Multi-element limitation: Simulation and measurements suggest that P is more limiting than n in young northern hardwood ecosystems

Background/Question/Methods

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. 

Results/Conclusions

The Multi-Element Limitation (MEL), which now includes P, light, and water as well as N and carbon, 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 observations in replicate young (26-30 years) and mature (>100 years) stands in the Bartlett Experimental Forest, New Hampshire, support these predictions.  Fine roots foraged preferentially for P in young stands and for N in mature stands, according to ingrowth into nutrient-amended cores.  Foliar retranslocation of P exceeded that of N in yellow birch and sugar maple, especially in young stands.   Experimental nutrient manipulations are planned to test the prediction that young stands are commonly more P limited and mature stands are more N limited in this widespread forest type.