Dynamics of Nitrogen Mineralization and Nitrification in Hemlock and Hardwood Treefall Gaps

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Abstract
In this study I have attempted to integrate two major topics: (1) the role of disturbance regimes in structuring communities, and (2) the effects of various disturbances on ecosystem—level processes such as nitrogen cycling. In an old—growth, northern hard—wood—hemlock forest in upper Michigan, USA, I measured net N (nitrogen) mineralization rates in small (90—250 m2), recent (0—3 yr) treefall gaps. These gaps were paired with plots in adjacent, undisturbed forest; half of the plots were dominated by eastern hemlock and half by sugar maple. Net N mineralization was measured using monthly on—site soil incubations. I made corresponding measures of other soil characteristics, moisture, and canopy opening (square metres) in each plot. The nitrogen mineralization rate varied with physical differences that existed between forest plots and gap plots, as well as with canopy species dominance. In gaps, N mineralization was higher under hemlock (forest, X = 95.3; gap, X = 107.3 kg°ha—1°yr—1) and lower under maple (forest, X = 102.0; gap, X = 88.5 kg°ha—1°yr—1). Nitrification of N was more than double in gaps under hemlock (forest, X = 27.4; gap, X = 69.8 kg/ha). Nitrification and soil pH were positively correlated in forest plots. I discuss the implications of these results for community compositional change and relevance to current hypotheses. These include hypotheses of mineralization rate change with disturbance, the role of nitrification in climax communities, and the influence of conifer canopy and litter on forest floor processes. Observed differences in these factors at this small—scale, single treefall level are compared with other large—scale whole—ecosystem studies. Implications for species adapted to this disturbance pattern are suggested. I hypothesize an interrelationship for the various observed changes in gaps and a mechanism for the exploitation of these increased resources by plants