3000 years of human-vegetation interaction near the Las Cruces biological station in southwestern Costa Rica : paleoecological evidence from Laguna Zoncho

There is growing evidence that prehistoric indigenous groups affected tropical forests of Costa Rica. Costa Rica has a diverse array of tropical forest communities and the severity and extent of human impacts in these sites are not well understood. There are some studies that have helped elucidate these impacts. Sediment cores from Lagunas Bonilla and Bonillita show over 2600 years of forest disturbance and maize cultivation on the shores of the lakes. Studies at La Selva Biological Station reveal that this lowland tropical rainforest reserve was also subject to prehistoric forest clearance and maize cultivation.

In this thesis, I analyze a 5.9 meter sediment core from Laguna Zoncho in southwestern Costa Rica. The analysis of this sediment core from a previously unstudied location in Costa Rica provides insight into how indigenous groups affected vegetation at mid-elevations (1190 m). This sediment core records 3000 years of environmental history and provides more evidence to support the concept that indigenous groups altered the landscape. Laguna Zoncho is only 2 km from the Las Cruces Biological Station, so the results of this study are relevant for understanding the extent of past human disturbance that the Las Cruces Forest may have experienced. The archaeology of this region of Costa Rica is less well understood than in other parts of Central America, and this study contributes to a better general understanding of the timing of indigenous occupation.

The presence of the pollen of Xea mays throughout the entire length of the sediment core is evidence of 3000 years of human occupation at Laguna Zoncho. This finding complements and extends some limited prior archaeological work. Finding evidence of human-induced vegetation disturbance in a sediment core from a lake with archaeological sites is not surprising. What is of more interest is the timing, length, and apparent intensity of the human disturbance. Pollen percentages of forest species and disturbance taxa fluctuate dramatically down the core, representing various periods of intense human disturbance and regrowth of mature forest. Changing abundance of charcoal in the sediment core points to variations in the fire regime, which may reflect changes in climate as well as human activities.

Human disturbance is widespread on the Las Cruces landscape today, with coffee fields and pasture having replaced much of the forest that existed a scant 50 years ago. When Italians settled this part of the Costa Rican frontier, they found evidence of indigenous agricultural fields. Continuous indigenous occupation of Laguna Zoncho may have continued well into the twentieth century. Based on the pollen and charcoal evidence of forest clearance and burning at Laguna Zoncho, I suggest that the current clearing of the forest is a cycle of human presence in an already altered landscape.