The world's oldest man-made biological experiment

Biological experiments are often short-lived due to logistical or resource-related challenges, and short-term observations are extrapolated to make long-term predictions. However, the effects of experimental treatments on biological communities and processes take time to develop. Consequently, the robustness of conclusions drawn from observations increases with the duration of the experiment. Scattered throughout central Laos, thousands of large stone jars have been left behind from ancient burial rituals. The most famous sites in the Xiengkhouang province are collectively referred to as the Plain of Jars. These jars form a massive biological experiment: for ~2000 years, rainwater has interacted with geology to create unique yet replicated aquatic ecosystems influenced by different tree coverage. The layout of these jars, with clusters of up to several hundred jars separated by several kilometers, allows for controlled testing of multiple questions within ecology and evolution. Here, we report for the first time how these ancient mesocosms can be used to test ecosystem responses to local abiotic variation and disturbance. We show that tree cover dominates every jar ecosystem’s state, and that variations in tree cover density create gradients in oxygen and nutrient concentrations among jar ecosystems. These findings show that litter contribution to aquatic ecosystems leads to higher nutrient content and reduces oxygen concentration, even in systems under different long-term selection in the oldest man-made ecosystems ever analyzed. This first biological analysis provides a fundamental understanding of a unique environment and offers trajectories for future exploration.