To understand the temporal dynamics of a forest, long-term direct observations are required. My study examined the long-term persistence of trembling aspen (Populus tremuloides Michx) and white spruce (Picea glauca (Moench) Voss) in the boreal mixed-wood forests of Riding Mountain National Park. A set of 266, disturbance-free, permanent sample plots were established in 1947 (stand age = 120 years) and followed through time for 55 years. My results indicate that although the density and basal areas of aspen do decline over the 55-year period, a successful regeneration and establishment occurs around 140 years. The long-term persistence of aspen is a result of clonal reproduction following the canopy breakup beginning around 130 years or earlier. This implies that the long-term persistence of both aspen and spruce occur and the expected succession to softwood dominance does not occur.

To understand the temporal dynamics of a forest, long-term direct observations are required. My study examined the long-term persistence of trembling aspen (Populus tremuloides Michx) and white spruce (Picea glauca (Moench) Voss) in the boreal mixed-wood forests of Riding Mountain National Park. A set of 266, disturbance-free, permanent sample plots were established in 1947 (stand age = 120 years) and followed through time for 55 years. My results indicate that although the density and basal areas of aspen do decline over the 55-year period, a successful regeneration and establishment occurs around 140 years. The long-term persistence of aspen is a result of clonal reproduction following the canopy breakup beginning around 130 years or earlier. This implies that the long-term persistence of both aspen and spruce occur and the expected succession to softwood dominance does not occur.

In this thesis I examined the impacts of variable retention harvesting on residual tree mortality and natural regeneration of white spruce [Picea glauca (Moench (Voss)] in northern Alberta. The VR was done in four overstory canopy compositions (ranging from deciduous dominated to conifer dominated) and at six rates of canopy retention (2%, 10%, 20%, 50%, 75% and 100%). After 10 years there was 32.9 % mortality of aspen (Populus tremuloides Michx.) and 16.9 % mortality of spruce in the VR cuts. Mortality of individual trees was greater with low density of trees, in the conifer stands and for trees with short live crowns, which are large and trees near machine corridors. Natural regeneration of spruce was greatest with higher availability of seed trees (>30 ha-1) and on machine corridors, where stocking reached 74%. By contrast, stocking was 14% on retention strips, when seed tree density was 11 seed trees ha-1. / Forest Biology and Management

Two aspects of drought resistance were investigated on wet and dry ecotypes of three conifer species: 1) the relative importance of drought avoidance and drought tolerance mechanisms in resisting drought stress was assessed on Douglas-fir (Pseudotsuga menzieseii) and lodgepole pine (Pinus contorta) seedlings, and 2) the effects of drought on root hydraulic conductance and low temperature, on root water flow rates Were assessed on first-year seedlings of Douglas-fir, lodgepole pine and white spruce (Picea glauca). To study drought avoidance, Douglas-fir and lodgepole pine seedlings were grown in sealed containers in wet (522% water content) or dry (318% water content) peat/vermiculite soil in a factorial treatment design. Dry weights, water use, and root length were determined for seedlings at each of five harvests and stomatal conductance and shoot water potentials were measured during the last 12 weeks of the experiment. Lodgepole pine seedlings had greater dry matter production, water use, stomatal conductance and new root length than Douglas-fir seedlings. New root weight of lodgepole pine seedlings exceeded that of Douglas-fir seedlings during the last five weeks of the experiment, and specific root length of new roots was higher for lodgepole pine seedlings throughout the experiment. Douglas-fir seedlings showed higher water use efficiency (WUE) than lodgepole pine seedlings, although water uptake rates per unit of root dry weight showed little difference between species. Soil water treatment influenced specific root length of new roots, water uptake per unit of new root length, and WUE in Douglas-fir seedlings more than in lodgepole pine seedlings. To study drought tolerance, Douglas-fir and lodgepole pine seedlings were grown under drought and well-watered conditions. At each of three harvests a pressure-volume curve was produced for each seedling. Douglas-fir maintained a lower osmotic potential at full saturation [special characters omitted] and lower turgor loss point [special characters omitted] than lodgepole pine under both watering regimes,. Both species had lower [special characters omitted] when drought-stressed. Douglas-fir appears to be a more conservative species, maintaining low stomatal conductance and tolerating drought conditions, whereas lodgepole pine avoids drought by producing large amounts of roots to exploit the soil resource. To study root hydraulic conductance (Lproot) and water flow rates through roots (WFRR), water flow was measured through de-topped roots of Douglas-fir, lodgepole pine, and white spruce seedlings in a pressure chamber. In a drought experiment, seedlings were grown in sandy soil in a greenhouse under drought and well-watered conditions during their first growing season and, in a low temperature experiment, seedlings were grown in sandy soil in growth chambers at 25/20°C (day/night) and 15/10°C, In the drought experiment, water flow through roots was measured at three pressures. No differences in Lproot were found for Douglas-fir and white spruce seedlings grown under the two watering regimes, however, lodgepole pine seedlings had reduced Lproot when grown under drought conditions. Welk watered seedlings of lodgepole pine and white spruce had higher Lpr00t in 1989 than in 1990 whereas Douglas-fir seedlings had the same Lproot in both years. In the low temperature experiment, WFRR was measured at 1.0 MPa and temperatures of 20°C for 24 hours or 20°, 12°, and 4°C for 18, 15, and 15 hours respectively. At 20°C, white spruce seedlings had higher WFRR than the other two species. Lodgepole pine and white spruce seedlings grown in the 1S°/10°C growth chamber had higher WFRR than seedlings grown in the 25°/20°C growth chamber. Water flow rate decreased with temperature in all three species. After correcting for viscosity, all seedlings had lower WFRR with reduced temperature, except for Douglas-fir and white spruce seedlings grown at 15°/10°C which had the same WFRR at 20°C and 12°C. Therefore, Douglas-fir and white spruce seedlings were found to become less sensitive to low temperature (chilling) stress when pre-conditioned at low temperatures. In the drought and low temperature studies, dry weight biomass of white spruce was lowest but white spruce had a greater specific root length than lodgepole pine and Douglas-fir. In the drought study, biomass production in seedlings from wet ecotypes of each species was more reduced when drought-stressed than seedlings from dry ecotypes. / Graduate

This research assesses whether a refinement of the temporal resolution of tree-ring data can improve our understanding of the radial growth-climate relationship. Two study sites in Northern Labrador were chosen, one coastal (Nain), and the other inland (Kamestastin). In Nain, microcore samples were taken weekly from the same five white spruce (Picea glauca) trees over the 2014 growing season. After cross sections were made and stained, the resulting 10µm thin radii provided a direct view of active ring development. In coastal Labrador, radial growth was initiated during the last week of June 2014, and ceased by August 25th. Circumference band dendrometers were installed on white spruce trees at both the Nain and Kamestastin sites. The dendrometers were used to measure micrometre-scale changes over the 2014 growing season. Analogous records of temperature were collected with equal temporal resolution, from an Environment Canada climate station (#8502800), and via a programmable data logger (UX120-006M, Onset HOBO). Correlation function analysis determined the relationship between daily temperature variables and daily variations in stem size. A strong relationship was found between minimum daily temperature and daily stem size at both sites over the eight week long growing season. Traditional dendrochronological sampling methods were utilized to retrieve tree cores from white spruce and eastern larch (Larix laricina) in Nain and Kamestastin. Site-specific master growth chronologies were created using crossdating and standardization techniques. After establishing long term records of monthly temperature and accumulated growing degree-days (GDD) at both study sites, a linear regression analysis was undertaken to determine the suitability of these two variables as predictors of annual-radial growth. An accumulated June/July GDD index was identified as an overall better predictor of annual ring-width than mean monthly temperature variables in northern Labrador. Exploring radial growth on an intra-annual scale helped to improve our understanding of the complex radial growth-climate relationship in Labrador. This allows for a strengthening of tree rings as a proxy climate indicator in remote regions of the northern boreal forest. The findings from this thesis provide the tools necessary to improve upon long-term climate reconstruction and forecasts of boreal forest structure in the face of climate change.

Mémoire (M.Ress.Renouv.)-- Université du Québec à Chicoutimi, 1993. / Document électronique également accessible en format PDF. CaQCU

High latitude and altitude ecosystems are currently undergoing rapid and unprecedented warming in response to anthropogenically induced climate change. Subarctic, alpine regions are particularly vulnerable to increases in air temperature and changing precipitation regimes, which have caused cascading hydrological and ecological impacts. In addition to changing flow regimes, thawing permafrost, and declining glaciers, widespread changes in vegetation composition, density and distribution have been observed across northern regions. Specifically, treeline is advancing with increasing latitude and altitude and shrubs are increasing in height, extent, and density. Despite widespread documentation of this northern greening, few field-based studies have evaluated the hydrological implications of these changes. Quantification of total evapotranspiration (ET) across a range of vegetation gradients is essential for predicting water yield, yet challenging in cold alpine catchments due to heterogeneous land cover. Direct field-level measurements of transpiration (T) and evaporative partitioning across subarctic, alpine ecosystems and species are rare, yet essential to assess sensitivities and hydrological response to changing climate drivers. This thesis presents six years of surface energy balance components and ET dynamics and two years of sap flux measurements and critical zone stable water isotope sampling at three sites along an elevational gradient in a subarctic, alpine catchment near Whitehorse, Yukon Territory, Canada. These sites span a gradient of thermal and vegetation regimes, providing a space-for-time comparison for future ecosystem shifts: 1) a low-elevation boreal white spruce forest (~12-20 m), 2) a mid-elevation subalpine taiga comprised of tall, dense willow (Salix) and birch (Betula) shrubs (~1-3 m) and 3) a high-elevation subalpine taiga with short, sparse shrub cover (< 0.75 m) and moss, lichen, and bare rock. We utilize both mass flux measurements and stable water isotopes to evaluate the timing, magnitude, sensitivities, and sources of plant water uptake across these vegetation covers. Total ET decreased and interannual variability increased with elevation, with mean May to September ET totals of 349 (±3) mm at the forest, 249 (±10) mm at the tall, dense shrub site, and 240 (± 26) mm at the short, sparse shrub site. The shrub sites exhibited similar ET losses over 6 years despite differences in shrub height and abundance, although daily rates were higher at the tall shrub site in the peak growing season. From May to September, ET:R ratios were the highest and most variable at the forest (2.19 ± 0.37) and similar at the tall, dense shrub (1.22 ± 0.09) and short, sparse shrub (1.14 ± 0.05) sites. In the mid-growing season, mean T rates were greater at the dense shrub site (2.0 ± 0.75 mm d-1) than the forest (1.47 ± 0.52 mm d-1). During this time, T:ET was lower at the forest (0.48) than at the tall, dense shrub site (0.80). During the growing season between the two years, 2020 was considerably wetter and cooler than 2019. At the tall shrub site, during the mid-growing season (July 1-Aug 15), T dropped considerably in 2020 (-26%), as T was suppressed during the short, wet growing season. In contrast, T at the forest was only moderately suppressed (-3%) between years in this same period. Evapotranspiration was more strongly controlled by air temperature during the early and late season at the forest, while ET at the shrub site was more sensitive to warmer temperatures in the mid-growing season. At the shrub sites, ET was energy limited with no observed soil moisture limitation on T. While 2H and 18O of volume weighted precipitation became more depleted with elevation, the opposite was true in xylem water, where 2H and 18O became more enriched with elevation. Plant water uptake was more reflective of snow water at the forest site than both shrub sites, particularly early in the year and during dry periods. Near-surface bulk soil water had more negative lc-excess at the forest throughout the season and with depth, highlighting increased contributions from soil evaporation. This study combined direct measurements of sap flux, ET, and critical zone isotopes to provide new details on multi-year plant-soil-water dynamics, critical zone water cycling, and species-specific plant water uptake patterns in seasonally frozen soils, which have not previously been reported in cold regions. Our results suggest that advances in treeline will increase overall ET and lower interannual variability; however, the large growing season water deficit and stable water isotope signature at the forest indicates strong reliance on soil moisture from late fall and snowmelt recharge and the potential for plant water stress. Differences between the shrub species were apparent in the sap flux and stable isotope measurements, highlighting the need to further evaluate species specific responses and feedbacks when predicting hydrological fluxes across subarctic ecosystems. Overall, our results suggest that predicted changes in vegetation type and structure in northern regions will have a considerable impact on water partitioning and will vary in a complex way in response to changing precipitation timing, phase and magnitude. / Thesis / Candidate in Philosophy

Accurate and reliable predictions of site index (height of dominant trees at a reference age, usually 50 years at breast-height) for timber crop species is essential for silvicultural site-specific decision making. Site index can be predicted from site quality once the relationship between site index and site quality has been quantified. Site quality is defined as the sum of all environmental factors affecting the biotic community, such as the factors directly influencing the growth of vascular plants (light, heat, soil moisture, soil nutrients, and soil aeration). Since these factors vary greatly in time, indirect estimates of site quality have widely been used as predictors for site index in various multiple regression models. Trembling aspen (Populus tremuloides Michx.) is the most widely distributed broadleaf species in British Columbia, especially in the Boreal White and Black Spruce (BWBS) biogeoclimatic zone. Growing this species for sustainable timber production requires a good understanding of its productivity attributes and accurate predictions of its growth. This extension note presents (1) relationships between trembling aspen site index and some indirect measures of site quality, and (2) site index prediction models using the indirect measures of site quality as predictors.

Depuis quelques années la sylviculture intensive prend une grande ampleur afin de rapprocher la source de fibres des usines, d'accroître la productivité des plantations, et de diminuer la pression de coupe sur les forêts naturelles. Toutefois un débat sur le type d'aménagement optimal des plantations oppose l'aménagement mono- et pluri-spécifique. Il est important de mieux comprendre le fonctionnement de ces écosystèmes pour en effectuer une bonne gestion et d'optimiser les services écosystémiques que ces plantations fournissent. La décomposition des litières et le recyclage des nutriments sont des processus complexes essentiels au fonctionnement des écosystèmes. Ainsi mon projet de thèse visait à mieux comprendre l'influence de la mixité de deux essences forestières, à savoir l'épinette blanche et le peuplier hybride, en comparaison à des plantations pures sur le processus de décomposition des litières et le stockage de carbone. Les résultats de cette étude ne montrent pas d'amélioration du processus de décomposition avec le mélange du peuplier et de l'épinette ou de leurs litières. En revanche, le mélange de ces deux espèces en plantation tamponne les effets contrastés du peuplier et de l'épinette observés dans les plantations monospécifiques. De plus, le stockage de carbone et la productivité du peuplier sont améliorés dans les plantations mixtes par rapport aux plantations monospécifiques. Les herbacées semblent être bénéfiques pour la diversité d'organismes et favorisent la libération d'azote des litières d'arbres. Cet aspect pourrait contrebalancer l'effet négatif de la présence d'herbacées qui entrent en compétition avec les arbres pour les ressources. / The use of trees under intensive management is particularly important for rapid fibre production and for reduce cutting pressure on natural forests in boreal regions. However, a debate on the best type of plantation management opposes the mono-and pluri-specific management. Despite the possible antagonistic effects on productivity, it seems that mixed plantations would have benefits on soil properties, environmental stability, but also to maintain biodiversity and aesthetic value. It is important to better understand the functioning of these ecosystems to make good management in order to optimize ecosystem services of these plantations. Litter decomposition and nutrient cycling are essential process for the ecosystems functioning. My thesis project was to better understand the influence of the mixing of two tree species planted in comparison to monospecific plantations, namely white spruce and hybrid poplar, on the litter decomposition process and carbon storage. The results of this study showed no improvement in the decomposition process with the mixture of poplar and spruce in plantation or by their litters mixture. However, the mixture of the two species in plantation buffers the contrasting effects of poplar and spruce observed in monospecific plantations. In addition, carbon storage and productivity of poplars are improved in mixed plantations compared to monospecific plantations. Herbaceous litter appears to be beneficial for the abundance of organisms and promote the release of nitrogen from tree litter. This could offset the negative effect of the presence of grasses that compete with trees for resources.

Trembling aspen (Populus tremuloides Michx.) is one of the most common tree species in the boreal and temperate forests of North America. It grows on many different sites and associates with a variety of tree species. In BC, aspen is frequent throughout all submontane and montane continental forested zones. Relationships between environmental factors and forest productivity have been the subjects of many studies. Most of these studies, using various topographic, soil, physical and chemical properties as independent variables, had limited success in accounting for the variation in SI over a large geographic area. The objectives of this study were (1) to quantify relationships between aspen SI and environmental factors at two spatial scales, and (2) to develop predictive SI models from easily measurable environmental factors.