Macroscopic fossil charcoals as proxy of a local fire linked to conifer-rich forest from the late Pliocene of northwestern Yunnan, Southwest China

Abstract

Yunnan at southeastern margin of the Tibetan Plateau is subject to frequent wildfires each year, while its wildfire history remains poorly known due to the lack of studies on palaeofire in the region. In this study, we report a local fire from the late Pliocene of northwestern Yunnan, based on macroscopic fossil charcoals recovered from the Sanying Formation of Lanping Basin. These sedimentary charcoals exhibit silky lustre in the light and complete homogenization of adjacent xylem cell walls, characterizing the result of incomplete combustion during the late Pliocene. Our preliminary taxonomic analysis indicates that the studied charcoals are dominated by conifers, suggesting higher importance of coniferous elements as fuel sources in the fire. We assert a conifer-rich source forest for the fire event by also considering plant remains of other types, i.e., needle fragments, small shoots, fruits and seeds, from the same sampling layer. Since conifers are commonly prone to wildfires, this type of forest might have a close link with the fire by serving highly flammable fuels. We consider that the regionally seasonal drought during the late Pliocene might also take responsibility, because in the dry season forest fuels such as ground litter would become ignitable after intensive desiccation. As modern wildfires in northwestern Yunnan are closely coupled with conifer-dominant forests and seasonally dry climate, we assume this correlation might have been established by the late Pliocene. Our study may bring attention to potential roles of wildfire on local and/or regional flora and vegetation evolution in this region.