What is the taiga, and why is the northern ring of forest all needles?

The taiga is the great ring of conifer forest around the top of the world, and it is made of needles because evergreen needles suit a short, cold summer. They lose little water, and since the tree keeps them, it can photosynthesize with old needles in late winter and spring, when the light is good but it is still too cold to grow new leaves. Narrow, conical crowns with drooping limbs shed the snow. Taiga, also called boreal forest or snow forest, is a biome of coniferous forests made up mostly of pines, spruces and larches. It covers 17 million square kilometres, 11.5 percent of Earth's land, with the largest areas in Russia and Canada.

Life here is hard. After ice caps and tundra, taiga has the lowest average temperatures of any land biome, generally between minus 5 and plus 5 degrees Celsius, and snow may lie for as long as nine months in the northernmost parts. Trees must grow rapidly in the short summer, when the days are long. Some sources give 130 days as a typical growing season. Very few species cope: spruce, fir, pine and the deciduous larch. The Dahurian larch of eastern Siberia tolerates the coldest winters of the Northern Hemisphere.

The word has a human story. Taiga is Russian, and it is believed to come from Turkic or Mongolic roots, where related words mean wooded or uninhabited forest lands.

The forest is big, and its carbon is bigger. Primary boreal forests are reported to hold 1,042 billion tonnes of carbon, more than currently found in the atmosphere. And the north is warming: in the last quarter of the twentieth century, the zone the boreal forest occupies saw some of the greatest temperature increases on Earth. In Siberia the taiga is converting from needle-shedding larch to evergreen conifers, which may accelerate warming, because evergreens absorb more of the Sun's rays.