Peter A. Scott

Establishment of white spruce populations and responses to climatic change at the treeline, Churchill, Manitoba, Canada

Studies on recruitment of white spruce from 1785 to present document the open forest succession at Churchill, Manitoba. Advance of trees onto land emerging from Hudson Bay due to isostatic uplift and permafrost intrusion has resulted in the development of two distinct systems. Open forest and forest-tundra are distinguished by seedling establishment, crown forms, and the growth patterns of the trees. Forest-tundra plots which demonstrate transition to open forest are documented. Where the two systems were established prior to the major climatic warming, they are resistant to invasion and there is little resultant change in the position of the treeline. Once started, population growth within the open forest system tends to be self generating and is little affected by subsequent climatic cooling. The mature open forest established prior to 1800 has shown a decline in seedling establishment during the climatic warming. These sites are characterized by extensive lichen cover.

CROWN FORMS AND SHOOT ELONGATION OF WHIlE SPRUCE AT IHE TREELINE, CHURCHILL, MANITOBA, CANADA

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-yrold 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 foresttundra 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.

Large-Scale Changes in Atmospheric Circulation Interpreted From Patterns of Tree Growth at Churchill, Manitoba, Canada

Growth layer analysis involving 11 treeline white spruce (Picea glauca) and two tamarack (Larix laricina) from Churchill, Manitoba, Canada, produced a data set with some 500,000 tree rings. A summary of these data is made using knowledge of tree growth and weather stresses. The growth trends conform well to Northern Hemisphere and Arctic temperature data sets and poorly to local temperatures. The summer position of the Arctic Front is close to the treeline and some periods of tree growth are dominated by arctic conditions while others are dominated by more temperate conditions. The period 1760 to 1820 shows intermittent dominance of temperate conditions while the period 1921 to 1970 shows a complete dominance of temperate conditions. All other years from 1715 to 1982 show a dominance of arctic conditions. Under the cool Arctic air mass the trends longer than 8 yr are suppressed and poorly defined relative to the trends in hemispheric temperature data. The presence of the Arctic Front to the north of Churc...