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How Roman farmers left their mark on nature

July 9, 2007
Special to World Science  

Be­neath the stout oak trees and plen­ti­ful an­i­mal life of the Tron­çais for­est in cen­tral France—prized for pro­vid­ing wood bar­rels for some of the best wines—there lies a less vi­si­ble boun­ty. Ar­chae­o­lo­gists in re­cent years have dug up an abun­dance of an­cient Ro­man set­tle­ments, pre­vi­ously hid­den by earth and for­est cov­er.

A Ro­man set­tle­ment ex­ca­vat­ed in the Tron­çais for­est, Cen­tral France. The for­est, a fa­mous source of oak wood, was long as­sumed to be an­cient and nev­er defor­ested, but 108 Ro­man set­tle­ments were re­cent­ly found there. (Pho­to cour­te­sy Laure Laüt)

Yet these ag­ri­cul­tur­al opera­t­ions, which col­lapsed along with the em­pire, have left a last­ing mark on the wild­life above: it’s much more di­verse in their im­me­di­ate vicin­ity than fur­ther off, re­search­ers have found. The dif­fer­ence, they say, is large­ly at­tri­bu­ta­ble to Ro­man fer­til­iz­a­tion prac­tices.

The find­ings are one of sev­er­al re­cent stud­ies in­di­cat­ing that farm­ers of times past world­wide, from New Eng­land to the Am­a­zon rainfor­est, have left si­m­i­lar lega­cies of in­creased bio­di­vers­ity. 

But while pre­vi­ous re­search had found such ef­fects last­ing for hun­dreds of years, the Tron­çais stud­ies in­di­cate they can per­sist strongly af­ter al­most two mil­len­nia, ac­cord­ing to sci­en­tists.

The discov­eries should prove use­ful in man­ag­ing bio­di­vers­ity and de­vel­op­ing con­serva­t­ion poli­cies, re­search­ers said; they can even serve as a guide to ar­chae­o­lo­gists, who can use lo­cal wild­life di­vers­ity as a clue to help pin­point an­cient set­tle­ments. 

But be­yond the prac­ti­cal uses, Etienne Dam­brine, lead au­thor of a pa­per de­scrib­ing the find­ings, wrote in an e­mail that he hopes the rev­ela­t­ion of such in­ti­m­ate links be­tween his­to­ry and na­ture “may make peo­ple dream.” Dam­brine, of the Na­tional In­sti­tute for Ag­ro­nom­ic Re­search in Cham­p­e­noux, France, and col­leagues de­tailed their find­ings in the June is­sue of the re­search jour­nal Ecol­o­gy. 

Prunus spinosa—al­so called black­thorn, a plant that pro­duces plums used in mak­ing sloe gin—was one of the plant species found to be more abun­dant near Ro­man set­tle­ments.

The rea­son the in­creased bio­di­vers­ity around an­cient set­tle­ments lasts so long, they wrote, may be that the hu­man ac­ti­vity mod­i­fies the soil and sets up chem­i­cal cy­cles that become self-sus­tain­ing. At Tron­çais, such cy­cles per­sisted de­spite a long his­to­ry of hu­man ex­ploita­t­ion—and over-ex­ploita­t­ion—of for­est re­sources up to the pre­s­ent.

The Ecol­o­gy study fo­cused on plant life. It found that the num­ber of plant spe­cies in­creased on av­er­age by 50 per­cent near set­tle­ments. More re­cent, un­pub­lished work is al­so re­veal­ing great­er bio­di­vers­ity among an­i­mals along with plants, Dam­brine said.

The 11,000-hectare (26,000-acre) for­est, with sandy soils, con­tains 108 known Ro­man set­tle­ments, most discov­ered in the past two dec­ades, ac­cord­ing to Dam­brine and col­leagues. The set­tle­ments are dat­ed to be­tween 1,600 and 2,000 years ago and are among hund­reds of Ro­man set­tle­ments be­lieved to lie in French for­ests.

Dam­brine and col­leagues stud­ied 10 of the Tron­çais set­tle­ments, and found an av­er­age of 25.3 plant spe­cies in plots with­in 100 me­ters (109 yards) of them. By com­par­i­son, there were 16.8 spe­cies in equal-sized plots fur­ther off, they re­ported.

The in­crease in spe­cies rich­ness was found es­pe­cially among var­i­ous flow­er­ing and oth­er shrubs, researchers said, plants whose soil re­quire­ments are con­sist­ent with the types of fer­tilizers the Ro­mans used.

“Latin au­thors re­peat­edly men­tion the need for reg­u­lar fer­til­iz­a­tion af­ter plow­ing, us­ing ash­es or an­i­mal ma­nure,” they wrote. “This fer­til­iz­a­tion in­volved a trans­fer of min­er­al el­e­ments from re­mote ar­eas, probably for­ests, to the cul­ti­vat­ed ar­eas sur­round­ing the farms, through cat­tle graz­ing and fu­el wood col­lec­tion. Do­mes­tic gar­bage, in­clud­ing some bro­ken ce­ram­ics, was re­dis­trib­ut­ed with ma­nure and ash­es.”

The chem­i­cal changes re­sult­ing from such ac­ti­vity were “pos­sibly ir­re­versible,” they added. Thus “un­der­stand­ing pre­s­ent pat­terns of bio­di­vers­ity re­quires the in­ves­ti­ga­t­ion of land-use his­to­ry on a much long­er time scale than pre­vi­ously thought.”

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Beneath the plentiful animal life and stout oak trees of central France’s Tronçais forest—prized for providing wood barrels for some of the best wines—there lies less obvious bounty. Archaeologists in recent years have dug up an abundance of ancient Roman farming settlements, previously hidden by earth and forest cover. Yet these agricultural operations, which collapsed along with the empire, have left a lasting mark on the wildlife above: it’s much more diverse in their immediate vicinity than further off, researchers have found. The difference, they said, is attributable to Roman fertilization practices. The findings are one of several recent studies indicating that farmers of times past worldwide, from New England to the Amazon rainforest, have left similar legacies of increased biodiversity. But while previous research had found such effects lasting for hundreds of years, the Tronçais studies indicate they can persist strongly after almost two millennia, according to scientists. The discoveries should prove useful in managing biodiversity and developing conservation policies, researchers said; they can even serve as a guide to archaeologists, who can use local wildlife diversity as a clue to help pinpoint ancient settlements. But beyond the practical uses, Etienne Dambrine, lead author of a paper describing the findings, wrote in an email that he hopes the revelation of such close links between history and nature “may make people dream.” Dambrine, of the National Institute for Agronomic Research in Champenoux, France, and colleagues detailed their findings in the June issue of the research journal Ecology. The reason the increased biodiversity around ancient settlements lasts so long, they wrote, may be that the human activity sets up chemical cycles that tend to sustain themselves once in motion. At Tronçais, such cycles persisted despite a long history of human exploitation—and over-exploitation—of forest resources up to the present. The Ecology study focused on plant life. It found that the number of plant species increased on average by 50 percent near settlements. More recent, unpublished work is also revealing greater biodiversity among animals along with plants, Dambrine said in an email. The 11,000-hectare (26,000-acre) forest, with sandy soils, contains 108 known Roman settlements, most discovered in the past two decades, according to Dambrine and colleagues. The settlements are dated to between 1,600 and 2,000 years ago and are among as many as 2,000 Roman settlements believed to lie in French forests. Dambrine and colleagues studied 10 of the Tronçais settlements, and found an average of 25.3 plant species in plots within 100 meters (109 yards) of them. By comparison, there were 16.8 species in equal-sized plots further off, they reported. The increase in species richness was found only among vascular or “higher” plants with stems, shoots and leaves, as opposed to the more primitive mosses, the researchers wrote. And whereas the further-off plants were of types more likely to thrive in acidic soils, those near the settlements were types that prefer neutral or nitrogen-rich soils, especiall various flowering and other shrubs. The findings were consistent with the types of fertilizers the Romans are believed to have used, the resaerchers added. “Latin authors repeatedly mention the need for regular fertilization after plowing, using ashes or animal manure,” they wrote. “This fertilization involved a transfer of mineral elements from remote areas, probably forests, to the cultivated areas surrounding the farms, through cattle grazing and fuel wood collection. Domestic garbage, including some broken ceramics, was redistributed with manure and ashes.” The chemical changes resulting from such activity were “possibly irreversible,” they added. Thus “understanding present patterns of biodiversity requires the investigation of land-use history on a much longer time scale than previously thought.”