Alps are mainly composed of four kinds of conifers: spruce, larch, Swiss pine, and mountain pine. The spruce forest is found mainly in the lower part of the subalpine zone on siliceous or sometimes calcareous subsoil when the layer of humus is sufficiently thick and acidic. The flora of the spruce forests is limited, except for mosses. The most characteristic species are Oxalis acetosella, Moneses uniflora, Lycopodium annotinum, Listera cordata and two kinds of berries: Vaccinium myrtillus (European blueberry) and Vaccinium vitis-idaea (lingonberry). The Swiss pine and larch forests are found at higher elevations, and are more open, which allows a much more varied flora to thrive despite harsher climatic conditions. Larch has loses its needles in the cold season, which allows it to resist snow loads well. The Swiss pine keeps its needles but also resists the weight of the snow well because its branches are short and robust; it is particularly resistant to bad weather because it grows the highest in altitude, in inhospitable areas where no other tree can compete with it. Because of these very harsh living conditions, its growth is slow. Other plants in these forests include Homogyne alpina, Melampyrum sylvaticum, and the rare Linnaea borealis. The mountain pine grows on thin soils which are not suitable for the other species. There are two subspecies: one that grows in the Western Alps and is commonly called hooked pine (Pinus mugo ssp. uncinata) and another in the Eastern Alps that is called reclining pine (Pinus mugo ssp. mugo). Both colonize steep slopes and scree and are accompanied by the winter heath (Erica carnea), Polygala chamaebuxus, Daphne striata, and alpine clematis (Clematis alpina). Alpine zone The Alps are eponymous of the alpine climate typical between the tree line up to the permanent snow line, roughly between . Rocks The plants growing on the rocks must find their nutrients in the little soil accumulated in the cracks. In general, rocks are a very hostile environment for plants, with very large temperature differences and strong winds. The plants that grow there are forced to adapt in order to survive: they thus develop long and strong rootsto better resist the wind and to extract water from the dry soil. They tend to have leathery or waxy leaves to slow down transpiration. In addition, the chemical composition of the rock greatly influences the flora: the flowers of limestone rocks will be