Climate and Soils

The climate ofthe boreal forest is continental and, importantly, for the growing season, there tends to be between 30 and 150 days of temperatures above 10 °C. Temperature lows can fall below —25 °C. Average annual precipitation is 38-50 cm, with the lowest amounts in the northern boreal forest, and greater frequency of precipitation during the summer season. Water is seldom limiting because of the generally flat topography and low rate of evaporation.

Permafrost can occur in the northern parts of this zone, the southern limit coinciding roughly with a mean air temperature of — 1 °C and snow depth of about 40 cm. The zone of permafrost generally starts at depths ranging from 1.5 to 3 m in the areas of the boreal forest where it occurs. Its occurrence limits soil processes to an upper active layer and impedes water drainage, leading to waterlogged soils. The soil decomposition rate in the taiga is slow, which leads to the accumulation of peat.

Several soil types characterize the boreal forest. The soils of a major part of the boreal forest, under a dense coniferous canopy, are heavily podzolized where the soil is permeable, and so it consists largely of Spodosols. Intense acid leaching forms a light ash-colored eluvial soil horizon leached of most base-forming cations such as calcium. Thus taiga soils tend to be nutrient poor. Gelisols are common in the north, where permafrost occurs. These are young soils with little profile development. Histosols, which are high in organic matter, form in non-permafrost wetlands, where decomposition is slowed by hypoxic conditions. These are often referred to as peatlands.

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