Successional changes in nitrogen availability as a potential factor contributing to spruce declines in boreal North America

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Abstract
This paper examines the hypothesis that the depression of soil nitrogen availability by litter from black and white spruce (Piceamariana and P. glauca), which is low in nitrogen and high in recalcitrant compounds, may lead directly to spruce decline or predispose the forest to dieback from other factors. A set of model simulations based on principles and information developed independently of current episodes of spruce decline demonstrates the importance of this mechanism to cycles of boreal forest declines. The model also shows that if litter of red spruce (P. rubens) has a similar effect on the nitrogen cycle, then the effect of acid deposition on red spruce decline must be considered in light of this intrinsic property of spruce ecosystems. Because of the confounded nature of ecosystem variables, a neutral modeling approach is a necessary framework for exploring the combined effects of nutrients, drought, changing climate, and pollution on forest diebacks, for directing site selection and field measurements, and for synthesizing results of field and laboratory experiments in an ecosystem context.