The interaction of fire, climate and vegetation in the boreal forest of Alaska-Yukon during the Holocene

Franklin-Smith, Leanne (2006) The interaction of fire, climate and vegetation in the boreal forest of Alaska-Yukon during the Holocene. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Using radiocarbon-dated sedimentary records with the temporal focus on two key vegetative transitions (deciduous-Picea and Picea-Pinus) and a climatic transition (cold and dry to moister), high resolution time series of charcoal-peak frequencies form lake sediments are used as a proxy of the local fire regime.

The regional vegetation transition from deciduous- to coniferous-dominated forest at ~10ka BP displays a clear sequence where the climate shift precedes the alteration in vegetation composition, to which the fire regime responds. The deciduous vegetation experienced low levels of burning, with a lower fire frequency than when Picea became dominant on the landscape, suggesting that Picea was excluded from the landscape due to moisture limitations rather than high fire return frequencies.

In the Yukon Territory, Pinus contorta (lodgepole pine) is migrating northwards and westwards towards Alaska, and is considered a potential invasive species to the northern boreal forest of Alaska under global warming. Lodgepole pine is a fire-dependent species that appears to thrive and spread when fires are intense and frequent. Analysis of stomata reveals lodgepole pine was present in the Southern Yukon forests, at least in low numbers, by ~6 ka BP, much earlier than conventional pollen records suggest. The main population expansion (represented by increased Pinus pollen from <5 to >15%) was regionally asynchronous, and occurred over 3 ka after the first appearance of Pinus. Contrary to expectations derived from flammability estimates and modern observations that pine stands burn particularly frequently, there is no clear, sustained increase in charcoal peak frequency in the late-Holocene Pinus zone; Pinus-Picea forests appear to have burned under a regime similar to that of the preceding Picea-dominated forests.