Fire as a key factor in the transition and maintenance of the oak savanna ecosystem in the Cedar Creek Ecosystem Science Reserve, Minnesota, USA

Savannas are wrongly perceived as degraded ecosystems whereas they represent a highly valuable landscape and cover 20% of the Earth’s surface. In the USA, Cedar Creek Ecosystem Science Reserve (Minnesota, USA) focuses on temperate savanna, unique in the continent. A prescribed burning program has been established in the early 60’s to include this key structural process into the savanna system. In this study, we reconstructed for the first time the fire regime imprints in this ecosystem and analyzed the link with the savanna vegetation since its establishment, about 4200 years before present day. Replacing first the boreal forest, then the oak/pine forest, following the glacial retreat and the climate warming of the Holocene (at respectively ca. 8000 years and 7200 years before present), this savanna system experienced a mFRI of 156.5 +/- 158.2 years.

At its establishment, the savanna presented the lowest recorded abundance of pine species while the abundancy of Poaceae dramatically increased in the understory thus defining the landscape characterized by low density of oak that is observed nowadays. In parallel, the fire regime of the mixedwood forest was characterized by less frequent fires associated with relatively high charcoal accumulation rates (mean Fire Return Interval of 280.9 years +/- 160.6 years) which transitioned towards higher frequency of fire events associated with low charcoal influx, while moving towards the savanna vegetation. In addition, the change of fire frequency is associated with a change in the fuel type burned, with dominance of ligneous fuel during the forested phase (W/L ratio of charcoal particles < 0.5) shifting to a more diversified fuel type around 3750 years BP (WL ratio > 0,5). Our results thus suggest that savanna dominated landscape is associated with frequent fires of a mixed fuel composition, reflecting the more diverse vegetation and the establishment of the herbaceous layer in sparse oak trees in the landscape.

Ultimately, frequent fires have maintained savannas for over 4 millennia thus highlighting that prescribed burning is a practice to be maintained in order to protect this ecosystem.