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As a temperature-delineated boundary, Arctic treeline is predicted to shift poleward and tree growth is expected to increase in response to rapid warming. The massive scale of the Arctic treeline magnifies these changes to impact energy balance, carbon balance, and climate-related feedbacks at local, regional, and global scales. Yet, not all sections of the Arctic treeline are reporting growth, suggesting factors other than temperature may be becoming more limiting as the climate continues to change. This dissertation investigates how water availability and tree size may modify the response to climate change of a dominant conifer species (white spruce, Picea glauca) growing at an Arctic treeline site in the Brooks Range, Alaska, USA. The first chapter examines the influence of temperature and water availability on population regeneration and individual tree growth during the 20th century. A climatic shift towards a warmer and drier climate after 1975 caused divergent responses of sapling regeneration and mature tree growth, suggesting that, while individuals have grown, this section of treeline has remained relatively stationary. The second chapter explores the present-day relationships between tree size, temperature, moisture availability, and tree growth by examining the response of intra-annual radial stem growth rate to changing environmental conditions at the Arctic treeline. Tree size and water availability play important roles in moderating the growth response to increasing temperature. Finally, in the third chapter, the environmental cues which trigger the onset of radial stem growth in spring are identified. The results suggest a combination of winter chilling and subsequent spring heat accumulation initiates onset, like trees growing at lower latitudes. However, the chilling and heating thresholds at this Arctic treeline site were far colder than those identified at lower latitudes, suggesting local adaptation to harsh Arctic winters and springs. Through these new findings, this dissertation advances our understanding of Arctic treeline dynamics and will help to predict the future of the Arctic treeline more accurately in a rapidly changing climate.

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The main goal of this thesis was to improve our understanding of the long-term effects of establishment treatments and climate change on lodgepole pine and white spruce growth in the western boreal forests. My dissertation also investigated the combined effects of climate and competition on white spruce and trembling aspen growth in boreal mixtures. In the first part of the thesis I evaluated the effects of site preparation treatments on growth of lodgepole pine and white spruce in north-eastern British Columbia. Results indicate that mechanical site preparation can provide yield gains of up-to 10 percent for pine and spruce at 60 and 80 years, respectively. These stands are showing a Type 1 growth response which implies that the treatment effect will eventually cease 90-100 years after planting. In the second part of the thesis I explored pine and spruce growth in relation to past climate and site preparation. Results indicate that up-to 45% and 37% of the respective variation in spruce and pine growth can be explained by selected climatic variables. Future projections indicated that height growth of young pine plantations in the sub-boreal zone could benefit (in the short term) from longer growing seasons by up-to 12% on untreated stands. Untreated young spruce plantations in the boreal zone may suffer height growth decreases of up-to 10% due to increased drought-stress. Vegetation control and mechanical site preparation treatments appear to mitigate effects of climate change to some extent. In the third part of the thesis I explored the combined effects of climate and trembling aspen competition on spruce and aspen growth using data from a long-term study in the boreal zone. Results indicate that climate variables and initial size of the tree can account for significant portions of the annual growth of spruce. Including an estimate of aspen competition in the equations improved the predictive ability of these models. Evidence of the inter-annual variability in aspen competitiveness on spruce and aspen growth indicates that the stress-gradient hypothesis can be applied in boreal mixedwood forests. / Forest Biology and Management

Site preparation treatments are often used prior to the planting of clearcut forest lands to improve planter access and to increase the number and quality of planting spots. Most mechanical site preparation treatments alter the configuration and material composition of surface soil materials, and can have marked effects on soil properties important to seedling survival and growth. Effects of some of these treatments on soil moisture, soil temperature, rates of nitrogen mineralization, and the establishment of Picea glauca x engelmannii seedlings were examined on fresh, moist, and wet sites in the moist cold subzone of the Sub-boreal Spruce Zone in west-central British Columbia. Four types of microsite alteration were investigated: forest floor removal (spot scalping), soil mounds over inverted sections of forest floor (inverted mounds), mineral soil mounds over a mineral soil surface, and inversion of the forest floor and mineral soil in place. Soil temperature was monitored continuously and soil moisture weekly at the 10-cm depth in 16 combinations of site and microsite treatment during two growing seasons. The response of seedling height and diameter growth was monitored for three growing seasons. Effects of altering soil temperatures through mechanical treatments on rates of nitrogen mineralization were examined by incubating a standard soil material in a range of microsites created by six combinations of site and mechanical treatment. Effects of substrate quality and soil temperature on rates of nitrogen mineralization were examined in paired mounded and untreated spots in fresh, moist, and wet sites. In all sites, early growing season soil temperatures in the seedling rooting zone were substantially warmer in inverted mounds than in other treatments. Spot scalping increased temperatures slightly relative to controls in the fresh site, but had little or no warming effect on moist and wet sites. Inverted mounds became substantially drier than other treatments during periods of low rainfall, particularly in the fresh site. After three growing seasons, seedling height growth was greatest in inverted mounds, irrespective of site. Amounts of nitrogen mineralized in a standard soil material during incubation for 77 days in the field were significantly greater for samples placed in inverted mounds than for those placed in other microsite treatments. There was a significant positive correlation between amounts of nitrogen mineralized during field incubations and degree hour sums calculated for associated microsite treatments and sites. Both substrate quality and soil thermal regime affected rates of N mineralization in samples from paired mounded and untreated spots, and an interaction was observed between the two factors. / Graduation date: 1991

White spruce (<i>Picea glauca</i> (Moench) Voss), black spruce (<i>P. mariana</i> (Mill.) BSP), and jack pine (<i>Pinus banksiana</i> Lamb.) are dominant conifer trees within the boreal forest. Rising CO2 concentrations may create hotter and drier conditions in the Southern Boreal Forest of Canada, and have negative impacts on germination and regeneration of conifers. Conifers vary in their germination requirements and may have different responses to climate change. Experiments were conducted to access the germination potential, variability among collections, and to predict the ability of these conifers to germinate under future climatic conditions. Twelve collections of white spruce and black spruce and ten collections of jack pine seeds were collected from the Boreal Plain Ecozone of Saskatchewan. Seeds of all collections varied in their dormancy characteristics and dormancy breaking requirements because no single stratification or light treatment stimulated germination in all three species. Seed dormancy was greatest in white spruce and least in black spruce. Germination tests at 5, 10, 12.5, 15, 17.5, 20, 25, 30, and 35°C were used to develop thermal time models. Each species had unique temperatures for optimal germination ranging from 20°C in white spruce, 20-25°C in black spruce, and 25-30°C in jack pine. The speed of germination under similar temperature regimes was fastest for jack pine, intermediate for black spruce, and slowest for white spruce. The base temperature for white spruce decreased (r=0.63, P=0.03) with increasing June precipitation while that of jack pine tended to increase with latitude (r=0.60, P=0.07) and April precipitation (r=0.58, P=0.08). No environmental variables correlated with germination of black spruce. The Canadian Global Climate Model, version 2, with emission scenarios predicted future temperature and precipitation at the sites where seeds were collected. Using the base temperature for germination as a guideline, temperatures suitable for germination in the spring are predicted to advance by a few weeks to a month and a half earlier with increased concentrations of CO2. Moisture availability may, however, control seed germination at these sites. Overall, jack pine and black spruce might better adapt to increasing temperature because of their high germination temperatures (>30°C). Variation in most germination parameters existed among collections, suggesting this variability can be used to select seed sources for reforestation or assisted migration in a changing climate.

White spruce (<i>Picea glauca</i> (Moench) Voss), black spruce (<i>P. mariana</i> (Mill.) BSP), and jack pine (<i>Pinus banksiana</i> Lamb.) are dominant conifer trees within the boreal forest. Rising CO2 concentrations may create hotter and drier conditions in the Southern Boreal Forest of Canada, and have negative impacts on germination and regeneration of conifers. Conifers vary in their germination requirements and may have different responses to climate change. Experiments were conducted to access the germination potential, variability among collections, and to predict the ability of these conifers to germinate under future climatic conditions. Twelve collections of white spruce and black spruce and ten collections of jack pine seeds were collected from the Boreal Plain Ecozone of Saskatchewan. Seeds of all collections varied in their dormancy characteristics and dormancy breaking requirements because no single stratification or light treatment stimulated germination in all three species. Seed dormancy was greatest in white spruce and least in black spruce. Germination tests at 5, 10, 12.5, 15, 17.5, 20, 25, 30, and 35°C were used to develop thermal time models. Each species had unique temperatures for optimal germination ranging from 20°C in white spruce, 20-25°C in black spruce, and 25-30°C in jack pine. The speed of germination under similar temperature regimes was fastest for jack pine, intermediate for black spruce, and slowest for white spruce. The base temperature for white spruce decreased (r=0.63, P=0.03) with increasing June precipitation while that of jack pine tended to increase with latitude (r=0.60, P=0.07) and April precipitation (r=0.58, P=0.08). No environmental variables correlated with germination of black spruce. The Canadian Global Climate Model, version 2, with emission scenarios predicted future temperature and precipitation at the sites where seeds were collected. Using the base temperature for germination as a guideline, temperatures suitable for germination in the spring are predicted to advance by a few weeks to a month and a half earlier with increased concentrations of CO2. Moisture availability may, however, control seed germination at these sites. Overall, jack pine and black spruce might better adapt to increasing temperature because of their high germination temperatures (>30°C). Variation in most germination parameters existed among collections, suggesting this variability can be used to select seed sources for reforestation or assisted migration in a changing climate.

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No description available.

Knowledge of optimum concentrations and ratios of major nutrients in tree foliage is required to assess the nutrient status of the boreal forest. This thesis aimed at determining foliar nutritional standards for white spruce (Picea glauca (Moench) Voss), black spruce (Picea mariana Mill. B.S.P.) and jack pine (Pinus banksiana Lamb.) and to assess the nutrient status of forest stands across Canada for all major nutrients (N, P, K, Ca, Mg and Mn) using critical values (CVA) and compositional nutrient diagnosis (CND). Trees were sampled at three locations in Ontario and Quebec to cover a gradient of soil fertility levels. A boundary line approach was used in combination with quadratic regression models to assess the relationship between growth and foliar nutrient concentrations or CND scores when free of the effects of interacting environmental factors. (Abstract shortened by UMI.)