Multi-millennial interactions between fire frequency and tree abundance in North American boreal forests

Background/Question/Methods

Wildfire is the major naturally occurring disturbance for much of the boreal forest in North America, greatly influencing forest structure and function. Climate appears to be the main process triggering fire and local factors are seen to play a significant role in the spatial distribution of fire. In order to investigate the relationship between climate, local factors, and fire in the mixedwood boreal forest over the last 10,000 years, macroscopic sedimentary charcoal and plant macroremains from two lakes in northwestern Ontario, Canada, were analyzed. Our specific objectives were: (1) to examine whether climate or local factors are the primary driver of regional fire frequency, and (2) to identify and analyse the relationships between fire frequency and dominant forest vegetation at each site. We compare the distribution of Fire Return Intervals (FRIs) and the synchronicity of fire events among sites to determine whether climate or local factors control the regional fire regime. We reconstruct the local vegetation history and assess species specific responses to FRI at each site. 

Results/Conclusions

Each site displayed a unique history of fire and vegetation. Different FRI distributions and independence in the occurrence of fire events were detected between each site in the 10,000 to 4000 cal. yr BP time period. Between 4000 cal. yr BP and the present, similar FRIs were observed with fires occurring independently between sites. Results indicate that local factors strongly influenced the regional fire regime at the centennial to millennial time-scale. At both sites, we found that Pinus banksiana and Betula papyrifera, were not significantly correlated to FRI suggesting that these species were able to maintain their local abundance at the millennial time-scale irrespective of changes in fire frequency. Picea mariana and Larix laricina, were most abundant at short FRIs. We conclude that the onset of neoglacial cooling likely created conditions that were favourable for the development and growth of coniferous species which increased landscape flammability. Given the importance of local factors versus climate in the region it is likely that the increase in coniferous vegetation is responsible for the observed reduction in regional FRIs after ca. 4500 cal yr BP. Results highlight the influence of local factors as important drivers of FRI at centennial to millennial scales.