Spatiotemporal patterns of tundra fires: late-Quaternary charcoal records from Alaska

Abstract

Abstract. Anthropogenic climate change has altered many ecosystem processes in the Arctic tundra and may have resulted in unprecedented fire activity. Evaluating the significance of recent fires requires knowledge from the paleo-fire record because observational data in the Arctic span only several decades, much shorter than the natural fire rotation in Arctic tundra regions. Here we report results of charcoal analysis on lake sediments from four Alaskan lakes to infer the broad spatial and temporal patterns of tundra fire occurrence over the past 35 000 years. Background charcoal accumulation rates are low in all records (range = 0–0.05 pieces cm-2 year-1), suggesting minimal biomass burning across our study areas. Charcoal peak analysis reveals that the mean fire return interval (FRI; years between consecutive fire events) ranged from 1648 to 6045 years at our sites, and that the most recent fire events occurred from 882 to 7031 years ago, except for the CE 2007 Anaktuvuk River Fire. These mean FRI estimates are longer than the fire rotation periods estimated for the past 63 years in the areas surrounding three of the four study lakes. This result suggests that the frequency of tundra burning was higher over the recent past compared to the late Quaternary in some tundra regions. However, the ranges of FRI estimates from our paleo-fire records overlap with the expected values based on fire-rotation-period estimates from the observational fire data, and thus quantitative differences are not significant. Together with previous tundra-fire reconstructions, these data suggest that the rate of tundra burning was spatially variable and that fires were extremely rare in our study areas throughout the late Quaternary. Given the rarity of tundra burning over multiple millennia in our study areas and the pronounced effects of fire on tundra ecosystem processes such as carbon cycling, dramatic tundra ecosystem changes are expected if anthropogenic climate change leads to more frequent tundra fires.

Arctic tundra fires: natural variability and responses to climate change

“…Consistent with observations from the past 60 years, charcoal data from Alaska show that the frequency of wildfires has varied greatly across space and time ( Figure 2). Within the extent of the AR Fire, no fire occurred in the previous 6500 years (Chipman et al 2015). This extreme rarity of tundra burning is supported by data from other sites; at Tungak Lake in southwestern Alaska, only five fires occurred in the past 35 000 years (Chipman et al 2015).…”

“…Similarly, the lateglacial (14 000-10 000 years ago) tundra in north-central Alaska burned at frequencies close to those of the modern boreal forests, with mean return intervals of 140-150 years (Higuera et al 2008). A notable feature that has emerged from the accumulating paleofire records is that the broad spatial patterns of tundra fires observed in recent decades have been in place for thousands of years (Chipman et al 2015).…”

“…Furthermore, tundra fires may have increased over time in terms of size and biomass consumed. On the North Slope of Alaska, where fires have been extremely rare over the past 10 000 years (Hu et al 2010;Chipman et al 2015), the Anaktuvuk River Fire (AR Fire) occurred in 2007, at a time when the region was unusually warm and dry (Jones et al 2009;Hu et al 2010). This single fire burned a 1039km 2 area, more than doubling the total area burned during the previous 60 years in the region.…”

“…Within the extent of the AR Fire, no fire occurred in the previous 6500 years (Chipman et al 2015). This extreme rarity of tundra burning is supported by data from other sites; at Tungak Lake in southwestern Alaska, only five fires occurred in the past 35 000 years (Chipman et al 2015). In stark contrast, in the Noatak ecoregion tundra fires have occurred regularly, with mean fire-return intervals (the time interval between individual fire events) at four lakes ranging from 135 to 309 years over the past 2000 years (Higuera et al 2011a).…”

“…Historical observations of tundra fires reveal that tundra burning has increased in northern Alaska and decreased farther south over the past few decades (Rocha et al 2012). No unambiguous evidence exists to demonstrate that fire frequencies were higher in the past 60 years as compared with fire frequencies during the late Quaternary; tundra-fire frequencies overlap statistically between those two periods for each of the ecoregions where charcoal records have been obtained for paleofire reconstruction (Chipman et al 2015). Nonetheless, some tundra ecoregions have not experienced burning for several centuries or even millennia (Chipman et al 2015).…”