<P> On coasts and isolated mountains the tree line is often much lower than in corresponding altitudes inland and in larger, more complex mountain systems, because strong winds reduce tree growth . In addition the lack of suitable soil, such as along talus slopes or exposed rock formations, prevents trees from gaining an adequate foothold and exposes them to drought and sun . </P> <P> The arctic tree line is the northernmost latitude in the Northern Hemisphere where trees can grow; farther north, it is too cold all year round to sustain trees . Extremely cold temperatures, especially when prolonged, can freeze the internal sap of trees, killing them . In addition, permafrost in the soil can prevent trees from getting their roots deep enough for the necessary structural support . </P> <P> Unlike alpine timberlines, the northern timberline occurs at low elevations . The arctic forest--tundra transition zone in northwestern Canada varies in width, perhaps averaging 145 kilometres (90 mi) and widening markedly from west to east, in contrast with the telescoped alpine timberlines . North of the arctic timberline lies the low - growing tundra, and southwards lies the boreal forest . </P> <P> Two zones can be distinguished in the arctic timberline: a forest--tundra zone of scattered patches of krummholz or stunted trees, with larger trees along rivers and on sheltered sites set in a matrix of tundra; and "open boreal forest" or "lichen woodland", consisting of open groves of erect trees underlain by carpet of Cladonia spp . lichens . The proportion of trees to lichen mat increases southwards towards the "forest line", where trees cover 50 percent or more of the landscape . </P>

Which biome is found on the mountain above the tree line