Most clonal plants exhibit a modular structure at multiple levels. At the level of the organs, they are characterized by functional modules, such as, internodes, leaves, branches. At the level of the genetic individual (clone or genet), they possess independent evolutionary and physiological units (ramets). These evolutionary units arise through the widespread phenomenon of clonal reproduction, achieved in a variety of ways including rhizomes, stolons, bulbils, or lateral roots. The focus of this study was Populus tremuloides, trembling aspen, a dioecious tree that reproduces sexually by seed and asexually through lateral roots. Local forest patches in western populations of Populus tremuloides consisted largely of multiple genotypes. Multi-clonal patches were dominated by a single genotype, and in one population (Riske Creek) we found several patches (five out of 17) consisting of a single genotype. A second consequence of modularity is that during the repeated cycle of ramet birth, development and death, somatic mutations have the opportunity to occur. Eventually, the clone becomes a mosaic of mutant and non-mutant cell lineages. We found that neutral somatic mutations accumulated across 14 microsatellite loci at a rate of between 10^-6 and 10^-5 per locus per year. We suggest that neutral genetic divergence, under a star phylogeny model of clonal growth, is an alternative way to estimate clone age. Previous estimates of clone age couple the mean growth rate per year of shoots with the area covered by the clone. This assumes a positive linear relationship between clone age and clone size. We found, however, no repeatable pattern across our populations in terms of the relationship of either shape or size to the number of somatic changes. A final consequence of modularity is that during clonal growth, natural selection is relaxed for traits involving sexual function. This means that mutations deleterious to sexual function can accumulate, reducing the overall sexual fitness of a clone. We coupled neutral genetic divergence within clones with pollen fitness data to infer the rate and effect of mildly deleterious mutations. Mutations reduced relative sexual fitness in clonal aspen populations by about 0.12x10^-3 to 1.01x10^-3 per year. Furthermore, the decline in sexual function with clone age is evidence that clonal organisms are vulnerable to the effects of senescence.

Magistro darbe tiriama kai kurių abiotinių veiksnių įtaka hibridinės drebulės adaptacijai ex vitro sąlygomis. Darbo objektas – Smulkiadantės tuopos x drebulės (Populus tremuloides x Populus tremula) hibridas. Darbo tikslas – Nustatyti temperatūros ir substrato pH įtaką hibridinės drebulės mikroūglių adaptacijai ex vitro sąlygomis. Darbo metodai – Temperatūros ir pH įtaka adaptacijai ex vitro sąlygoms. Darbo rezultatai: Mikroūglių be šaknų, paruoštų perkėlimui į daigintuves, poveikis skirtinga temperatūra parodė, kad geriausiai mikroūgliai adaptavosi prieš tai palaikyti 12 val. +4 ºC temperatūroje. Genotipų reakcija į skirtingas temperatūros sąlygas skyrėsi nežymiai. Adaptuojamus ex vitro sąlygose hibridinės drebulės mikroūglius rekomenduojama laistyti tirpalais, kurių pH (5,5–6) artimas arba šiek tiek didesnis už fiziologinį. Augimo reguliatoriai, PBZ ir ABR, gali būti naudojami hibridinės drebulės mikroūglių adaptacijos ex vitro sąlygose pagerinimui, ypač mažiau adaptacijai palankiomis sąlygomis (kai pH 5). / This master thesis researches some effects of abiotic factors on adaptation of the hybrid aspen under ex vitro conditions. Object of the research – Hybrid aspen (Populus tremuloides x Populus tremula). Purpose of the research – Defining the impact of temperature and pH substrate on adaptation of the hybrid aspen under ex vitro conditions. Research methods – The impact of temperature and pH substrate on adaptation under ex vitro conditions. Results of the research: The effect of different temperature on rootless micro plantlets prepared for planting shows that the best results of adaptation are achieved when micro plantlets are disposed to temperature of +4 ºC for 12 hours. The reaction of genotypes to different temperature conditions is of no significant importance. For better adaptation under ex vitro conditions, it is recommended to water micro plantlets with solutions that have the same pH (5,5 – 6) or minimally larger than physiological saline. Plant growth regulators, such as PBZ and ABR, can also be used for better adaptation results under ex vitro conditions, especially under less favourable conditions (when pH is 5).

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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.

Regional variation in nitrogen (N) deposition increases plant productivity and decreases species diversity, but landscape- or local-scale influences on N deposition are less well-known. Using ion-exchange resin, we measured variation of N deposition and soil N availability within Elk Island National Park in the ecotone between grassland and boreal forest in western Canada. The park receives regionally high amounts of atmospheric N deposition (22 kg ha⁻¹ yr⁻¹). N deposition was on average higher ton clayrich luvisols than on brunisols, and areas burned 1 – 15 years previously received more atmospheric N than unburned sites. We suggest that the effects of previous fires and soil type on deposition rate act through differences in canopy structure. The magnitude of these effects varied with the presence of ungulate grazers (bison, moose, elk) and vegetation type (forest, shrubland, grassland). Available soil N (ammonium and nitrate) was higher in burned than unburned sites in the absence of grazing, suggesting an effect of deposition. On grazed sites, differences between fire treatments were small, presumably because the removal of biomass by grazers reduced the effect of fire. Aspen invades native grassland in this region, and our results suggest that fire without grazing might reinforce the expansion of forest into grassland facilitated by N deposition.

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.

The objective of this thesis was to determine how aspen (Populus tremuloides Michx.) root systems and suckering are affected by decking area (site of log processing and storage) disturbances and seasonal timing of these disturbances. In a field study, summer-built log decks reduced regeneration by half compared to fall-built decks, and if decks were built in the fall, 11 month and 1.5-3 month storage were similar in their impact. A growth-chamber study examined the timing of traffic-induced wounding of the root system and simulated log storage on aspen root systems and suckering. For both summer and winter treatments the combination of root wounding and log storage killed nearly the entire root system and prevented suckering. Root wounding and log storage alone caused a 35-40% reduction in living root mass, carbohydrate reserves, and sucker growth. Sucker numbers were reduced by one half for the winter but were unaffected for the summer.