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Mary Arthur


Forest floor calcium (Ca) declines in northern hardwood forests are of interest because Ca availability may limit future forest growth. In the 1990s investigations into Ca pools and fluxes across a northern hardwood forest chronosequence showed decreases across stand age in the mass of forest floor base cations and litterfall cation concentrations. I undertook a study of factors that influence litterfall chemistry to develop a better understanding of base cation biogeochemistry in developing northern hardwood forests. Although Ca, potassium (K), and magnesium (Mg) concentrations in litterfall significantly declined with stand age, species composition and soil nutrient availability also influenced litterfall chemistry. The interplay between these factors, such as the distribution of species with different nutrient demands in areas of base cation availability or limitation, influences forest floor chemistry and may impact the time in which soil Ca depletion occurs. Additionally, I examined the quantity and qualities (species composition, decay class, size, and nutrient concentrations and contents) of woody debris across the same northern hardwood forest chronosequence. The objectives of this study were to quantify cation pools in woody debris, describe how the qualities of these woody debris pools change with stand development, and establish a baseline for long-term measurements of woody debris accumulation and chemistry with stand age. Decomposition of woody debris transfers base cations to the forest floor, and may be partially responsible for forest floor Ca gains in young stands. Calcium and K pools in woody debris were most strongly influenced by woody debris biomass, whereas Mg content was influenced by both woody debris biomass and species composition. In young stands, woody debris pools were strongly influenced by management practices during harvest. By 20 years since harvest woody debris pools reflected processes of stand development. For example, from 20 to nearly 40 years since harvest woody debris biomass and species composition was dominated by mortality of early successional species. Woody debris in more mature stands reflected small-scale stochastic disturbances including disease and single-tree mortality. Litterfall and woody debris are each components of within-stand cation cycling that will be critical to future forest growth with limited Ca availability.