Date of Award
Doctor of Philosophy (PhD)
Ocean Engineering and Marine Sciences
Robert van Woesik
Richard B. Aaronson
Andrew G. Palmer
Humid montane forests in the tropical Andes form hotspots for biodiversity, endemism, and conservation. Threats to the forests from deforestation are amplified by ongoing climate change, which could further reduce available habitat area by about 57 – 80%. These forests are known to have withstood considerable climate change in the past, and an emerging view suggests that they have also been shaped by a long interaction with people. Although climate-driven species range shifts are broadly known, questions remain about how past climate change has affected the species composition of these forests. The interaction of people with forests on the eastern flank of the Andes has not been investigated as fully as counterpart habitats in the lowlands or in the high Andes. For many years it was thought that these steep forested slopes were too harsh for pre-Columbian human occupation, yet new archaeological evidence suggests that these forests have been occupied for thousands of years. Despite the discovery of more archaeological sites, much has yet to be learned of the impacts of pre-Columbian humans on humid montane forests. Here I present the palaeoecological records of two lakes in the humid montane forest, and create a model predicting the likelihood of pre-Columbian occupation to answer four key questions: Do major changes in the abundances of plant taxa align in the Andes with known climate events? Did humid montane forests in the Andes recover after pre-Columbian human abandonment? Where were pre-Columbian humans most likely to occupy the Andes? What environmental factors influence pre-Columbian human occupation in the Andes? Lake Progreso, located in northern Peru, provides a 12,700-year history of humid montane forests, capturing almost the entirety of known pre-Columbian human occupation and major Holocene climate events. The fossil pollen and charcoal record from Lake Progreso indicated the presence of a fully formed humid montane forest in the basal sample, supporting hypotheses that vertical migrations were largely complete and that these forests existed in near-modern form before the onset of the Holocene, despite cooler temperatures (c. 5 – 7°C) during the previous glacial period. During the Mid-Holocene Dry Event (MHDE, 9000–4000 cal yr BP) the fossil pollen suggested that vegetation changed little at this site, but pre-Columbian occupation began during this climatic event. It is likely that the MHDE created ideal conditions at Lake Progreso for pre-Columbian occupation. Maize was present from 4000 cal yr BP until 370 cal yr BP, when the lake catchment was abandoned. The area then recovered, indicating reforestation immediately prior to European arrival (c. 370 cal yr BP, AD 1588). The forest around Lake Progreso today likely does not have any ecological legacies from pre-Columbian human impacts, but instead shows impacts from modern day cattle farming and ongoing climate change. Lake Comoran, located in the humid montane forests of Ecuador, provides a detailed history of occupation and abandonment over the past 2300 years. At Lake Cormoran, pre-Columbian activity is present from 2300 – 1550 cal yr BP. Alnus agroforestry is inferred during this time, about 1000 year earlier than has been found at any other site. The end of regular maize cultivation at 1350 cal yr BP provides a refined age estimate for the eruption of the Sangay Volcano and the ensuing abandonment of the Upano Valley by the Upano culture. Archaeological estimates show that pre-Columbian humans occupy the area again from c. 1150 to 700, but phytolith maize data suggest that this occupancy could have continued until c. 530 cal yr BP. After this abandonment at c. 530 cal yr BP, the forest regrows, and the palm Dictyocaryum becomes dominant. Although the sediment prior to pre-Columbian occupation was not recovered, it is possible that the increase in Dictyocaryum represented either a return to a pre-human occupation forest composition or was a secondary ecological legacy induced by conditions resulting from the human occupation. Comparisons of Lake Progreso and Lake Cormoran with other mid-elevation lakes showed the spatial variability of pre-Columbian occupation in the Andes, but few studies have attempted to quantify this variability or identify major influences in pre-Columbian occupation. Using compiled archaeological data and a suite of environmental variables, I generated an ensemble species distribution model (SDM) that incorporated generalized additive models, random forest models and Maxent models to reconstruct spatial patterns of pre-Columbian people that inhabited the Tropical Andes, within the modern countries of Bolivia, Peru, and Ecuador. Within this region, elevation, mean annual cloud frequency, distance to rivers and precipitation of the driest quarter were the environmental variables most closely related to human occupancy. My model indicated that c. 11% of our study area (65,368 km2) was likely occupied by pre-Columbian humans. My model also showed that 30 of 351 forest inventory plots, which were used to generate ecological understanding of Andean ecosystems, were likely occupied in the pre-Columbian period. In previously occupied sites, successional trajectories may still be shaping forest dynamics, and those forests may still be recovering from the ecological legacy of pre-Columbian impacts. My ensemble SDM links palaeo- and neoecology and can also be used to guide both future archaeological and ecological studies. Lake Progreso, Lake Cormoran, and my ensemble SDM link temporal and spatial data, providing insights into past climate change and pre-Columbian human occupation. Although pre-Columbian humans did not occupy all the humid montane forest, they did occupy some locations for thousands of years, especially in areas with ideal conditions or during climate events that created ideal conditions. Overall, I find evidence of widespread, though not ubiquitous human impacts in the tropical humid montane forests. I do not document direct ecological legacies that are still impacting the modern forest but, rather find evidence of recovery of species composition, and possible indirect ecological legacy effects.
Sales, Rachel Kathleen, "The Impacts of Holocene Climate and Pre-Columbian Humans on Neotropical Andean Forests" (2022). Theses and Dissertations. 1339.
Available for download on Tuesday, May 07, 2024