Global climate change affects all areas of the Earth, with some regions experiencing greater impacts than others. In particular, the Arctic treeline—that is, the southern limit of the Arctic Bioclimate Zone—is being affected to various degrees by the present-day shifts in global climate patterns. Bioclimate (or bioclimatic) zones are broad areas of vegetation that correspond to mean annual temperatures at various latitudes and altitudes. Located predominantly between 60° and 70° north latitude, and stretching across North America from northern Alaska to the coast of Labrador, and across Eurasia from Scandinavia to eastern Siberia, the Arctic treeline forms the biogeographic boundary between the boreal forests to the south (dominated by coniferous trees) and tundra vegetation to the north (dominated by small shrubs, mosses, lichens, and bare ground). The Arctic treeline is not a sharply delineated boundary; instead, it is a transitional zone in which continuous forest cover gives way to increasingly scattered fragments of forest and increasingly smaller individual trees northward. See also: Arctic Circle; Arctic Ocean; Climate modification; Forest; Global climate change; Global warming; Plant geography; Tree; Tree growth; Tundra; Vegetation and ecosystem mapping

The circumpolar Arctic treeline map, circling through the northern regions of North America and Eurasia for approximately 13,350 km (8300 mi). The treeline border (the boundary line between the red and white areas on the map) marks a transitional zone in which continuous forest cover gives way to increasingly scattered fragments of forest and increasingly smaller individual trees northward. In general, the forest–tundra transition is being affected by global climate change. The region beyond the treeline is shown in red; the Arctic Ocean is shown in light blue; and non-Arctic areas are shown in white. (Credit: U.S. Fish and Wildlife Service)

Trees are not capable of growing in areas north of the Arctic treeline. The climate is too harsh and the prolonged cold temperatures freeze the internal sap of trees [typically, trees are viable only in locations having a mean growing-season temperature above 6.4°C (43.5°F)], making growth impossible. In addition, the summers are too cool and too short in these regions for adequate photosynthesis to occur, so proper tree growth and reproduction cannot take place. Another factor is that permafrost in the soil prevents tree roots from attaining enough depth to provide the required structural support. See also: Permafrost; Photosynthesis; Physiological ecology (plant); Root (botany); Temperature

Presently, in upper latitudes of the Northern Hemisphere, the climate is undergoing dramatic shifts, including increasing temperatures that are warming more than two times quicker than the global average. Along with shifts in temperature, there are concomitant shifts in the vegetation of these areas. In some cases, the treeline is advancing northward, and the tundra is gradually disappearing, with vegetation becoming greener and shrubbier. However, in other instances, trees are being eliminated because of another effect of global warming: namely, as temperatures rise, the extra heat is drying up these areas, creating opportunities for invasive insects and forest fires to destroy trees. In fact, researchers have observed that the Arctic treeline is moving northward in some areas and southward in other areas, depending on environmental conditions in those areas. Still, the overall trend favors the advancing northern movement of the treeline, with some investigators predicting that 50% of the current tundra could be converted to greener vegetation by the year 2100. See also: Forest fire; Invasion ecology