Peter Scott

Studies of northern circumpolar tree line have related tree distribution to numerous environmental controls, such as permafrost, moisture, heat, radiation, and frontal circulation patterns (Barry, 1967; Bryson, 1966; Hare and Ritchie, 1972; Hustich, 1966; Larsen, 1989). Recent modeling experiments indicate that the circumpolar tree line could be shifted north tens to hundreds of kilometers if anticipated magnitudes of global warming due to increases in atmospheric greenhouse gasses are realized (e.g., Emmanuel et al., 1985; Rizzo and Wiken, 1992). Detecting and migitating the negative impacts of such vegetation shifts requires an understanding of the dynamics of tree line-climate interrelations. In this chapter we review some of the data and current concepts regarding climate-vegetation dynamics at the circumpolar tree line. In an effort to evaluate the mechanisms responsible for forest development we review selected northern European and North American studies of northern forests, the tree line, and forest-tundra, and evaluate the dominant influences and processes of forest development at the forest edge. We synthesize this information to suggest how the tree line might react in real terms to future climate warming.

Annual radial increment throughout the stem, and height increment of individual white spruce trees at Churchill, Manitoba were reconstructed through measurement of ring widths on sections taken at close intervals throughout the stem. For the period around the eruption of Tambora in 1815, four trees each from open-forest and forest-tundra sites provided data. On each site, one tree was less than 1 m in height in 1816, the others ranging from 3 to 6 m. Growth of the larger trees, as indicated by height and radial increment, was generally declining over the two decades prior to 1816. In the upper stem, particularly of the forest-tundra trees, radial increment was least in 1818. Effects were less severe in the lower stem and recovery in open-forest trees had begun in 1818 after a low in 1817. Net-height gain of the forest-tundra trees during 1816-20 was one-third that of the previous five years, whereas in open forest trees it more than tripled relative to reduced growth in the previous five years. In combination with the radial-increment data, this suggests the occurrence of conditions in 1816, or possibly late summer of 1815, that led to damage of the terminal bud and upper crown with loss of foliage and (or) reduction of foliar efficiency and production of new foliage. Such effects were much less severe on open forest trees. The decline in overall tree growth was statistically significant in 1817-18 compared with the variability in tree growth for 10 years prior to 1815. Comparisons made with the period around 1835 (eruption of Coseguina) show subsequent growth reductions were greater than after Tambora.