Ecology of the past

Amazonia – natural wilderness or abandoned parkland?

By Judith Kirschner (currently studying for MSc Earth Sciences: Geo-ecological Dynamics track at the University of Amsterdam)

In the 4^th edition of our “Amsterdam Paleoecology Club” (APC), we discussed ‘A 6900-year history of landscape modification by humans in lowland Amazonia’ by Bush et al. (2006)¹. The high-resolution record presented in this paper shows impressively that what we might perceive as native rainforest today could rather be a since a long time actively modified landscape.

The chronology from Lake Sauce (Peruvian Andes) suggests a continuously forested landscape under significant anthropogenic impact over the last 6900 years. Indicators of human activity are taken to be the varying presence of crop pollen (Zea mays) throughout the record, combined with the continuous occurrence of charcoal. Two extra-large fire events are dated at c. 6700 and between 4500-4230 cal BP, probably enhanced by the relatively drier climate of the mid-Holocene. However, it is not clear yet if human actions formed a response to climate change or were part of social and cultural changes.

The paper raises the interesting question:

• Why is there a lag of several hundreds of years between the initiation of human action (first large charcoal spike) and first crops to be recorded in the sequence?

This effect has been observed before^2,3,4,5 and might result from an observational limitation, i.e. the fact that Zea pollen are poorly dispersed and hard to detect^6,7 in records. Alternatively, a human behavioral explanations could be invoked to suggest that either: (i) it took several phases of burning until the landscape was suitable for agriculture, or (ii) that early burning was done for the purpose of improved hunting and that the planting of crops came much later.

Another remarkable point is the peak intensity of Maize agriculture from c. 3380-700 cal BP. This time period coincides with more sophisticated pre-Columbian societies possibly practicing trade and local specialization away from the studied location. However, the abandonment centuries before the arrival of Europeans might suggest that other dynamics driving population declines or movement at this location.

Our critical discussion in the APC also raised a range of different questions:

• Does the massive charcoal input into the chronology necessarily point towards large-scale fire events?
• How can we recognize a fully recovered forest in a pollen diagram?
• While recovery usually takes around 200-300 years after a heavy disturbance⁸, many small fire events extended forest regrowth here until around 2000 years ago, so does the forest ever really recover?
• How appropriate was the use of Cyperaceae:Poaceae ratio as an indicator for wetness?

The answers to these questions are subject to ongoing debates.

REFERENCES

¹ Bush, M.B., Correa-Metrio, A., McMichael, C.H., Sully, S., Shadik, C.R., Valencia, B.G., Guilderson, T., Steinitz-Kannan, M. and Overpeck, J.T., 2016. A 6900-year history of landscape modification by humans in lowland Amazonia. Quaternary Science Reviews, 141, pp.52-64. DOI: 10.1016/j.quascirev.2016.03.022

² Bush, M.B. and Colinvaux, P.A., 1988. A 7000-year pollen record from the Amazon lowlands, Ecuador. Vegetatio, 76(3), pp.141-154. DOI: 10.1007/BF00045475

³ Bush, M.B., Silman, M.R., De Toledo, M.B., Listopad, C., Gosling, W.D., Williams, C., De Oliveira, P.E. and Krisel, C., 2007. Holocene fire and occupation in Amazonia: records from two lake districts. Philosophical Transactions of the Royal Society B: Biological Sciences, 362(1478), pp.209-218. DOI: 10.1098%2Frstb.2006.1980

⁴ Carson, J.F., Whitney, B.S., Mayle, F.E., Iriarte, J., Prümers, H., Soto, J.D. and Watling, J., 2014. Environmental impact of geometric earthwork construction in pre-Columbian Amazonia. Proceedings of the National Academy of Sciences, 111(29), pp.10497-10502. DOI: 10.1073/pnas.1321770111

⁵ Bush, M.B., Miller, M.C., De Oliveira, P.E. and Colinvaux, P.A., 2000. Two histories of environmental change and human disturbance in eastern lowland Amazonia. The Holocene, 10(5), pp.543-553. DOI: 10.1191/095968300672647521

⁶ Kennedy, L.M. and Horn, S.P., 1997. Prehistoric Maize Cultivation at the La Selva Biological Station, Costa Rica 1. Biotropica, 29(3), pp.368-370. DOI: 10.1111/j.1744-7429.1997.tb00438.x

⁷ Lane, C.S., Cummings, K.E. and Clark, J.J., 2010. Maize pollen deposition in modern lake sediments: A case study from Northeastern Wisconsin. Review of Palaeobotany and Palynology, 159(3-4), pp.177-187. DOI: 10.1016/j.revpalbo.2009.12.003

⁸ Foster, R.B., 1990. Long-term change in the successional forest community of the Rio Manu floodplain. In: Gentry A.H. (ed.) Four Neotropical rainforests. Yale Univ. Press, New Haven, CT, pp.565-572. (link to book)