the Pacific Ocean coasts of North America and Asia), into coniferous temperate rainforests where oak and hornbeam appear and join the conifers, birch and Populus tremula. Glaciation The area currently classified as taiga in Europe and North America (except Alaska) was recently glaciated. As the glaciers receded they left depressions in the topography that have since filled with water, creating lakes and bogs (especially muskeg soil) found throughout the taiga. Soils Taiga soil tends to be young and poor in nutrients, lacking the deep, organically enriched profile present in temperate deciduous forests. The colder climate hinders development of soil, and the ease with which plants can use its nutrients. The relative lack of deciduous trees, which drop huge volumes of leaves annually, and grazing animals, which contribute significant manure, are also factors. The diversity of soil organisms in the boreal forest is high, comparable to the tropical rainforest. Fallen leaves and moss can remain on the forest floor for a long time in the cool, moist climate, which limits their organic contribution to the soil. Acids from evergreen needles further leach the soil, creating spodosol, also known as podzol, and the acidic forest floor often has only lichens and some mosses growing on it. In clearings in the forest and in areas with more boreal deciduous trees, there are more herbs and berries growing, and soils are consequently deeper. Flora Since North America and Eurasia were originally connected by the Bering land bridge, a number of animal and plant species (more animals than plants) were able to colonize both land masses, and are globally-distributed throughout the taiga biome (see Circumboreal Region). Others differ regionally (typically with each genus having several distinct species), each occupying different regions of the taiga. Taigas also have some small-leaved deciduous trees, like birch, alder, willow, and poplar; these grow mostly in areas further south of the most extreme winter weather. However, the Dahurian larch tolerates the coldest winters of the Northern Hemisphere, in eastern Siberia. The very southernmost parts of the taiga may have trees such as oak, maple, elm and lime scattered among the conifers, and there is usually a gradual transition into a temperate, mixed forest, such as the eastern forest-boreal transition of eastern Canada. In the interior of the continents, with the driest climates, the boreal forests might grade into temperate grassland. There are two major types of taiga. The southern part