HIGH-FREQUENCY SHIFTS IN SAVANNA ECOSYSTEM STATES REVEALED BY SEDIMENT CORE PROXY RECORDS FROM LAKE TANGANYIKA (AFRICA)

Enhancing resilience to climate change in the Afrotropics requires robust predictions of ecosystem responses to environmental variability. Yet, high-resolution paleoecological records, invaluable for understanding African vegetation dynamics, are scarce for this region. We used a radiocarbon-dated sediment core from central Lake Tanganyika to examine terrestrial ecosystem responses to hydroclimate and fire dynamics over the last two millennia. Analysis of phytoliths and macrocharcoal datasets show multi-decadal to multi-centennial oscillations between open grasslands and woodlands within the Zambezian miombo region, shifting from stable to unstable states depending on disturbance levels. The transition from woodlands to grasslands was primarily driven by decreased moisture availability, increased fire activity (natural and anthropic), and ecosystem feedback mechanisms. Our findings offer new constraints on the timing and vegetational structure response to major events of global change over the Common Era in the Lake Tanganyika watershed, including the relatively dry Medieval Climate Anomaly (ca. 1000 to 1250 CE), dominated by grasslands, and the two phases of the Little Ice Age (LIA). The early LIA (ca. 1250 to 1530 CE) was characterized by relatively cold and wet conditions that promoted tree encroachment, while the main LIA (ca. 1530 to 1850 CE) had drier conditions and intensified fire activity, leading to the expansion of open grasslands. These significant shifts in grassland-woodland communities were chiefly controlled by hydroclimate and rapid ecosystem interactions, which we discuss in detail. Our dataset reveals that in tropical savannas around Lake Tanganyika, fire activity can act as both a disruptive force (e.g., opening landscapes and preventing tree encroachment) and a stabilizing force (e.g., maintaining a grassland stable state). Understanding these changes in vegetation structure and function is crucial for developing successful biodiversity conservation strategies and protecting the ecosystem services provided by the miombo woodlands, which are essential to the livelihoods of millions of people. This study also paves the way for using phytolith analysis as terrestrial vegetation proxies in the Lake Tanganyika Drilling Project.