Nutrient dynamics in sugar maple forests and assessment of long-term sensitivity to nitrogen deposition

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Globally, humans have altered the nitrogen (N) cycle to a greater degree than the much more publicized carbon (C) cycle. In Canada, the highest N deposition occurs in southern Ontario, where forest cover is dominated by sugar maple; a species known to be sensitive to enhanced N deposition. Potential impacts of high N deposition on other ecosystem components (lichens, herbs) in southern Ontario are unknown, but current N deposition levels are within the range that has been shown to affect plant and lichen species in the UK and western USA. The effect of N enrichment on forests, however, is not straight forward, and possible outcomes including enhanced nitrate leaching (acidification), declines in soil respiration and a reduction in fine root density may cause deficiencies in other mineral nutrients. In order to estimate the threshold deposition input above which adverse impacts begin to occur, the critical load (CL) for nutrient-N can be determined. There are two approaches for estimating the critical load of N; an empirical approach that links measured deposition with an observed impact, or a process-based approach, which takes into account soil and plant processes, enabling an assessment of long-term chronic N accumulation in soils. These models are appealing to policy makers as they provide an estimate of the 'time to effect', and are thus more readily linked to emission targets. The objective of this work is to undertake an integrated assessment of the impacts of current and predicted N deposition on hardwood forests, with a view to establishing appropriate critical N loads to protect these ecosystems. This will be achieved through six complimentary research projects that investigate N impacts on 1) lichens; 2) herbaceous species; 3) sugar maple chemistry; 4) sugar maple growth and N cycling; 5) biogeochemical cycling and soil respiration and 6) an integrated assessment of the current and predicted impacts of N deposition through biogeochemical modeling. This study will result in the training of six graduate students and at least 4 undergraduate students and will provide the sound scientific basis that is necessary to aid policy decisions with respect to future N emissions in Canada.