Abstract/Summary

Southern Brazil’s Araucaria Forests are unique, iconic, and vanishing. Part of a globally important biodiversity hotspot, they are threatened by 20th -Century deforestation and 21st -Century anthropogenic climate changes. Natural climate variability and Indigenous people also affected Araucaria Forests over many millennia, but considerable uncertainty remains over the type and scale of their impacts. Interweaving ecological models and data on past, present and future human, climate and vegetation dynamics, this thesis seeks to better understand the roles of climatic and non-climatic factors – including topography, human land use, and fire – in shaping Araucaria Forests over the last 21,000 years and into the late 21st Century. Results show that 21st -Century anthropogenic climate change will likely bring greater disruption to Araucaria Forests than the last 21 millennia of natural variability. Key species will experience major range contractions and novel warm-adapted forests will replace long-established floristic associations, echoing and exceeding changes from the Holocene onset 12,000 years ago. Araucaria populations will likely find shelter in small-scale microrefugia, but more than a third of these have already lost their natural vegetation cover and few remnants are well protected. But results also show that climate only partially controls Araucaria Forest dynamics. 21,000 years ago, low atmospheric CO2 concentrations helped grasslands dominate woody vegetation, a dynamic which continued until recent millennia. Araucaria Forests then flourished late in the Holocene, as climate changes tipped landscapes over fire-suppression thresholds, triggering runaway forest expansion. Although subtle human impacts can be difficult to see in fossil pollen data, pre-colonial Indigenous communities did significantly shape Araucaria Forest structure and composition with fire, crop cultivation, and enriched Araucaria populations. This understanding of how past climatic and non-climatic factors combined to shape contemporary Araucaria Forests provides vital information for their future – from non-linear responses to climate shifts to conservation strategies and pathways for sustainable resource use.