overlaps tundra biome subpolar coniferous forest biome polar tree-line ecotone An ecotone which bridges a subpolar coniferous forest biome and a tundra biome. Arctic tree-line ecotone This assumes, of course, that the planet's or planetoid's poles receive less stellar radiation and/or thermal energy than equatorial regions. Major vegetational ecotones such as that between the northern edge of the Boreal Forest in North America (or taiga in Russia) and the arctic tundra, known as the arctic tree-line ecotone, are controlled largely by large-scale airmass contrasts and are sensitive to climatic change, as noted by R. A. Bryson (1966). Harvey Nichols, Ph.D. , April 23, 2017 Emeritus Professor of Biology. tree line ecotone