Recent Climate Change Impacts on the Boreal ... - Wiley Online Library

Geography Compass 4/2 (2010): 67–80, 10.1111/j.1749-8198.2010.00310.x

Recent Climate Change Impacts on the Boreal Forest of Alaska Monika P. Calef* University at Albany

Abstract

The vast boreal forest biome is experiencing rapid climate warming with multiple impacts on the ecosystem which include direct vegetation changes; alterations to the ground thermal and hydrological regimes; and increases in disturbance by fire, insects, and disease. This cold biome serves as one of the major carbon storage reservoirs on this planet and there is much concern that soil warming will result in the release of massive amounts of carbon into the atmosphere where it can cause a significant feedback to global climate change. Climate predictions and observations point to interior Alaska as one of the regions experiencing the most drastic warming and can thus serve as an important harbinger of ecosystem response to climate change.

1 Introduction On average, global surface temperatures have increased by 0.8 C in the past century and by 0.6 C in the past 30 years alone which makes this the warmest (or very nearly so) it has been during the entire Holocene (the past 10,000 years) and within 250 years in upland forests (Fastie et al. 2003). Annual area burned varies by several factors; however, there are roughly two large fires per decade or every 4 years (Kasischke et al. 2002, 2006). This could be explained up to 75% by large-scale atmospheric weather patterns (teleconnections) and monthly weather (Duffy et al. 2005). Fires were increasingly severe as fire size increased (large fires burn hotter), on flat terrain (absence of barriers to fire spread) and in purely coniferous (needle-leaved) stands (Duffy et al. 2007). Although black spruce generally benefits from fire, more severe fires enhanced aspen recruitment and growth instead while inhibiting spruce regeneration. With conifer self-replacement impeded, the succession at the site developed toward deciduous dominance instead (Johnstone and Kasischke 2005). All around the world, people alter the natural fire regime; in interior Alaska, they are responsible for an approximately 10-fold increase in fire ignitions within 5 km of settlements and roads, but account for only 5% of area burned (Calef et al. 2008). Human-caused fires remain small because they often are set outside the fire season when conditions are not conducive to fire spread, in vegetation types with low flammability (i.e., not in black spruce forest), or in locations where fires are immediately suppressed (DeWilde and Chapin 2006). Humans also alter the fire regime via suppression which decreases total area burned; however, its exact effectiveness is still under much debate (Cumming 2005; Johnson et al. 2001; Miyanishi and Johnson 2001; Ward et al. 2001) despite the enormous expenditure on fire fighting. Lightning-ignited wildfires occur generally from mid-June, when convective thunderstorm activity is frequent, until either late summer when rain or fall snow showers arrive; thus, a delay in summer rains leads to a larger annual area burned. Owing to the highly flammable nature of black spruce, its dominance in the forest increases fire initiation (Krawchuk et al. 2006) as well as fire frequency and total area burned (Rupp et al. 2002). Several modeling approaches predict an increase in area burned in the near future in response to general climate warming in Alaska (Bachelet et al. 2005; Balshi et al. 2009) as well as nearby Alberta, Canada (Tymstra et al. 2007). If future warming recreates conditions analogous to the medieval warming period, higher fire activity can be expected (Yalcin et al. 2006). A decrease in fire return interval to less than 25 years significantly ª 2010 The Author Journal Compilation ª 2010 Blackwell Publishing Ltd


76 Climate change in Alaska

reduced conifer replacement in north-western Canada resulting in an aspen-dominated forest (Johnstone and Chapin 2006). As a deciduous forest is much less flammable than a spruce forest, this would likely reduce future fire frequency and size. There is much controversy in the boreal literature, if long-term fire suppression eventually leads to a more flammable landscape and thus more frequent, more severe, and larger fires; something currently happen in much of the (lower) Western United States. Humans are conducting a giant experiment without a control: for example, the forests surrounding Fairbanks were extensively logged during the gold rush of the early 20th century and have been recovering ever since; in 1986, a fire suppression policy was instituted in Alaska that classified all land areas into one of four fire management designations. The fire suppression designation dictates how rapidly and intensively a fire is fought; while climate change has been impacting climate patterns and vegetation growth and thus indirectly the fire regime probably since 1900. Thus it is unclear, what exactly is responsible for the observed increase in area burned in highly protected forests near Fairbanks since 2000 (Calef in preparation). Likewise, it is too early to say if this is just a fluke or a warning sign for what the future holds. Carbon released during a fire contributes atmospheric greenhouse gases and thus accelerates global warming whereas the blackening of the ground leads to increased absorption of sunlight (and thus ground heating). However, the removal of the tree canopy and understory vegetation, accompanied by a switch from dark spruce to snow-covered ground and eventually lighter colored deciduous vegetation, cause a larger fraction of the incoming solar radiation to be reflected (high albedo) rather than absorbed into the ecosystem (Chapin et al. 2008). Measurements of absorbed energy before and after fire differed especially during spring and summer and remained different for three decades (Randerson et al. 2006). When evaluating all these impacts of fires on the climate of the Northern Hemisphere, each large fire had a net local cooling effect for roughly 55 years (Randerson et al. 2006). 3.3

DISTURBANCE BY INSECTS AND DISEASE

Large-scale insect and disease infestations by bark beetles, defoliators, and fungi (for details on species, see Malstro¨m and Raffa 2000) are responsible for extensive stand diebacks in the circumpolar boreal forest. From 1993 to 1998, cumulative insect damage in the Alaskan boreal region alone was approximately 1.6–2 million ha, as much as 30% greater than the 1.5 million ha burnt state-wide (Malstro¨m and Raffa 2000). Multi-year spruce bark beetle (Dendroctonus rufipennis) outbreaks on the Kenai Peninsula from 1992 to 2000 killed 90% of the region’s spruce (National Assessment Synthesis Team 2001; Soja et al. 2007). Recent warming has led to an increase in infestations because (i) drought-stressed trees are more susceptible to attack (Karl et al. 2009); (ii) warmer summers have shortened insect life cycles by as much as 50% (Berg et al. 2006); (iii) milder winters allow more pests to survive; and (iv) a warming climate allows pest species to migrate North thereby expanding their range (Volney and Fleming 2000). Large-scale tree infestations can alter the accumulation and distribution of fuels (McCullough et al. 1998) by leaving behind dead branches and trees, which get more flammable with every passing year. Canadian fire risk modeling therefore classifies the damage left behind by massive infestations as an extreme fire risk (Forestry Canada Fire Danger Group 1992; Stocks 1987). After massive spruce budworm (Choristoneura fumiferana) infestations in western Ontario, Canada, dead and broken branches and tree tops became entangled as they fell thus creating ladders for ground fires which turned them ª 2010 The Author Journal Compilation ª 2010 Blackwell Publishing Ltd


Climate change in Alaska 77

into much more damaging crown fires (Fleming et al. 2002); subsequently, 7.5% of the infested area in dry western Ontario burned within 3–9 years post-infestation (Fleming et al. 2002). 4 Conclusion Unlike many other parts of the globe, the vast boreal forest ecosystem is already in the midst of climate change with observable impacts on vegetation, soils, and disturbance regimes (Soja et al. 2007). All these alterations of the natural system affect carbon deposition versus release and thus have global implications (Chapin et al. 2000). Warming, for example, has led to an earlier arrival of spring thaw and thus has extended the growing season by 2–4 days from 1988 to 2000 (Euskirchen et al. 2006). Longer growing seasons lead to warmer soils which translates into increased amounts of formerly locked-up soil carbon available for decomposition and release. Satellite data suggest an extensive decline in forest productivity (browning) in the circumpolar boreal forest which is in sharp contrast to the greening observed in tundra areas (Bunn et al. 2007). This observed decrease in boreal forest greenness can be attributed to several factors in Alaska (most of which hold true in other boreal forest areas as well): (i) reduced photosynthesis owing to tree drought stress and mortality (especially for treeline white spruce; Barber et al. 2000; Lloyd and Fastie 2002; Wilmking et al. 2004); (ii) more widespread insect and disease infestations as a result of milder winters and trees already being weakened by drought stress (Niemela¨ et al. 2001); and (iii) increases in wildfire frequency and severity as a consequence of warmer and drier summer weather (Kasischke and Turetsky 2006) and potentially following large-scale tree infestations (Stocks 1987). 5 Short Biography Monika Calef is interested in exploring vegetation dynamics using Geographic Information Systems (GIS) and modeling. She studied climate change response of boreal forest vegetation in interior Alaska (paper in Journal of Biogeography) and the human impact on wildfire in Alaska both in terms of ignition (paper in Earth Interactions) and suppression (paper in preparation for International Journal of Wildland Fire). New research interests include interactions of land use change and climate change in the Hudson River valley of eastern New York State. She holds a BA in Geography and German from Augustana College in Sioux Falls, South Dakota; an MS in Environmental Studies from Ohio University, Athens; and a PhD in Environmental Sciences from the University of Virginia. She worked as a Postdoctoral Research Associate at the University of Alaska, Fairbanks, and is currently a faculty member at University at Albany. 6 Note * Correspondence address: Monika P. Calef, Department of Geography and Planning, University at Albany, SUNY, 1400 Washington Avenue, Albany, NY 12222, USA. E-mail: [email protected].

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