Impacts of Climate and Landform on Boreal Forest Fire Regimes over the last 6000 years

Background/Question/Methods In the Alaskan boreal forest climatic warming over the past 50 years has coincided with a doubling of area burned over this same period.  In contrast, paleoecological studies show that area burned increased with cooling and moistening during the middle Holocene, which has been attributed to the regional expansion of highly flammable Picea mariana (black spruce) forests. Thus millennial-scale climatic controls of fire regimes over the Holocene are more complex than recently observed climate-fire links. Here we present six fire-history records spanning the last 6000 years from the Copper River Basin (CRB) in south-central Alaska. P. mariana was a dominant component of the regional vegetation throughout the study period, allowing us to examine the effect of climate on fire regimes independently of vegetation change. The climatic history of the CRB was constrained by a lake-level reconstruction and a new midge-inferred July temperature record from the region. Mean fire-return intervals (FRIs) were calculated from peaks in macroscopic charcoal (>180µm) records for all sites, and the distributions of FRIs were compared between three periods with broadly contrasting climatic conditions: warm/dry (6000­­-3800 yr BP), cool/moist (3800-1800 yr BP), and cold/wet (1800-0 yr BP).

Results/Conclusions Our results highlight strong interactions between landform and the responsiveness of the local fire regimes to climatic forcing. The six study sites differ greatly in the density of lakes within a 5-km radius, which likely reduce fire spread by serving as firebreaks. Sites in landscapes with low lake density (1-3.5% cover, n=4) had similar FRIs across the three climatic zones. This finding suggests that fire regimes were insensitive to late Holocene climatic change and contrasts with climate-fire relationships observed over the past several decades.  However, landscapes with a high lake density (5 and 21% cover, n=2) were sensitive to these same climatic changes: mean FRIs were higher during the cold/wet vs. warm/dry period (354 vs. 278 yr; p = 0.05).  During wetter periods, higher lake levels likely increased water body extent, which reduced burning in landscapes with high lake densities.  Thus, the impacts of climate change on fire regimes appear to have been a function of the efficacy of firebreaks: landscapes with a high density of lakes were more sensitive to the relative shifts in lake density than landscapes with a low lake density.  These results imply that landscape heterogeneity will be an important factor modifying the response of the fire regime to ongoing and projected climatic warming.