Is P More Limiting Than N in Young Northern Hardwood Forests?.

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 extended the Multi-Element Limitation (MEL) model to include P, light, and water as well as N and carbon, and we applied it to simulate secondary succession in northern hardwood forests.  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. To test for differences in N and P availability and acquisition as a function of forest age, we made field measurements in replicate young (26-30 years) and mature (>100 years) stands in the Bartlett Experimental Forest, New Hampshire.  Foliar retranslocation of P exceeded that of N in yellow birch and sugar maple, especially in young stands.  Phosphatase activity was higher in young forests than in mature forests, and fine roots foraged preferentially for P in young forests and for N in mature forests. Net N mineralization rates were higher in 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.  Microbial N:P ratios in the mineral soil in both old and young forests averaged 29:1, suggesting P limitation to microorganisms that may cause competition for P with plants.  These results indicate that P could now be more limiting than N especially in young stands.