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.[6][7] Southern slopes have some cover of black spruce, Picea mariana, marking the northern limit of those trees.[8] As the global mean temperature increases, tree line has been observed to move further north, changing the boundaries of where these trees are found.[9] An increase in shrub abundance is also being experienced in areas which were previously dominated by tundra, impacting the ecology of the area[10][11].