Crown Forms and Shoot Elongation of White Spruce at the Treeline, Churchill, Manitoba, Canada

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Abstract
Five crown forms of treeline white spruce are used to define open forest, forest-tundra, and tundra regions at Churchill, Manitoba, Canada. Shoot elongation of 25-, 50-, and 100-yr-old open forest trees was studied on a transect through an area of forest succession during 1983. During 1984, shoot elongation of different crown forms was examined relative to ground temperatures, air temperatures, and precipitation. Stem and branch elongation of open forest trees was greatest and occurred over the longest period of time, followed by forest-tundra trees, and tundra trees. The increasing density of spruce in the zone of invading open forest is associated with moss development that moderates the ground temperature, increasing the lower root zone temperature early in the growing season and maintaining lower temperatures in the upper root zone later in the season. Continued moss accumulation coincides with permafrost formation into the root base. On the forest-tundra and tundra, moss development is patchy. Early warming of the peat surface results in drying that impedes heat influx to the lower root zone. The temperatures in the lower root zone appear limiting to forest-tundra trees and restricting to tundra trees at the start of the growing season. Shoot elongation is over before the upper root zone becomes very hot. Treeline crown forms are grouped according to similar patterns of elongation and common types of development. The wind-abraded crown forms are caused by slow growth that leaves a tree exposed to wind effects for a longer period of time.