Although temperate forests have long been thought to be primarily nitrogen limited, resource optimization theory suggests that ecosystem productivity should be co-limited by multiple nutrients. In northeastern North America, air pollution and forest harvesting disturbance elevate N availability and contribute to the likelihood of P limitation. We explored the relative limitation by N and P in northern hardwood forests by simulating productivity with the Multiple Element Limitation Model (MEL) and by testing several indices of nutrient availability and acquisition, in replicate young (26-30 years) and mature (>100 years) forests in the Bartlett Experimental Forest (BEF), NH. The model predicted a greater response of aboveground productivity to N+P than N or P alone. In older stands, MEL predicted a greater response to N than to P addition, but in younger stands, the supply of N from detritus was predicted to be sufficient to create P limitation. Field measurements were consistent with this pattern of limitation. Fine roots of trees foraged preferentially for P in young forests and for N in mature forests. Phosphatase activity and net N mineralization rates were higher in soils of young than mature forests. Resin-available P did not differ between young and mature forests, but lower bicarbonate-extractable P in young forests suggests transfer of organic P to available pools, consistent with the idea of higher P mobilization in young forests. Foliar retranslocation was higher for P than for N in two of three tree species, and microbial N:P ratios averaged 29:1 in the mineral soil, suggesting P limitation to microorganisms that may cause competition for P with plants. Our results suggest that P could now be more limiting than N especially in young northern hardwood forests.