Fire Ecology and Regime Shift due to Climate Change


Fire Ecology and Regime Shift due to Climate Change

The historic fire regime has been well studied on the Kenai Peninsula (Berg and Anderson 2006, Anderson et al. 2006).  Prior to this past century, major fires of unknown origin took place in 1871, 1883, 1891, and 1910 (Lutz 1960), burning much of the Tustumena Benchlands.  In 1947 and 1969, two large human-caused fires burned 310,000 acres and 86,000 acres, respectively, in the Kenai Lowlands, converting much of the mostly mature black spruce forest to birch and aspen.  Since the 1990s, ~ 140,000 acres of mature and beetle-killed white and Lutz spruce have burned in several fires around Skilak Lake and south of Tustumena Lake.   

Historically, two distinct fire cycles occur in spruce on the Kenai landscapes.  Black spruce forests, primarily in the Kenai Lowlands, increase in flammability with age (DeVolder 1999).  A revised estimate of the mean fire return interval over the entire study area for the past 300 years is 89 ± 43 years (1 SD) years (range 25-185 years) based on 1,022 basal cross-sections and 771 increment cores of lowland black spruce (Berg, pers. comm.).  Twelve fires were dated as occurring in 1708, 1762, 1801, 1828, 1883, 1834, 1849, 1867, 1874, 1884, 1888, and 1898. 

In contast, white, Lutz and Sitka spruce forests on uplands burn on a much longer time interval.  In the virtually monospecific stands of Lutz spruce on the southern Kenai Peninsula, the mean fire return interval for the past 2,500 years was estimated to be 515 ± 355 (1 SD) years (range 105–1642 years);  the mean time-since-fire was estimated at 605 ± 413 years (median 444 years, range 90–1518 years).  These estimates are based on a sample size of 112 radiocarbon dates of soil charcoal at 22 sites from Anchor Point to Nikiski (Berg and Anderson 2006).  In the Swanson River Road area, a lake-sediment charcoal study at Paradox Lake estimated the mean fire return interval to be ~200 years, perhaps due to the presence of more black spruce in the valley bottoms.  On the south side of Kachemak Bay, there is no charcoal evidence that the old-growth Sitka spruce forests have ever burned in the 2,200 years that Sitka spruce has been in the area.  On the Refuge north of Kachemak Bay, there are no spruce forests more than 200 years old, even though some of those forests have not burned for at least 1,500 years.  It appears that spruce bark beetle outbreaks recycle these forests much more frequently than does fire (Sherriff et al.  2011).  It is rare to find spruce trees more than 300 years old, although these species typically live to be 500 - 600 years in other parts of their range.

In mixed white and black spruce and hardwoods forests, the mean fire return interval for the Paradox Lake area ~10 km north of Sterling was estimated at 130 ± 66 years (1 SD) years with 35 fires separated by intervals of 40 to 270 years occurring during  the last 4,600 years since the arrival of black spruce on the landscape.  This estimate is based on sedimentary charcoal in a 9m core taken from Paradox Lake (informal name) at a depth of 15.8 m, supplemented with a 70-cm short core of near-surface sediments.  The total charcoal fire history record spanned ~13,000 calendar years; mean fire return intervals were longest during the shrub-herb tundra phase (138 ± 65 yr), decreased after expansion of Betula kenaica, Salix and Populus (77 ± 49 yr) and Picea glauca (81 ± 41 yr), and increased again with the arrival of P. mariana (130 ± 66 yr)(Anderson et al. 2006). 

The presence of almost 1 million acres of beetle-killed spruce forest on the Kenai Peninsula has raised the specter of catastrophic wildfire.  However, Berg and Anderson (2006) used 121 radiocarbon-dated soil charcoal samples to reconstruct the regional fire history of the last 2,500 years and found no relation between fire activity and past spruce bark beetle outbreaks.  On average, one fire occurred for every 10 spruce bark beetle outbreaks in these forests.  Nevertheless, a run of warm summers since 1987 has created a spruce bark beetle outbreak of unprecedented scale, and at least three major fires with high rates of spread in recently beetle-killed timber have occurred.  This suggests that, with a future warmer climate and increasing human use of the landscape, fire and beetle kill may be well more closely associated than in the past (Berg and Anderson 2006). 

The effects of climate change on vegetation composition have been modeled for the Kenai Peninsula using two different approaches.  Rupp and Mann (2002) simulated fire-induced vegetation change using ALFRESCO and refuge biologists have used a climatic envelope approach with RandomForest™.  Although very different assumptions underlie these two approaches, both models suggest the following salient outcomes:

  • conversion of softwood to hardwood presumably due to more frequent and hotter fires;
  • rising treeline (primarily mountain hemlock) with concomitant loss of alpine tundra;
  • increasing herbaceous cover (grasslands) particularly south of Caribou Hills (note: parcelization and the resulting deforestation will also increase grasslands along the WUI); and
  • loss of old growth softwood forests (note:  Berg suggests consecutive above-average summer temperatures will keep spruce bark beetles sustained metabolically and thereby Sitka, white and Lutz spruce from maturing in the foreseeable future).

In addition, Global Climate Change (CGM) model projections of future boreal forest climates suggest that fire burn area will increase as fire regimes change (Podur & Wotton 2010, Amiro et al. 2006). Altered fire regimes may cause a synergistic increase in invasion potential.  Greater fire frequency may create more habitat for invasions, and invasions in turn can contribute to more frequent fires.  Although the interaction of climate, fire regime, and invasions is not well understood in Alaska, there are well-documented examples of established invasive plants increasing fire frequency. Cheatgrass (Bromus tectorum) invasions in the western US  has resulted in native vegetation loss, less predictable livestock and wildlife forage crops, and higher fire control costs (Epanchin-Neill et al. 2009). Before cheatgrass invasion, shrub-regenerating fire occurred every 60 to 110 years; major fires now occur every three to five years (ELI 2002). Buffelgrass (Pennisetum ciliare) causes higher intensity, lower interval fires that decrease native plant richness (McDonald & McPherson 2011).

Although there is no empirical evidence yet to suggest that the historic mean fire return interval in spruce is changing in response to rapid climate change, it is apparent that the fire regime may be altering in unexpected ways.  In the aftermath of spruce bark beetle-induced deforestation, grassland (Calamagrostis canadensis) fires have burned in April on the southern part of the Peninsula in recent years.  Lightning caused the 2005 Irish Channel fire that burned 1,100 acres of mountain hemlock (Morton et al. 2006), an event so rare in this forest type that charcoal evidence of a historic fire regime has not been detected.  The Refuge maintains three long-term plot-based data sets for monitoring the effects of wildfire:  NPS Fire Monitoring Handbook (FMH) plots, Hakala plots, and the Forest Inventory & Analysis (FIA) plots.