The phosphorus status of northern hardwoods differs by species but is unaffected by nitrogen fertilization

Northern hardwood forests in the eastern US exhibit species-specific influences on nitrogen (N) cycling, suggesting that their phosphorus (P) cycling characteristics may also vary by species. These characteristics are increasingly important to understand in light of evidence suggesting that atmospheric N deposition has increased N availability in the region, potentially leading to phosphorus limitation. We examined how P characteristics differ among tree species and whether these characteristics respond to simulated N deposition (fertilization). We added NH₄NO₃ fertilizer (50 kg ha^-1 year^-1) to single-species plots of red oak (Quercus rubra L.), sugar maple (Acer saccharum Marsh.), eastern hemlock (Tsuga canadensis (L.) Carr.), American beech (Fagus grandifolia Ehrh.), and yellow birch (Betula alleghaniensis Britt.), in the Catskill Mountains, New York from 1997 to 2007. Species differences were observed in foliar, litter and root P concentrations, but all were unaffected by a cumulative N fertilization of 550 kg/ha. Similarly, measures of soil P availability and biotic P sufficiency differed by species but were unaffected by N fertilization. Results suggest species exhibit unique relationships to P as well as N cycles. We found little evidence that N fertilization leads to increased P limitation in these northern hardwood forests. However, species such as sugar maple and red oak may be sufficient in P, whereas beech and hemlock may be less sufficient and therefore potentially more sensitive to future N-stimulated P limitation.