Future climate changes will alter disturbance regimes worldwide with important implications for many ecological and social systems. In west central British Columbia, Canada, fire and insect disturbances have shaped the historic character of Douglas-fir (Pseudotsuga menziesii var. glauca Beissn. Franco) dominated forests. However, since AD 1900 fire suppression and other forest management practices have led to denser forests and conifer encroachment into grasslands. Considering climate changes in interior British Columbia are expected to result in warmer and drier conditions, understanding the influence of climate on forest disturbances is crucial for land managers tasked with both mitigating the effects of disturbance and promoting resilience in forest ecosystems. This research focused on developing multi-century, annually-resolved records of fire and western spruce budworm outbreaks to evaluate: the historic climate conditions related to these disturbances; the influence of grassland proximity on disturbance-climate relationships; and, whether western spruce budworm outbreaks were related to fire activity. At the landscape scale, a detailed study in the Churn Creek Protected Area revealed spatially variable stand structure and fire-climate relationships at a low elevation forest-grassland ecotone over the interval AD 1600 to 1900. This finding suggests the site was characterized by fires of mixed-severity dominated by frequent, low-severity, fires related to positive antecedent moisture conditions punctuated by widespread fires of moderate to high severity related to intervals of persistent drought. At the regional scale, the influence of interannual climate variability and large-scale patterns of climate variability (e.g. El Nino Southern Oscillation) was evaluated using new and existing records of fire history and multiple climate pattern reconstructions. Regional fire activity was shown to be significantly related to interannual climate variability, and no consistent patterns between regional fire years and the individual phases or phase combinations of large-scale patterns of climate variability were detected. The findings suggest that the spatial expression of large-scale climate patterns translates into weak and undetectable terrestrial effects related to fire activity in this region. The influence of grassland proximity on disturbance history was investigated using site-level and regional tree-ring reconstructions of western spruce budworm outbreaks and fire activity based on four sites adjacent to grasslands and four sites not adjacent to grasslands between AD 1600 and 1900 (fire) and AD 1600 and 2009 (western spruce budworm). Fires affecting grassland proximal sites were more frequent than fires occurring in forests not adjacent to grasslands, and the character of western spruce budworm outbreaks was generally consistent among all sites. Fire activity was related to both warm, dry and cool, wet conditions in the fire year and/or year(s) preceding the fire depending on proximity to grasslands, suggesting climate conditions associated with both fine fuel growth and drying are key determinants for fire activity. The initiation of western spruce budworm outbreaks was significantly related to drought and this relationship was enhanced at sites adjacent to grasslands. At the site-level and regional scale, no consistent association was found between the initiation of western spruce budworm outbreaks and fire years indicating the historic interaction between these disturbances is weak or non-existent. / Graduate

Vast amounts of carbon are stored forest soils, a product of decaying organic matter. Increased CO2 in the atmosphere is predicted to lead to increasing global temperatures, and more extreme moisture regimes. Such increases in mean temperature could accelerate the rate of organic matter decay in soils and lead to additional release of CO2 into the atmosphere, thus exacerbating climate change. However, due to its impact on plant metabolism, high atmospheric CO2 concentrations may also lead to greater condensed tannins (CT) and reduced nitrogen (N) content in leaf litter. This reduction in litter quality has the potential to slow decay of organic matter in soil and therefore offset the accelerated decay resulting from a warmer climate. My research aimed to quantify the effects of climate and litter chemistry, specifically CT and N, on litter decay, N mineralization and associated microbes in the field. Strings of litterbags were laid on the forest floor along climate transects of mature Douglas-fir stands of coastal British Columbia rain-shadow forests. In-situ climate was monitored alongside carbon and nitrogen loss over 3.58 years of decay along three transects located at different latitudes, each transect spanning the coastal Western Hemlock and Douglas-fir biogeoclimatic zones. Microbial communities in the decaying litter and in forest soils were also analyzed using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE). Microbial biogeography at field sites was partially influenced by climate, soil characteristics and spatial distance, but did not improve best fit decay models using climate and litter chemistry variables. Litter with greater initial CT and smaller N concentration slowed down early decay (0 - 0.58 yr) and net N mineralization. Warmer temperatures accelerated later decay (0.58 - 3.58 yr) and net N mineralization. Water-soluble CT were rapidly lost during decay, while other forms of CT were likely responsible for slower decay. The composition of fungal communities on decaying litter was affected by initial concentrations of CT and N. On a yearly basis, the slower decay of litter with high CT and reduced N content can offset accelerated rates of decay associated with warmer temperatures. Concurrent shifts in microbial communities and net N mineralization suggest potential benefits to trees. / Graduate / 2017-12-19

Graduation date: 2006 / Canopy gap formation is a major factor contributing to maintenance of overstory species diversity and stand structure in forests and may be integral to development of understory shrub and herb layers as well. Acknowledgement of gap formation as a fundamental feature of natural forests has led to consideration of gaps as an option in forest management regimes. This study examined understory vegetation communities across canopy gaps created as a part of the Density Management Study (DMS), which investigates the effectiveness of a thinning regime in promoting late-successional habitat development in young Douglas-fir forests of western Oregon. Patterns in understory vegetation community composition in and around 0.1 and 0.4ha gaps created as a part of the DMS treatment were investigated. The primary goal of this research was to investigate the potential role of canopy gap creation in fostering heterogeneity in understory vegetation communities, and to examine the extent of gap influence on the surrounding thinned forest matrix. Tree species distributions have been shown to partition across gaps in tropical forest systems through differential responses of species to gradients in resource availability, a pattern known as gap partitioning. In temperate forests, understory vegetation communities are much more diverse than the overstories, and display a greater array of habitat requirements. Therefore, understory communities may be more likely than overstories to exhibit gap partitioning in these forests. Patterns in understory community composition across gaps suggest that gap partitioning has occurred. The strength of this partitioning effect appears to differ between gap sizes, as smaller gaps showed a less powerful effect. Abundance of ruderal species was strongly related to gap partitioning in larger gaps, while smaller gaps were dominated by competitor species. Partitioning may be related to an interactive relationship between harvest-related ground disturbance and resource gradients. Therefore, considerations of gap partitioning processes should take into account intensity and spatial distribution of ground disturbance in relation to resource gradients. In addition, conditions necessary for the expression of gap partitioning in understory vegetation communities may be rare in natural gaps in this region. The influence of gaps on understory vegetation communities in the surrounding forest appears to be relatively small. This small influence extent may help explain the lack of a stand level response to gap formation in these stands. Larger gaps exhibit a slight influence on the understory plant community in the surrounding forest to the north of the gap. In small gaps, there seemed to be an influence of the surrounding forest on gap interiors, resulting in an area of influence smaller than the physical gap area. This relationship may indicate that the area of gap influence on understory vegetation may not scale linearly with physical gap size. Species diversity was higher in gap interiors than in surrounding thinned forests. However this effect was partially due to the presence of exotic species, which showed an affinity for gap interiors. Late successional associated species were negatively related to gap interiors, but only in the larger gap size. Gap creation appears to be promoting small scale species diversity in these stands, but creation of large gaps may also promote the establishment of exotic species and may have a negative effect on late successional associated species. However, any and all of these effects may be transient, as understory communities will be strongly affected by overstory re-establishment, and related changes in resource availability. In general, gap formation may influence small-scale stand heterogeneity as evidenced by understory plant communities, but this effect may rely strongly on the nature of gap formation and intensity of disturbance related to this formation.

In the wetter climates associated with the coastal forests of northwestern North America, coarse woody debris (CWD) accumulations in the form of snags, downed boles, and large branches can be large in natural forest ecosystems. Seedlings often regenerate on stumps and downed logs in the understory of old-growth coastal forests. The question remains though, whether CWD is a necessary component for seedling survival and growth in forests managed for commodity production. This study addresses one concern of forest managers: is there an immediate nutritional or moisture supply advantage conferred by CWD for the survival and growth of seedlings in the coastal climate of British Columbia? We compared survival and growth of seedlings planted in decaying wood compared to non-woody humus forms and mineral soil under heavy shade and full light conditions. Low light environments are of particular interest since reports of the strong association between CWD and regeneration has primarily referred to understory seedlings and saplings in old-growth forests.

Knowledge of ecological characteristics of trees, sites and tree growth on different sites is fundamental for silvicultural decision-making and planning. With the biogeoclimatic ecosystem classification in place, silvicultural management in British Columbia has been given an ecological foundation; however, relationships between growth and site have not yet been fully investigated. The purpose of this study was to determine how height growth of Douglas-fir within the drier portion of the CWH zone varies with site. We adopted site index (m @ 50 yr bh) as a species-specific measure of forest productivity, recognizing that it indicates height growth performance at a selected point in time. If forest productivity is correlated with ecological measures of site quality, what site factors should be used to quantify the relationships? Because of compensating effects, the numerous site factors can be reduced to four primary (synoptic) factors that directly affect plant establishment and growth: climate (light and temperature), soil moisture, soil nutrients, and soil aeration (not used in this study).

Western larch (Larix occidentalis Nutt.) is a locally important species in the Nelson Forest Region, and to a lesser extent, in the Kamloops Forest Region. Its range extends from west of the Rockies to Okanagan Lake, and north to Salmon Arm, in the IDF, ICH, MS, and ESSF biogeoclimatic zones. Prior to this study, the site index curves developed for western larch in western Montana were used to model height and estimate site index in British Columbia. It has been suggested that these curves may not adequately reflect the height growth patterns of western larch in BC. Differences could arise from genetics, different methods of selecting sample trees, or climatic differences. The objective of this project was to produce accurate height growth models for western larch in BC.

Western spruce budworm (Choristoneura occidentalis (Freeman)) is a native defoliator of the Interior Douglas-fir (Pseudotsuga menziesii var. glauca (Mirb.) Franco) forests of British Columbia, Canada. This thesis used dendrochronology and the software program OUTBREAK to reconstruct a defoliation history of Douglas-fir for nineteen forest sites near Kamloops in central British Columbia. By comparing the radial growth response of non-host ponderosa pine trees to Douglas-fir trees growing in nearby but separate stands, seven western spruce budworm outbreaks were distinguished over the past 300 years. Although there is considerable variation in the timing and duration of these western spruce budworm events at the stand level, synchronous outbreaks have occurred at approximately 43-year intervals. Climate variation appears to have been important to budworm outbreaks in the 20th century. Notable outbreaks tended to occur during years of early springs with average air temperature, following winters with lower than average precipitation. Based on this finding, it is proposed that with high overwintering survival, increased population growth rates, and a longer growing season, the extent of future outbreaks will shift northward and may increase in size.

Western spruce budworm (Choristoneura occidentalis (Freeman)) is a native defoliator of the Interior Douglas-fir (Pseudotsuga menziesii var. glauca (Mirb.) Franco) forests of British Columbia, Canada. This thesis used dendrochronology and the software program OUTBREAK to reconstruct a defoliation history of Douglas-fir for nineteen forest sites near Kamloops in central British Columbia. By comparing the radial growth response of non-host ponderosa pine trees to Douglas-fir trees growing in nearby but separate stands, seven western spruce budworm outbreaks were distinguished over the past 300 years. Although there is considerable variation in the timing and duration of these western spruce budworm events at the stand level, synchronous outbreaks have occurred at approximately 43-year intervals. Climate variation appears to have been important to budworm outbreaks in the 20th century. Notable outbreaks tended to occur during years of early springs with average air temperature, following winters with lower than average precipitation. Based on this finding, it is proposed that with high overwintering survival, increased population growth rates, and a longer growing season, the extent of future outbreaks will shift northward and may increase in size.

Face à la toxicité des éléments traces métalliques (ET) dans l’environnement, les plantes ont développé différentes stratégies aujourd’hui exploitées en phytoremédiation. Les arbres paraissent plus efficaces que les herbacées pour la phytoextraction de par leur importante biomasse et leur système racinaire plus profond. Ces travaux ont donc été menés sur des espèces ligneuses largement répandues dans la région Limousin, plus particulièrement des conifères: le Douglas, le mélèze hybride et ses deux parents, le mélèze d’Europe et le mélèze du Japon. Plusieurs modèles de culture ont été utilisés pour permettre, par des approches complémentaires, une meilleure appréhension des réponses de ces conifères aux ET : des plantules cultivées in vitro et des germinations ou des arbres âgés de 2 ans cultivés sous serre. Les conifères ont été exposés soit à un seul ET, le Cadmium (Cd), soit à plusieurs ET (As, Pb, Sb) présents dans des sols collectés sur deux anciens sites miniers. Les résultats ont montré que, parmi les espèces étudiées, le Douglas présentait le taux d’accumulation le plus élevé en Cd. Cependant, la quantité stockée dans la biomasse aérienne est restée faible par rapport à d’autres espèces d’arbres déjà utilisées en phytoextraction. Par ailleurs, la caractérisation chimique des pectines de la paroi a mis en évidence des modifications quantitatives et qualitatives suggérant un processus d’exclusion du Cd. Au niveau intracellulaire, les résultats ont montré que les polyamines et les oligopeptides riches en thiols ne semblaient pas impliqués dans la compartimentation du Cd. En revanche, une accumulation de proline a été observée en réponse au Cd et à d’autres ET, suggérant une implication de la proline comme molécule antioxydante et / ou chélatrice d’ET. L’ensemble des résultats a mis en évidence la nécessité d’optimiser la croissance des conifères et la phytodisponibilité des ET du sol avant de pouvoir proposer ces conifères, et notamment le Douglas, pour la phytoextraction d’ET. / Given the toxicity of trace elements (TE) in the environment, plants developed varions strategies used currently for phytoremediation. Trees appear more efficient than herbaceous species for phytoextraction as they have a larger biomass and a deeper root system. The work was thus performed on woody species widespread in Limousin region, particularly conifers: Douglas fir, hybrid larch and its two parents, European larch and Japanese larch. Several culture models were used to allow, through complementary approaches, a better understanding of the response of these conifers to TE: in vitro grown plantlets and seedlings or 2-year-old trees grown in greenhouse. Conifers were exposed to a single TE, cadmium (Cd), or to several TE (As, Pb, Sb) present in soils collected from two former mining sites. Among investigated species, results showed that Douglas exhibited the highest rate of Cd accumulation. However, the amount stored in aboveground biomass remained low compared to other tree species already used in phytoextraction. Furthermore, the chemical characterization of cell wall pectins highlighted quantitative and qualitative modifications suggesting a Cd exclusion process. At the intracellular level, results showed that polyamines and thiol-rich oligopeptides did not appear to be involved in Cd compartmentation. However, a proline accumulation was observed in response to Cd and other TE, suggesting an involvement of proline as antioxidant molecule and / or TE scavenger. Overall, results highlighted the need to optimize conifer growth and soil TE phytoavailability before to propose these conifers, particularly Douglas, for the phytoextraction of TE.

Potential uses of dry weight information for simulation, and sources of data, for simulation of forest fuels were discussed. Tree and crown component dry weights of 23 Douglas fir, 18 western hemlock, and 23 western red cedar trees expanded to 314 "model" trees were investigated. Data were collected on the University Research Forest near Haney, British Columbia. Multiple regression techniques were used for the analysis. The best four independent variables are listed and the possibility of retaining a single independent variable, with its implications, are discussed. All predicting equations were conditioned to zero intercept and statistically tested for significance by the F-test. Of the independent variables tested, the combined variable of breast height diameter squared times total tree height was most closely related to tree component dry weights. The product of diameter and crown width squared was found to be the best single variable to describe crown component dry weights for Douglas fir. The dry weights of western hemlock and western red cedar crown components were most closely associated with breast height diameter times crown length. Reliable estimates of tree component dry weights were obtained. Crown components were highly variable and widely dispersed about the mean which resulted in less accurate estimates. / Forestry, Faculty of / Graduate