Increasing Vulnerability of Alaskas Boreal Forest as a Result of Climate Warming and the Changing Fire Regime

Eric S. Kasischke and F. Stuart Chapin III

The boreal region extends across the Earth's land surface between around 50°N and 67°N and covers some 15 million square kilometers. The biomes found in this region include forests, wetlands, peatlands and sub-alpine/alpine tundra. Most of this region has an average annual surface temperature around 0°C, with long, cold winters (average January low temperatures <-30°C) and short, warm summers (average July high temperatures >20°C). This temperature regime has resulted in the formation of permafrost (defined as any soil that has remained below 0°C for more than two years) throughout a significant portion of the boreal region. In turn, the presence of permafrost results in poor site drainage, low tree growth rates, and decreased rates of soil decomposition, leading to the development of deep organic layers lying on top of the ground surface.

A pronounced increase in the rate of summer-time warming (between 0.3 and 0.4°C per decade) began in the early 1960s throughout the western Arctic and Alaska (Chapin et al, 2005). While rates of precipitation have also increased, the rise in available soil water has not been sufficient to offset requirements for evapotranspiration, leading to lower tree growth in some regions due to moisture stress (Barber et al, 2000). Climate warming has also contributed to increases in insect outbreaks (Werner et al, 2006) and area burned as a result oflightning-ignited wildfires throughout the North American boreal forest (Gillett et al, 2004). Most scientists and federal and state land managers recognize that the recent warming in the high northern latitudes is likely to continue for the foreseeable future.

Fire is ubiquitous throughout the boreal region, with most of the burned area resulting from ignitions from lightning strikes (Dissing and Verbyla, 2003). Fire records show that during the 1990s, three million hectares of land surface were affected annually by fires in the boreal regions of North America (Kasischke and Turetsky, 2006), while satellite data show that between 10 and 15 million hectares per year have burned in Russia (Sukhinin et al, 2004) over the past decade. Depending on regional climate, sources of ignition and forest type, fire return intervals range between 50 and 200 years in boreal forests.

Paleoecological studies have provided insights as to how climate change has affected vegetation cover over the 12,000 years since the most recent recession of the Pleistocene ice sheets (Lloyd et al, 2006). The mosaic of forest cover in Interior Alaska — which is dominated by black spruce (Picea mariana), but also contains significant components of white spruce (Picea glauca), quaking aspen (Populus tremuloides), balsam poplar (Populus balsimifera) and paper birch (Betula neoalaskana) — has been present for the past 6000 years. Prior to that (6000 to 10,000 before present (BP)), warmer and slightly drier conditions resulted in white spruce being the dominant tree species. From 10,000 to 12,000 BP, warm and dry conditions resulted in a landscape where black and white spruce were only minor components of the landscape — during this period, vegetation was dominated by deciduous tree stands and shrub and grass-herb dominated ecosystems.

Given the above understanding of how past variations in climate shaped the composition of boreal forests it is fairly certain that the present mosaic of vegetation cover throughout interior Alaska, as elsewhere throughout the Arctic region, will change as temperatures continue to rise during the 21st century. However, a number of factors will influence the rate and direction of change, including variations in patterns of precipitation, changes to the fire regime, the degree to which insect outbreaks occur in different forest types, the rate of soil warming of areas underlain by permafrost, and the patterns of dispersion or human introduction of vegetation species not currently resident in this region. While some of these factors can be lessened through management intervention (for example, fire suppression, control of insects and pathogens, and prevention of the introduction of invasive species), many will remain outside of human control. Slowing the rate of climate change and influencing management strategies are the key priorities of those who depend on the many ecosystem services provided by Alaska's boreal forests, particularly Native Peoples.

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