The objective of this thesis was to understand the spatial patterns and processes responsible for canopy and understorey spider (Arachnida: Araneae) diversity at multiple spatial scales in north-temperate hardwood forests. I sampled tree trunks (sticky traps) and foliage (beating) of sugar maple and American beech tree canopies and their understorey saplings in old growth forests near Montreal, Quebec. Results show the composition of canopy and understorey assemblages differed significantly, and so did sugar maple and American beech canopy assemblages. Each stratum was also dominated by different species. The rank-abundance distribution of species from each habitat wsa also verticaly stratified because it fit different distribution models. Different factors likely structure assemblages in both habitats, particularly since the canopy is a less stable environment. Spiders from canopy and understorey foliage were tested in a laboratory for their propensity to balloon. General linear models indicated that small sized web-building spiders of the RTA and Orbicularia clades have the highest propensity to balloon. Small bodied species initiated ballooning regardless of the habitat they were collected in or their developmental stage. My results support the mixed evolutionarily stable strategy theory and indicate the absence of risk-spreading in the dispersal strategy of canopy spiders. My last chapter focused on dispersal capacity and diversity patterns of spiders at multiple spatial scales. Analyses of the species diversity of limited and high dispersal capacity species subsets through nested-multivariate ANOVA, additive diversity partitioning, and species-abundance distribution curves all point towards species-sorting processes as the main driver of local community spider diversity at the tree and stand spatial scales. Mass-effects and patch-dynamic processes drive site and regional scale diversity patterns. This thesis demonstrates that spiders provide good models to test many biological hypotheses. The research chapters of this thesis test hypotheses on the vertical stratification of forest spider diversity, the evolution of local dispersal adaptations, and the importance of dispersal capacity on species diversity patterns through a metacommunity framework.

The dynamics in Fagus-Acer forest have been frequently investigated, a particular interest being the replacement patterns and coexistence of the two dominant species, Fagus grandifolia Ehrh. and Acer saccharum Marsh. This thesis examines whether the community pattern and dynamics in a Fagus-Acer forest at Mont. St. Hilaire, Quebec, are consistent with predictions made by the disturbance hypothesis. This conceptual model explains the mechanisms underlying the coexistence and replacement patterns of the two dominant species. While the hypothesis suggests disturbance frequency and interspecific differences in growth under variable light conditions to be the key factors that determine the dynamics in Fagus-Acer forests, I demonstrate that edaphic factors and regeneration of Fagus by root sprouts can also play a significant role. Based on the findings, I propose an alternative model for the coexistence and replacement patterns in Fagus-Acer forest to better account for the influence of edaphic factors and of Fagus root sprouts.

This thesis used novel methodologies in dendrochemistry to observe past nutrient and Al change in relation to incremental stem xylem growth to predict current and future forest health. The methods included (1) sequential digestion of wood tissue to remove the elemental fraction that is mobile across tree rings leaving the structurally intrinsic, residual (or less mobile) ion fraction for analysis and (2) transformation of elemental concentrations into multivariate ratios (compositional nutrient diagnosis (CND)) over a time series. Sampling of trees represented a gradient in acidity resilience using three regions of southern Quebec (St. Lawrence Lowlands; Lower Laurentians; and Appalachian Highlands) and three species (red maple ( Acer rubrum L.); sugar maple (Acer sacharum Marsh.); and American beech (Fagus grandifolia Ehrh.). The elemental residual fraction had differences from the mobile fraction over time for Ca, Mg and Mn, but not for K or Al. The base rich Saint-Lawrence region had the highest and slightly increasing incremental stem xylem Al of the regions yet had stable or increasing growth. By contrast the acid sensitive Appalachian region had the greatest increase in Al accompanied by a decrease in growth beyond 1970. The Appalachians also had the highest Mn, which had an adverse effect on growth of sugar maple. The acid-resilient species American beech had stable or decreasing Al while having stable or increasing growth in contrast to the less resilient sugar and red maple. The nutrient poor Laurentian region had a persistent deficiency of K over time but no relationships with Al. Aluminium had a general negative correlation with the other canons. Although Mn had the highest levels in red maple for each region, it appears to be limiting growth. The changes in wood chemistry and growth over time appear to be driven by the resilience of the region or species to increasing acidic load in the ecosystems.

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L'association mycorhizienne implique nombre de plantes et de champignons, étant sans doute la symbiose mutualiste la plus importante et la plus répandue au sein des écosystèmes terrestres. Étant donné que la plupart des arbres forment des mycorhizes arbusculaires ou des ectomycorhizes qui se distinguent par leur écophysiologie, il est judicieux de caractériser les forêts en fonction de leur dominance mycorhizienne afin d'en mesurer les impacts sur les processus écologiques. Ainsi, l'objectif de cette thèse est de quantifier les influences de la dominance mycorhizienne en forêt sur les propriétés abiotiques et biotiques du sol ayant un impact à l'échelle locale sur deux processus associés : la décomposition de la matière organique et la régulation de la diversité végétale. Les forêts étudiées, de dominance mycorhizienne très contrastée, présentent des propriétés physico-chimiques et des communautés microbiennes distinctes au niveau du sol, mais des patrons de distribution verticale des microorganismes du sol d'une similarité inattendue. Dans ces forêts nordiques décidues, la décomposition de la matière organique est favorisée dans les couches supérieures du sol, notamment grâce à la présence du réseau fongique et d'autant plus lorsque les ectomycorhizes prédominent, ce qui prouve l'aspect déterminant du contexte local. L'établissement d'arbres mycorhiziens arbusculaires peut être limité par la combinaison des conditions abiotiques et biotiques édaphiques de la forêt boréale, qui est dominée par les ectomycorhizes, contrairement aux forêts à dominance partagée entre mycorhize arbusculaire et ectomycorhize, où la diversité est favorisée à l'échelle de la communauté. Cette thèse démontre le rôle déterminant, au niveau local, exercé par la dominance mycorhizienne sur les processus écologiques, et soulève l'importance de l'hétérogénéité biotique et abiotique du sol pour mieux saisir le fonctionnement des écosystèmes terrestres. / Mycorrhizas, which involve plants and fungi, are probably the most important and widespread mutual symbioses in terrestrial ecosystems. Since most trees form arbuscular mycorrhizas or ectomycorrhizas that are ecophysiologically distinct from each other, it is useful to characterize forests according to their mycorrhizal dominance in order to measure their respective impacts on ecological processes. The objective of this thesis is to quantify the impacts of forest mycorrhizal dominance on the abiotic and biotic properties of the soil, which influence at the local scale two associated processes: the decomposition of organic matter and the maintenance of plant diversity. The forests studied have opposite mycorrhizal dominance exhibit distinct soil physico-chemical properties and microbial communities, but more similar vertical distribution patterns of microorganisms than expected. Decomposition is favored by organic matter in the upper soil layers, but also by the presence of the fungal network, especially when ectomycorrhizas predominate, illustrating the importance of the local environmental context. Establishment of arbuscular mycorrhizal tree may be limited by the combination of abiotic and biotic edaphic factors of the boreal forest, which is ectomycorrhizal-dominated, in contrast to forests with shared dominance between arbuscular mycorrhizas and ectomycorrhizas, where tree species diversity is favored at the community level. This thesis demonstrates the decisive role, at the local scale, played by mycorrhizal dominance on ecological processes, and raises the importance of soil biotic and abiotic heterogeneity to better understand the functioning of terrestrial ecosystems.