Three sugarbushes were selected for the study of root distribution. Saint-Hippolyte was the best site for the distribution and production of fine roots. A high water-table and an abrupt change in texture limited the extension of the root system to the top 40 cm at the Vaudreuil site. In Tingwick, an acid mineral B horizon with a low Ca saturation restricted the production of fine roots in that horizon. / A sugarbush in Norbertville was used as a replacement for the Vaudreuil site for the study on root chemistry. The Saint-Hippolyte site had the best overall nutrient status. The molar ratio Ca/inorganic Al and Mg/inorganic Al in solution never reached values low enough to be considered toxic for the growth of fine roots. However, Al seems to increase the imbalance between Ca and K and may be responsible for the low foliar P found at Norbertville. Aluminium may also inhibit Ca uptake by roots due to very low Ca content in Tingwick.

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Sugar maple (Acer saccharum Marsh.) has experienced diebacks in many parts of its range; these declines have caused researchers to consider the many connected factors governing tree health. Altered availability of base cations due to acidic deposition and climatic extremes (drought, late winter freeze-thaw cycles) have been identified as two main causal stressors. To predict the role of climate on Ca nutrition, I investigated the relationship between the climatic factors of temperature, precipitation, water availability and windstorm on temporal changes to depth of Ca source of sugar maple. I hypothesized that the Ca uptake depth would be deeper after windstorms, or under dry or cold conditions, and shallower under warm or wet conditions. The ratio of Ca/Sr can be used to track the source of Ca. Calibration of the relationship between soil and plant Ca/Sr was done by analyzing Ca/Sr ratios in soil extracts of various strengths (H2O, NH4Cl, and ammonium EDTA) and in seedlings of sugar maple grown from natural regeneration on 37 differing sites. The relationships between soil extract Ca/Sr ratios and leaf and stem Ca/Sr ratios were linear, and the slopes of these relationships were different. These findings support the use of Ca/Sr as a tracer to Ca and they highlight the need to calibrate the technique for the plant tissue and the soil extractant used. This technique was then applied to study Ca uptake depth and its relationship with climate in the Hermine watershed, a sugar maple dominated forest in southern Quebec. The Ca/Sr was measured in soil, incremental cores of wood, and leaves. The ratio of Ca/Sr from nine soil profiles decreased with soil depth (r2 = 0.320, p<0.05), providing a signature for depth. Wood cores from ten trees were analysed in five or ten year increments from 1940 to 1999 to provide a long-term trend of Ca nutrition. Sugar maple leaves from 1995-2005 from the same 25 trees were analyzed to determine the annual dynamics of Ca uptake depth and its relationship with climate. Results suggest that the majority of tree Ca is taken from the lowest part of the rooting zone, regardless of the climatic conditions that occurred during this study, although the minor variations were related to the April temperature (r2 = 0.208, p = 0.023) and the June, July and August combined total precipitation (r2 = 0.581, p = 0.006). The effect of the windstorm on Ca uptake depth was only apparent in the area of the forest most visibly affected; Ca uptake was deeper.

Eight fertilization treatments were applied in May 1987 to two sugar maple stands. This was followed by soil and foliar sampling and decline evaluation in mid-summer 1987 and foliar sampling and decline evaluation in mid-summer 1988. / Trees selected for sampling purposes on average showed evidence of light to moderate decline. Foliar nutrient status was found to be poor at both sites, with Ca, Mg, K and P at or very near to deficiency levels although foliar molar ratios for Ca/K, Ca/Mg and Ca/Al were well within their respective critical ranges. / Fertilization had significant effects on several elemental concentrations of the soil and foliage at the Entrelacs sites. Effects in general showed an increase in base cation concentrations when those elements were supplied in high enough quantities in the fertilizers. Fertilization had no significant effect on decline levels. / Positive correlations were obtained between soil B horizon levels and foliar Ca, Mg and K and between soil FH horizon levels and foliar K and Al.

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Two studies were carried out in the Morgan Arboretum of McGill University to explore the agroforestry potential of some native tree species of Southern Quebec. In the first study, soil chemical characteristics under basswood (Tilia americana L.), white ash (Fraxinus americana L.) and bitternut hickory (Carya cordiformis Wang. K. Koch) in relation to sugar maple (Acer saccharum Marsh.) were assessed in natural stands. Soil pH was highest under white ash and was lowest under bitternut hickory. Soil NO$ sb3 sp-$ was low under basswood compared to white ash. Soil pH and exchangeable soil Ca$ sp{2+}$ and Mn$ sp{2+}$ decreased with an increase in basal area and exchangeable soil K$ sp+$ decreased (p = 0.07) with an increase in the proportion of sugar maple relative to total basal area. / In the second study, the effects of black walnut (Juglans nigra L.) on growth and nutrient content of lettuce (Lactuca sativa L.), kale (Brussica oleracea L.), parsley (Petroselinum crispum) and Swiss chard (Beta vulgaris L.) in an alley cropping system were determined. The experiment was carried out in a randomized complete block design with repeated measures. All vegetables survived in the black walnut plantation and in the open field. The order of sensitivity to growth under black walnut was Swiss chard $>$ kale $>$ lettuce $>$ parsley. Low light intensity in the plantation likely decreased plant dry weights and nutrient content. Generally plant N, P and K concentrations were higher in the plantation while Ca, Mg and Mn concentrations were higher in the open field, possibly due to an antagonistic effect of high soil K$ sp+$ content in the plantation and to inhibitory effects of black walnut.

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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.