Polish wetlands started changing in the 1300s when Catholic crusaders known as the Joannites settled a nearby village and leased land to peasants for farming grains.
(CN) — Inland wetlands are often considered unspoiled landscapes, but a new study shows how deforestation and agriculture in the Middle Ages transformed wetlands in Poland and imbued the vegetation with a record of societal change.
For earth scientists, Pawski Ług, a nature reserve in western Poland, is a rare gem.
Clearcutting around wetlands can upset their water balance and turn them into shallow lakes or terra firma.
But Pawski Ług’s fast-growing peatland, a type of wetland, surrounded by disturbed forests provide means to examine the 1,500-year evolution of the ecosystem, according to a team of Polish researchers who published their paper Thursday in the journal Scientific Reports.
The researchers say “surface vegetation surveys are usually not sufficient” to reconstruct an ecosystem’s baseline conditions, but the wetland vegetation they analyzed took them back centuries.
They dug into the peatland with corers in 2016 and extracted samples that contained peat moss stems ranging in age from 1,450 to 60 years old and froze the material in their lab.
In a painstaking study that took two years, they thawed out chunks of peat core, ascertained its age with radiocarbon dating and from every 5 centimeters took samples from which they created slides for microscopic analysis of the treasure trove it contained: pollen, charcoal fragments and testate amoebas—single-celled organisms found in wetlands and moist soil.
“By analyzing the composition of different peat layers, the authors were able to draw conclusions about the conditions that were present when each layer was formed,” the team said in a statement.
Once fed by streams and covered by waterlilies, Pawski Ług’s waterlogged wetlands started changing in the 1300s when Catholic crusaders from the Order of St. John of Jerusalem, Knights Hospitaller, known as the Joannites, settled the nearby village of Łagów.
The Brandenburg Margrave Ludwig V, an elected official of the Roman Empire, deeded the village and surrounding countryside to the order when he defaulted on a loan, and the Catholics leased large spreads to peasants for farming grains.
They cut down the dense forests of birch, oak, alder, beech, Scots pine and hornbeam trees around the wetlands to clear the land for farming and burn to heat their homes.
The researchers, led by Mariusz Lamentowicz, a professor of Earth sciences at Adam Mickiewicz University in Poland, believe deforestation may have lowered the water table so much the wetlands turned ombrotrophic, dependent on rain for moisture and no longer fed by streams, allowing peat moss to flourish and form giant floating mats.
When Scots pine became the dominant tree species around the mire, it created ideal conditions for peat moss as the trees littered the area with their acidic needles.
“Consequently, the soil experienced progressive soil acidification that also affected Pawski Ług and supported Sphagnum [peat moss] development in the basin. Sphagnum prefers acid conditions to grow, and it is also an important ecosystem engineer that acidifies habitat by itself,” the paper states.
Using the wetlands as a “natural archives,” the authors say they have shown how the transformation of the area’s culture from tribal to feudal affected the ecosystem.
For several centuries before the Joannites entered the scene, the area surrounding the peatland was sparsely populated by Slavic tribes who abstained from decimating the forest, though they did regularly spark controlled burns to clear space for small-scale farming and livestock grazing, the researchers say, pointing to small amounts of charcoal found in the older peat core.
Lamentowicz said in an email his team did not expect the peat core samples to so clearly show how clearcutting swiftly made way for an agrarian society and transformed the wetlands.
“We are astonished how sharp the signal is of the forest decline, and how fast the feudal economy eradicated pristine nature, creating anthroecosystems – novel ecosystems created by human activity,” he said.