Microbial Processes in the Alaskan Boreal Forest

Forest ecosystems typically occur in moderate environments where growing season rainfall is adequate to support tree growth and where nongrowing season conditions are not too extreme. The Alaskan boreal forests, however, occur at the limit of the forest biome, in an environment that is climatically extreme, with strong physical gradients. The seasonal variation in temperature is among the greatest on earth, with winter temperatures as low as –50ºC and summer growing season temperatures that can reach +30ºC (Chapter 4). The growing season is short, the climate is semi-arid, and growing season rainfall is limited. Forests exist in the region because evapotranspiration is also limited. Steep south-facing slopes can be too dry to support tree growth (Chapter 6). In contrast, in flat, low-lying areas, low evapotranspiration combined with permafrost produces wetlands despite the low rainfall. Regular drought makes the forest highly susceptible to fires. At large scales (many square kilometers), the boreal forest experiences regular, extensive fires that destroy whole stands, resetting succession (Chapter 17). This regular fire cycle produces a patchwork mosaic of forest stands in different successional stages across the landscape (Dyrness et al. 1986, Kasischke and Stocks 2000; Chapter 7). In large rivers (e.g., the Tanana), the cutting and filling of meander loops washes away some forest stands while depositing new silt bars for colonization and succession (Zasada 1986). At the landscape scale, the biogeochemical cycles in the boreal forest are therefore dominated by landscape structure (e.g., dry uplands vs. wet lowlands) and by disturbance (particularly fire). At smaller scales, however, the strong feedbacks between plant and soil processes control much of the functioning of individual forest stands, and possibly the rate of transition among successional stages. In this chapter, we discuss how microbial processes in the boreal forest produce unusual patterns of nutrient cycling that drive the overall functioning of boreal forest stands. Figure 14.1 illustrates the linkages between plant and microbial communities that dominate the functioning of the boreal forest soil system. In the feedbacks between plant and soil processes, plants drive the loop largely through inputs of organic materials.