The world’s largest terrestrial biome, consisting mainly of evergreen coniferous forests (chiefly pine, fir, and spruce), which occurs across subarctic North America and Eurasia and at high altitudes in temperate regions. In certain parts, such as northeastern Siberia, deciduous conifers and broadleaved trees, such as larch and birch, are dominant. Over the northermost taiga the ground is permanently frozen within about one metre of the surface, which prevents water from filtering down to deeper levels in the soil. This means that bogs may form in depressions. For at least six months of the year temperatures are below freezing but there is a short growing season lasting 3–5 months. Where average annual temperatures are higher, sunlight may melt the frozen soil during the summer. The soil in taiga areas is acidic and infertile. Compare tundra.