Alternative savanna states during the Last Interglacial: non-eucalypt dominance in northern Australia's monsoon tropics

This study presents a high-resolution reconstruction of Marine Isotope Stage 5 (MIS 5; 129–71 ka BP) vegetation dynamics from Girraween Lagoon in northern Australia's Top End, highlighting the central role of non-eucalypt taxa in shaping savanna structure and function. The pollen record reveals early-mid MIS 5 was characterised by intense monsoonal activity and widespread waterlogging, conditions that suppressed Eucalyptus / Corymbia but facilitated the expansion of sclerophyll non-eucalypts. This woody group formed diverse scrub-heath and mixed-forest mosaics acting as the functional equivalents of eucalypts: maintaining flammable vegetation, supporting grass-woody feedbacks, and thus preventing monsoon-forest from dominating despite favourable hydroclimate. Non-eucalypts were therefore pivotal in sustaining savanna processes during intervals when eucalypts were climatically disadvantaged. The non-eucalypts provided an alternative savanna state, challenging the traditional savanna-rainforest binary by demonstrating multiple stable woody configurations are possible. They also highlight the importance of non-eucalypts as alternative ecosystem engineers. The capacity of sclerophyll non-eucalypts to replace eucalypts across landscapes under wetter hydroclimatic extremes has significant implications for forecasting savanna resilience under future environmental change.