The Brooks Range forms the northernmost drainage divide in North America, separating streams flowing into the Arctic Ocean and the North Pacific. The range roughly delineates the summer position of the Arctic front. It represents the northern extent of the tree line, with little beyond isolated balsam poplar stands occurring north of the continental drainage divide. Trembling aspen and white spruce also occur north of the Brooks Range, though they are limited to sites that have been disturbed by human activity. Southern slopes have some cover of black spruce, Picea mariana, marking the northern limit of those trees. As the global mean temperature increases, tree line has been observed to move further north, changing the boundaries of where these trees are found. An increase in shrub abundance is also being experienced in areas which were previously dominated by tundra, impacting the ecology of the area.