Ca:Al Ratio Effects on Growth and Competitive Interactions of Northern Red Oak (Quercus rubra) and Yellow-Poplar (Liriodendron tulipifera)

in yellow-poplar; leaf [N] was significantly higher in red oaks grown at Ca:Al of 4 than 100. Yellow-poplar grew better in interspecific pairs than in intraspecific pairs. In contrast, competitive mode had no effect on red oak growth. Analysis of soil N pools indicated that red oak was unable to make use of the inorganic N added during biweekly nutrient solution additions. In contrast yellow-poplar was able to deplete soil N pools rapidly and its growth was probably N limited at Ca:Al ratio of 100. The critical Ca:Al threshold for growth decline appeared to be 4.0 for yellow-poplar. Yellow-poplar outcompetes red oak under N enriched conditions, but its competitive advantage decreases at lowered Ca:Al ratio. We hypothesize that the current shift in tree species composition observed in eastern North America may be the consequence of N enrichment, but that the direction of these shifts may reverse as lowered Ca:Al ratio and other effects of N saturation develop.