Vegetation and biome types in the Arctic are expected to shift or overtake each other in drastic ways. Tundra is the vegetation type expected to be the highly sensitive and vulnerable to future changes in the climate. Part of the rationale for this is that rising temperatures are expected to favor taller and denser types of vegetation, giving an advantage to forests in overtaking tundra (3). Shrinking of the tundra is expected to decrease its size to the smallest it has been in the past 21,000 years. Projections indicate that the overall losses of current tundra areas will be 40-57%, and only a 5% increase in tundra area due to colonization of new areas (14). Out of the sub-types of tundra, the prostrate dwarf-shrub tundra is predicted to have the greatest losses (60% of current habitat). This is related to the expected tendency for tundra biomes to have a shift in shrub species, with taller shrubs having a growing advantage in a warming climate (9).

Insect outbreaks are projected to cause greater damage to large areas of forest, which would continue a trend that has already been witnessed, such as the increased mortality caused in trees by spruce bark beetle outbreaks. Earlier arrival of spring temperatures without extra moisture input also causes problems for trees such as black spruce, as their needles will dry out without having extra incoming moisture. Due to this and other factors, the extent of area affected by, and the severity of, forest fires is expected to increase, which will add more pressure to old growth forests (6).

For the 21 st century, forest extent in the Arctic is expected to increase approximately 3 x 10^6 square km or an increase of 55% of over its present area (9). The forests are also likely to have an increase in canopy density and faster nutrient cycling due to the warmer climate, and these changes will likely “lead to increased water use and the development of more complex multi-layered tree and shrub canopies” (9). The treeline has been modeled to shift the northern limit of trees by 400 km during the 21 st century, which will outpace dispersion rates. In fact, in all but the coldest climate prediction scenario, evergreen needle-leaf forests will become the dominant biome type in the Arctic. Concurrent with this, tree species which are adapted for the coldest temperatures, such as Larix spp. in eastern Siberia are likely to decrease in importance, and the boreal forest mix will have greater numbers of cool-temperate forest trees (9).