, these data reveal the responses of past Andean society to ecological threat. These data are of particular relevance as possibilities for a new wave of agroforestry to sequester carbon in the Andes are considered.