The Aleutian Islands are treeless except for some plantations of Spruce on Unalaska. Their principal vegetation types are meadow and heath-shrub communities. In some places thickets of Willow (Salix barclayi) are interspersed with the subalpine meadows. The southern and southern coasts of Alaska have a moderate climate and very heavy precipitation (over 160 in. a year in some places). The vegetation is remarkable chiefly for great coniferous forests, composed of Sitka Spruce (Picea sitchensis), Western White Cedar (Chamaecyparis nootkatensis), Western Hemlocks (Tsuga heterophylla and T. mertensiana), and further south the Lodgepole Pine (Pinus contorta), Fir (Abies amabilis) and Western Arborvitae (Thuja plicata). The trees are in dense stands and grow to a great size. The ground cover is a heavy mat of mosses and there is commonly a thick undergrowth of Oplopanax horridum and species of Vaccinium, Rubus and Menziesia. The forests of the interior are much more open in character, and composed of White Spruce (Picea glauca), Birch (Betula papyrifera), Aspen (Populus tremuloides), Lodgepole Pine (Pinus contorta) and a few other species. In places the forest consists only of Picea glauca, sometimes interspersed with grassy meadows. Fires have been of widespread occurrence in both the distant and the recent past. In general, Pine and Aspen are 'fire trees' and do not reproduce themselves after a single generation following fire. Their abundance and size in relation, to the Spruce in any given stand are, therefore, a rough guide to the recent fire history of the stand. The only timber of much commercial value along the Alaska Highway is on the river flood plains and some of the foothill slopes. White Spruce grows to a height of 100 ft. or more with 18 to 24 in. d.b.h. on the flood plains, and the Alpine Fir and Spruces attain similar dimensions on the foothills.