Florida wildfires during the Holocene Climatic Optimum (9000–5000 cal yr BP)

Abstract

Fire is a key ecological driver in terrestrial ecosystems of the southeastern United States. The combination of plant species composition and fire poses the possibility of positive feedback loops in a landscape. How fire regimes will change in the future with global warming is uncertain. To better understand the main factors that control fire, Holocene fire history was studied using macroscopic charcoal (as a fire proxy) in sediment cores from two shallow lakes in north Florida, USA. Following the onset of lacustrine sedimentation in the early Holocene, there was a 95% decline in average charcoal concentration, from 28.3 to 1.4 charcoal particles cm−3, at ~ 7500 cal yr BP in a 3.9-m core from Newnans Lake (basal age: 8870 cal yr BP). At ~ 7000 cal yr BP in a 5.4-m core from nearby Lochloosa Lake (basal age: 9280 cal yr BP), particle density declined by 99%, from 113.8 to 1.4 charcoal particles cm−3. These declines in charcoal concentration are not artifacts of changes in bulk sedimentation, i.e. dilution, but instead reflect lower charcoal production. Newnans Lake and Lochloosa Lake averaged ~ 8.93 and ~ 40.42 charcoal particles cm−3, respectively over their entire records. Pollen records from the two study lakes do not display the Quercus to Pinus shift during the Holocene Climatic Optimum (9000–5000 cal yr BP) observed previously in cores from many studied lakes in the region. Instead, pine pollen, as a percentage of total oak and pine pollen, was high (> 50%) through the entire Holocene. Pine and oak pollen counts and the variety of arboreal pollen, however, were lower in early Holocene deposits. No relation was found between pine and oak percentages and macroscopic charcoal quantity, i.e. before, during, or after charcoal peaks. There were, however, qualitative differences between pollen assemblages within charcoal-rich and charcoal-poor depth intervals. Early to middle Holocene sediments contained pollen of Quercus spp., Pinus spp., Ambrosia sp., Amaranthaceae, Cyperaceae and Poaceae. Late Holocene sediments contained hydric and mesic pollen types including Taxodium spp., Carya sp., Ilex sp., and Liquidambar styraciflua. The flat, poorly drained topography around both lakes played a role in shaping Holocene plant communities and probably explains why a Quercus-to-Pinus shift seen elsewhere in Florida did not occur in the study region.