Researchers at McGill University used 2,000-year-old stone jars in Laos to observe long-term ecological processes, enhancing understanding of how strongly tree cover shapes small freshwater ecosystems. Their findings stand to help scientists predict how freshwater habitats will respond to environmental change, the researchers said.
The study showed that due to decomposing leaf litter, jars located beneath trees accumulated more nutrients and are dominated by oxygen-consuming organisms. Jars in open areas, by contrast, contained fewer nutrients and more oxygen-producing organisms such as algae.
A rare long-term experiment
The stone jars, carved from bedrock and placed at the Plain of Jars by people from an unknown culture, collect rainwater and form small aquatic habitats. Because they have remained in place since the Iron Age (1200 BC to 500 BC), they allow scientists to observe long-term ecological processes that are rarely captured in modern experiments.
"We use experiments to simplify the complexity of ecological systems, but they can only run for a feasible time span," said Lars Lønsmann Iversen, Assistant Professor of Biology at McGill and study co-author. "These stone jars have been standing in nature for 2,000 years, collecting water, and all the biology within these small aquatic habitats has been operating for that period. Suddenly, we're able to look at effects over very long timescales."
Laura Käse, lead author of the paper and a Postdoctoral Fellow at the University of Copenhagen, noted that even small differences in surrounding trees can change water chemistry.
"This helps us see how forests and small water bodies are connected over time," Käse said.
Sampling in a challenging landscape
The researchers sampled 39 jars across five sites, working with local scientists, heritage experts and community members.
They sampled the jars twice: once during the dry season and once during the wet season, and measured water chemistry, oxygen levels, nutrient availability and litter deposition to understand the ecological conditions within each self-contained habitat.
Iversen said the findings on how surrounding vegetation influences freshwater ecosystems were "to some extent confirmatory," but powerful because of their timescale.
"We know this process operates over months or years in typical experiments. Now we're showing it is still happening in small aquatic ecosystems that have been existing over 2,000 years," he said.
New questions about biodiversity and climate change
The team is now analyzing microbial DNA from water and sediment samples to determine whether the communities in the jars are a product of long-term selection or "reset" each year during the dry period. They are also investigating the rock type used for the jars, as a potential driver of freshwater biodiversity.
"We hope that within the next couple of years we'll be able to show with these jars that bedrock may be just as important for shaping freshwater biodiversity gradients as temperature and nutrient gradients," Iversen said.
Understanding these mechanisms is essential for anticipating how freshwater systems will respond to current and future human impacts and climate change, he added.
Cultural and scientific value
Because the jars are ancient burial vessels and protected UNESCO world heritage sites, the project relied on close collaboration with Laotian authorities, researchers and communities.
Iversen said he hopes the work highlights the scientific value of cultural sites that also contain long-term ecological records.
"The more we can highlight the presence of these jars and show their value, the better - not only for cultural purposes, but also as a source of information we can use to understand nature and how the world is changing," he said.
Added Käse: "Environmental changes often happen slowly. To make good decisions, we need to think in centuries, not just years."
About this study
"The world's oldest man-made biological experiment," by Laura Käse, Lars Lønsmann Iversen et al, was published in Ecography.
The research was funded by the VILLUM Foundation.
