It is commonly assumed that nitrogen (N) is the primary soil resource limiting the growth and productivity of temperate forests throughout northeastern North America.  Sustained inputs of N to forest ecosystems via atmospheric deposition is altering the N status of these ecosystems raising the possibility that other soil resources such as phosphorus (P) are increasingly limiting to tree growth and forest productivity.  The objective of this study was to determine whether nutrient limitation of tree growth changed from limitation by N to limitation by N and P, or P alone.  This study was conducted in two forest types common throughout the southern New England landscape, in sugar maple and white ash dominated stands growing on base rich parent material characterized by high rates of N cycling and high N availability, and red oak-beech-hemlock dominated stands growing on base-poor parent material characterized by slower rates of N cycling and lower N availability.  In 2003, N and P were added to replicate plots in each forest type in factorial combination at a rate of 150 and 50 kg ha-1 yr-1, respectively.  Diameter growth rates of all trees >10cm DBH were measured for two years using dendrometer bands. 
Results/Conclusions

After two years, diameter growth in the sugar maple-white ash forests remained strongly N limited.  There was no effect of P addition alone or in combination with N on diameter growth in these species.  In the red oak-beech-hemlock stands diameter growth in some species was stimulated by fertilization with N alone, while other species were unaffected by fertilization with either N or P.  The results of this study demonstrate that tree growth remains primarily N limited in southern New England, and that decades of atmospheric N deposition has not (yet) resulted in measurable P limitation.