Abstract
Fire is an essential component of tropical savannas, driving key ecological feedbacks and functions. Indigenous manipulation of fire has been practiced for tens of millennia in Australian savannas, and there is a renewed interest in understanding the effects of anthropogenic burning on savanna systems. However, separating the impacts of natural and human fire regimes on millennial timescales remains difficult. Here we show using palynological and isotope geochemical proxy records from a rare permanent water body in Northern Australia that vegetation, climate, and fire dynamics were intimately linked over the early to mid-Holocene. As the El Niño/Southern Oscillation (ENSO) intensified during the late Holocene, a decoupling occurred between fire intensity and frequency, landscape vegetation, and the source of vegetation burnt. We infer from this decoupling, that indigenous fire management began or intensified at around 3 cal kyr BP, possibly as a response to ENSO related climate variability. Indigenous fire management reduced fire intensity and targeted understory tropical grasses, enabling woody thickening to continue in a drying climate.
Highlights
The long-term response of fire to changing climate and human management in tropical savanna remains poorly understood despite the essential role fire plays in potentially maintaining bistable states and modulating biodiversity[1,2,3,4]
Our data show a strong coupling between monsoon intensity, vegetation, fire frequency, fire intensity, and vegetation burnt (δ13CSPAC) during the Holocene until ~ 3 cal kyr BP, after which fire regimes become decoupled from local vegetation and monsoon variability
The δ13CSPAC values range from − 15.7 to − 13.6‰ over this time, and indicate that almost all of the fuel load being burnt was derived from tropical grass (C4) biomass
Summary
The long-term response of fire to changing climate and human management in tropical savanna remains poorly understood despite the essential role fire plays in potentially maintaining bistable states and modulating biodiversity[1,2,3,4]. In the context of tropical northern Australian savannas, the impacts and causes of changing fire regimes remains enigmatic. It is still debated whether indigenous fire practices played a role in megafaunal extinction, or replaced the ecological function of megafauna[2,7]. We present a multi-proxy archive of changing environmental conditions directly comparable to independent records of fire frequency, fire intensity, and vegetation burnt. The carbon isotope composition of SPAC (δ13CSPAC value) is indicative of vegetation burnt in tropical s avannas[15] as grasses utilize the C 4 photosynthetic pathway Our integrated multi-proxy results provide a unique opportunity to directly compare changes in environmental parameters, fire frequency, fire intensity, and vegetation burnt. We suggest that indigenous fire practices began or intensified at that time, possibly as a response to changing climate variability and/or population increases
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