Plants at the summit: characterization of grasslands on cerros chatos of northeastern Uruguay

Flat-top hills (“cerros chatos”) stand out as distinct relief formations in northeastern Uruguay. These are elevations of about 300 m above mean sea level, generally formed by Jurassic sandstones, characterized by a flat top, a stone ledge of variable height, and a concave or stepped hillside. Little is known about the vegetation atop these hills, which consists mostly of sparse grasslands with little to no grazing. As such, the aim of this study is to make the first characterization of the vegetation at the top of flat-top hills. In order to achieve this, we describe the vegetation of three emblematic hills in the region: Del Medio Hill, Vigilante Hill and Miriñaque Hill. Plant species composition surveys were carried out in spring 2021 and autumn 2022, identifying and delineating distinct vegetation formations. 315 species belonging to 63 vascular plant families were identified, with Poaceae and Asteraceae ranking as the top two families in terms of species richness. Among the species found, 31 are included in the list of priority species for conservation in Uruguay. Furthermore, Achyrocline marchiorii represents a new record for the Uruguayan flora. Two vegetation formations were present in all three hills: the ledge formation, associated with the rocky outcrops around the perimeter of the hilltops; and the crest formation, associated with the deep soils and low rock cover at the center of the hilltops. A shoulder formation of colluvial origin was also identified in the Miriñaque Hill, showing moderate slope and rockiness. The uncommon flora found on top of these hills is threatened due to the rapid increase of afforestation in the region, and the presence of the alien species Cynodon dactylon, Melinis repens and Pinus taeda. Such evidences emphasize the need for urgent management plans to protect these hills, which are not covered by any conservation program on a national or global scale.