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Two studies were conducted to assess physiological responses of declining sugar maple (Acer saccharum Marsh.) in southern Quebec to wet acidic precipitation and increased soil fertility. Leaves of sugar maple were relatively acidic, had low external neutralization capacity (ENC = 5.1 $ mu$equiv. H$ sp{+} rm g sp{-1}$) and buffering capacity index (BCI = 103 $ mu$equiv. H$ sp{+} rm g sp{-1}$), and higher ENC/BCI compared to three other tree species. Deciduous species showed a higher BCI and a lower ENC in August. Our results suggest that sugar maple may be relatively sensitive to wet acidic deposition and that leaf buffering capacity is related to photosynthesis potential and seasonal change in foliar Ca concentration. Foliar analyses revealed that fertilization with a mixture of K, Ca, and Mg had significant effects on leaf concentrations of K, free amino acids (FAA) and the ratios of N to K, and FAA to N after two growing seasons. Our results suggest that base cation fertilization improved the N and K status of sugar maple and that leaf carbon partitioning appeared to be more dynamic in fertilized trees.

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.

Optimum nutritional levels for most commercial hardwoods of eastern Canada are unknown. This thesis dealt with the development of a method to determine nutritional standards using within site variation in tree growth and foliar chemistry. To this end, sugar maple (n = 87) and red maple (n = 39) trees were sampled in summer 2001 at the Station de biologie des Laurentides. Leaves were sampled for nutrients (N, P, K, Ca, Mg and Mn) and tree stems were measured for determination of basal area growth (BAG). Similar measurements for trees sampled annually during 1995--2001 were also used to measure the effect of annual variation on nutritional standards. A boundary line approach was used to assess tree growth response to nutrition using nutrient concentrations and Compositional Nutrient Diagnosis (CND) scores as predicting variables. A Basal Area Growth Index (BAGI) was computed using the live crown ratio to correct for the effect of stand density on BAG. An iterative and unbiased protocol was also developed to eliminate outliers. Optimum, critical and optimal range levels were derived from quadratic models significant at P < 0.15.

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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.

The study of biogeochemical cycles and their role in ecosystem function has helped to highlight the impacts of human activities on natural processes. However, our understanding of the effects of nitrogen (N) deposition on forested ecosystems remains limited due to the variable controls on N cycling. Soils, microclimate, and vegetation can influence rates and processes of N cycling, singly or in concert at multiple scales. Understanding how these factors influence N cycling across the landscape will help to elucidate the impacts of N deposition. The objectives of this study were to characterize variation in soils, microclimate and vegetation characteristics, and N cycling and decomposition dynamics across the landscape in a region impacted by N deposition. Relationships among these factors were explored to determine the main factors influencing N cycling and decomposition. Strong differences in net N mineralization and nitrification were found between forest stands with contrasting species composition and moisture availability. Nitrate production and leaching were related to sugar maple abundance, and base cation leaching was correlated with nitrate concentrations in soil solutions. Decomposition experiments were installed to examine the effects of substrate quality, microclimate and N availability on decay rates. Nitrogen amendments for the most part did not affect decomposition rates of wood and cellulose, and mass loss rates were correlated with microclimate and forest floor characteristics. In contrast, microclimate did not seem to affect leaf litter decay rates, and the results suggest that the presence of invertebrates may influence mass loss to a greater degree than moisture or litter quality. This work highlights the large degree of variability in N processing across the landscape and suggests that differences in microclimate and species composition may help to predict the impacts of chronic N deposition on N cycling and retention.