Climatic and landscape controls of the boreal forest fire regime: Holocene records from Alaska

1. The response of ecosystems to past and future climatic change is difficult to understand due to the uncertainties in the direction and magnitude of changes and the relative importance of interactions between climate and local factors. In boreal ecosystems such interactions may dictate the response to climatic change, but the interaction of climate, vegetation composition and the fire regime remains poorly understood. 2. Sediment cores from lakes in south-central Alaska were analysed for lithology, macrofossils, pollen and charcoal to investigate the relationships between moisture availability, species composition and mean fire return intervals (MFI). 3. Macrofossil and lithological evidence suggests that variations in effective moisture occurred over the past 7000 years and that the regional climate has been wetter during the past c. 3800 years than before. 4. Boreal forests existed in the region throughout the past 7000 years. Picea glauca and Picea mariana were the prevalent forest species around Chokasna Lake, whereas P. glauca and hardwood species (e.g. Betula) co-dominated the landscape around Moose Lake. Picea mariana replaced P. glauca as the dominant Picea species around Chokasna Lake at c. 2000 BP. 5. MFI was > 500 years before 3800 BP, except from 5800 to 5000 BP at Moose Lake and 5400 to 4550 BP at Chokasna Lake, when values were c. 200 years. MFI decreased to c. 200 years during the late-Holocene at both sites and to c. 150 years after 2000 BP at Chokasna Lake. 6. At both sites, fires occurred more frequently under wetter climatic conditions. Our results therefore support other recent studies demonstrating that warmer/drier climatic conditions do not necessarily induce greater fire importance. A combination of increased ignition by lightning strikes and seasonal-moisture variability probably resulted in more frequent fires under wetter conditions.