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Sexual dimorphism in life history traits may influence the distribution of the sexes,population sex ratios, the maintenance of sex in populations, and the evolutionarypotential of a species. In bryophytes, sexual dimorphism in traits related to growth andreproduction may be responsible for female-biased population sex ratios and a lack ofsexual reproduction. I examined the roles of natural selection in maintaining sexualdimorphism in the context of impacts on bryophyte population sex ratios, usingMarchantia inflexa as a model system. My studies included an assessment of amongpopulationvariation in habitat use by the sexes, comparison of phenotypes betweensingle-sex and both-sex populations, a field study of natural selection, and a comparisonof the influence of selection on asexual and sexual fitness components.The sexes of M. inflexa were sexually dimorphic in investment in growth, asexualand sexual reproduction. The sexes were spatially separated in populations, but thesexes overlapped in habitat use. Populations differed in growth, asexual reproductionrates, degrees of sexual dimorphism, and strength of among-trait correlations. Plantsfrom single-sex and both-sex populations differed in investment in growth and asexualreproduction, but the two population types showed the same degree of sexualdimorphism. Thus, local environment may be more influential than the presence of theopposite sex in maintaining sexual dimorphism.Selection on sexually dimorphic traits was both sex-specific and environmentallydependent. Between-sex correlations were not significant in the greenhouse but weresignificant in the field thus, evolution and expression of sexual dimorphism in nature maybe constrained by among-trait and between-sex correlations. Additionally, femalesincurred a cost of plasticity that males did not. Because there was a negative trade-offbetween sexual and asexual fitness, overall lifetime selection may result in a differentpicture of how the sexes experience selection. The combination of sex-specific andenvironment-dependent selection, and sex-specific costs to plasticity may not onlymaintain sexually dimorphic traits but also ensure the persistence of both sexes in apopulation.

Chez les plantes vasculaires, le métabolisme des phénylpropanoïdes conduit à la synthèse de précurseurs de biopolymères structuraux tels que la lignine, ainsi que de nombreux composés antioxydants et anti-UV. Ce métabolisme phénolique est apparu lors de la colonisation des terres par les plantes et a été critique pour leur adaptation à ce nouvel environnement. Physcomitrella patens, une bryophyte phylogénétiquement proche des premières plantes terrestres, est un bon modèle pour l'étude de certains caractères ancestraux. P. patens est dépourvue de lignine. En combinant des approches phylogénomique, génétique et biochimique, ce travail démontre le rôle essentiel de deux BAHD hydroxycinnamoyl tranférases dans le métabolisme phénolique de la mousse et la formation de précurseurs de la cuticule, une couche hydrophobe qui recouvre les parties aériennes de la plante et lui confère une imperméabilité. Il suggère également que deux hydroxycinnamoyl transférases sont requises pour la formation des composés phénoliques solubles accumulés par la mousse. Une exploration préliminaire du métabolisme des flavonoïdes chez ce modèle révèle d’autre part le caractère incomplet et primitif de cette voie métabolique. / In vascular plants, the phenylpropanoid metabolism leads to the synthesis of precursors of structural biopolymers such as lignin and of essential antioxidants and UV screens. The phenolic pathway leading to these compounds appeared upon plant land colonization and is thought critical for their adaptation to this new environment. Physcomitrella patens is a bryophyte, an early-diverging land plant and thus a good model to reveal ancestral traits. P. patens is devoid of lignin. Combining phylogenomic, genetic and biochemical approaches, this work demonstrates the essential role of two BAHD hydroxycinnamoyl transferases in the moss phenolic metabolism and in the formation of precursors of the cuticle, a hydrophobic layer, covering and conferring impermeability to the aerial parts of the plant. It also suggests that two nonredundant hydroxycinnamoyl transferases are required for the formation of the soluble phenolic compounds accumulated in moss. A preliminary exploration of the flavonoid metabolism in this model in addition reveals primitive features of this metabolic route.

Rapid climate change in Arctic regions is of concern due to important feedbacks between the Arctic land surface and the global climate system. A large amount of organic carbon (C) is currently stored in Arctic soils; if decomposition is stimulated under warmer conditions additional release of CO2 could result in an accelerating feedback on global climate. The strength and direction of Arctic C cycle - climate feedbacks will depend on the growth response of vegetation; if plant growth increases some or all of the extra CO2 emissions may be offset. Currently the Arctic is thought to be a small net sink for CO2, the expected balance of terrestrial C sinks and sources in the future is unknown. In this thesis I explore some of the critical unknowns in current understanding of C cycle dynamics in Arctic vegetation. Quantifying gross primary productivity (GPP) over regional scales is complicated by large spatial heterogeneity in plant functional type (PFT) in Arctic vegetation. I use data from five Arctic sites to test the generality of a relationship between leaf area index (LAI) and canopy total foliar nitrogen (TFN). LAI and TFN are key drivers of GPP and are tightly constrained across PFTs in Low Arctic Alaska and Sweden, therefore greatly simplifying the task of up-scaling. I use data from Greenland, Barrow and Svalbard to asses the generality of the LAI-TFN relationship in predicting GPP at higher Arctic latitudes. Arctic ecosystems are unique among biomes in the large relative contribution of bryophytes (mosses, liverworts and hornworts) to plant biomass. The contribution of bryophytes to ecosystem function has been relatively understudied and they are poorly represented in terrestrial C models. I use ground based measurements in Northern Sweden to fill an existing data gap by quantifying CO2 fluxes from bryophytes patches in early spring and summer, and develop a simple model of bryophyte GPP. Using the model I compare bryophyte GPP to that of vascular plants before, during and after the summer growing season, finding that productive bryophyte patches can contribute up to 90 % of modelled annual GPP for typical vascular plant communities at the same site, and that the relative magnitude of bryophyte GPP is greatest in spring whilst the vascular plant canopy is still developing. Understanding how GPP relates to plant growth is important in relating remotely sensed increases in Arctic ‘greenness’ to changes in plant C stocks. I use a 13C pulselabelling techniques to follow the fate of recently fixed C in mixed vascular and bryophyte vegetation, with a focus on quantifying the contribution of bryophytes to ecosystem carbon use efficiency (CUE). I show that bryophytes contribute significantly to GPP in mixed vegetation, and act to increase ecosystem CUE. I highlight the importance of including bryophytes, which do not have roots, in aboveground: belowground partitioning schemes in C models. To further explore C turnover in bryophytes, I use the results of a second 13C labelling experiment to develop a model of C turnover in two contrasting Arctic mosses (Polytrichum piliferum and Sphagnum fuscum). I find significant differences in C turnover between Polytrichum piliferum which respires or translocates about 80 % of GPP, while Sphagnum fuscum respires 60 %. This analysis is the first to explicitly model differences in C partitioning between Arctic bryophyte species. Finally, I discuss the implications of each chapter for our understanding of Arctic C dynamics, and suggest areas for further research.

<p>Bryophytes, by their haploid dominant life cycle, possess several unique qualities ideal for study of mating patterns. In particular, the possibility of intragametophytic selfing in some species, and the vegetative propagation of gametes allow for a unique window into the haploid stage that is intractable in other groups. Despite these advantages, there have been relatively few studies on mating patterns bryophytes in natural populations. <italic>Sphagnum</italic> (peatmoss) is an excellent case study in the interactions between sexual condition, ecology, and mating patterns. In the first Chapter, we use microsatellites to characterize the genetic diversity and mating patterns in fourteen species of <italic>Sphagnum</italic>, diverse in sexual condition (separate vs combined sexes in the haploid stage) and ecology (microhabitat variance along the water table). We find that genetic diversity and mating patterns are related only in species with separate sexes, that sexual condition and ecology have interacting effects on inbreeding coefficients, and that inbreeding depression is not a common phenomenon in <italic>Sphagnum</italic>. In the second Chapter, we conduct an intensive survey of one population of <italic>Sphagnum</italic> macrophyllum, to detect whether variance in haploid fecundity and mating success is related to diploid fitness. We find a relationship between mating success and fecundity (a signal of sexual selection), and fitness of the diploid generation is connected to the parentage of the haploid generation. Finally, in Chapter 3 we use phylogenetic comparative methods to track the phylogenetic signal in microhabitat preference in <italic>Sphagnum</italic>. We find extremely fast rates of evolution along the micronutrient gradient, but high phylogenetic signal along a hydrological gradient. Given that <italic>Sphagnum</italic> species living high above the water table have reduced water availability, phylogenetic signal in the hydrological gradient has macroevolutionary implications for mating systems in <italic>Sphagnum</italic>.</p> / Dissertation

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This thesis examined the efficacy of residual forest structure for the preservation and recovery of bryophytes five to six years after partial canopy harvest in boreal mixed-wood forests of northwestern Alberta, Canada. Bryophytes were sampled in two forest types that differed in pre-harvest abundance of broadleaf (primarily Populus tremuloides Michx. and P. balsamifera L.) and coniferous (primarily Picea glauca (Moench) Voss) canopy trees. In Chapter 2, epiphytic bryophytes growing on aspen (P. tremuloides) were characterized by species viability and nearest-neighbour relationships. Epiphyte assemblage structure showed increasing impact with declining retention owing to degradation of growing conditions for species on trees. Chapter 3 provided an analysis of species richness and abundance patterns in relation to residual canopy structure. Bryophytes generally benefitted from higher canopy retention; however, epixylic and epiphytic species were more sensitive to partial harvesting than species on other substrates, and liverworts were more sensitive than mosses. Liverworts exhibited higher among-site differences in richness as retention declined, which partly resulted from increasing numbers of local species extinctions. In an analysis of species-environment relations in partially-harvested forests in Chapter 4, forest moisture was reduced with any degree of harvesting in both forest types. Lower canopy retention and forest moisture levels were associated with reduced abundances of species with particular biological traits, such as limited reproduction and dispersal capacities. Their re-establishment after harvesting may be impeded because of biological and environmental limitations. Coniferous-dominated forests supported higher abundances of liverworts and species with greater moisture requirements than did mixed-wood broadleaf-coniferous forests, and are potentially important refuges of bryophyte source populations. Chapter 5 examined the capacity for bryophyte species to germinate from diaspore banks in forest soils. Species germinated readily from mineral soil samples obtained from harvested sites, including several perennials characteristic of intact forests. Diaspore banks may serve as a persistent source for species colonization at post-disturbance sites, but only under appropriate growing conditions and not for species that were most sensitive to harvesting. Overall, both amount and composition of forest structure retained after partial harvesting are important management considerations for ensuring conservation of the wide variety of bryophyte species in mixed-wood landscapes. / Conservation Biology

The placenta of two Sphagnum species was examined by electron microscopy. In contrast to all mosses so far investigated, neither sporophyte nor gametophyte placental cells of Sphagnum develop wall ingrowths. The sporophyte cells are highly vacuolate and the gametophyte cells close to them degenerate to produce a system of spaces filled with mucilage. Whether this type of placenta represents a primitive or derived condition in mosses is discussed.