Data from: 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. As a striking real-world
example and 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 the geological origin of each jar 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 O2 and nutrient concentrations
among jar ecosystems. These initial findings show that litter contribution
to aquatic ecosystems leads to higher nutrient content and reduces O2
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.