CA-AL RATIO EFFECTS ON GROWTH AND COMPETITIVE INTERACTIONS OF NORTHERN RED OAK (QUERCUS-RUBRA) AND YELLOW-POPLAR (LIRIODENDRON-TULIPIFERA)

To contribute to the understanding of the long-term effects of atmosph eric deposition on forests of eastern North America, we conducted a se t of greenhouse experiments designed to determine the effect of reduce d soil Ca:Al ratio on growth and competitive interactions of two commo n, co-occuring tree species. Second year seedlings of northern red oak (Quercus rubra), an ectomycorrhizal species currently declining in ab undance in eastern North America, and yellow-poplar (Liriodendron tuli pifera), an arbuscular mycorrhizal species currently increasing in abu ndance, were grown in intraspecific or interspecific pairs in either f ield soil or silica sand, and at Ca:Al ratio of 4 or 100. Overall rela tive growth rates (RGR) of the two species were similar; however, red oak allocated more new biomass to stem wood while yellow-poplar alloca ted biomass to leaves and roots. Reducing the Ca:Al ratio from 100 to 4 had no major effect on red oak growth but reduced RGR, leaf producti on, and N uptake significantly in yellow-poplar; leaf [N] was signific antly higher in red oaks grown at Ca:Al of 4 than 100. Yellow-poplar g rew better in interspecific pairs than in intraspecific pairs. In cont rast, competitive mode had no effect on red oak growth. Analysis of so il N pools indicated that red oak was unable to make use of the inorga nic N added during biweekly nutrient solution additions. In contrast y ellow-poplar was able to deplete soil N pools rapidly and its growth w as probably N limited at Ca:Al ratio of 100. The critical Ca:Al thresh old for growth decline appeared to be <4.0 for red oak and >4.0 for ye llow-poplar. Yellow-poplar outcompetes red oak under N enriched condit ions, but its competitive advantage decreases at lowered Ca:Al ratio. We hypothesize that the current shift in tree species composition obse rved in eastern North America may be the consequence of N enrichment, but that the direction of these shifts may reverse as lowered Ca:Al ra tio and other effects of N saturation develop.