THE PRODUCTIVITY OF TEMPERATE FORESTS ON GLACIATED SOILS HAS BEEN PRESUMED TO BE NITROGEN (N) LIMITED, ALTHOUGH THEORY SUGGESTS THAT ECOSYSTEM PRODUCTIVITY SHOULD BE CO-LIMITED BY MULTIPLE NUTRIENTS, ESPECIALLY INCLUDING PHOSPHORUS (P). EXPERIMENTAL TESTS OF N VS. P LIMITATION IN TEMPERATE FORESTS ARE NEEDED TO BETTER UNDERSTAND THE CAPACITY OF ECOSYSTEMS TO BALANCE THE ACQUISITION OF LIMITING RESOURCES. IN 2011 WE BEGAN A FULL-FACTORIAL NXP FERTILIZATION EXPERIMENT IN 13 STANDS OF YOUNG, MID-AGED, AND MATURE NORTHERN HARDWOODS IN THREE CONTRASTING SITES THAT SPAN A RANGE OF NATIVE SOIL FERTILITY. SURPRISINGLY, EARLY RESULTS SHOWED TREE GROWTH RESPONDING MORE TO P THAN TO N FERTILIZATION. WE PROPOSE TO CONTINUE THESE TREATMENTS TO TEST WHETHER P LIMITATION WILL PERSIST, POSSIBLY AS A CONSEQUENCE OF DECADES OF ANTHROPOGENIC N ENRICHMENT, OR WHETHER ADJUSTMENT OF EFFORT TO ACQUIRE N VERSUS P WILL RESULT IN CO-LIMITATION. WE WILL MEASURE ABOVEGROUND AND BELOWGROUND PRODUCTIVITY AND EVALUATE SHIFTS IN ALLOCATION OF EFFORT AFTER A DECADE OF NUTRIENT MANIPULATION. MECHANISMS THAT COULD ENABLE THE ECOSYSTEM TO ACQUIRE AND CONSERVE N AND P AND TO MAINTAIN STOICHIOMETRIC BALANCE IN THE FACE OF CHANGING NUTRIENT AVAILABILITY INCLUDE FOLIAR N AND P RESORPTION, DIFFERENTIAL ROOTING DEPTH, SOIL ENZYMES, MYCORRHIZAE AND CHANGES IN TREE SPECIES COMPOSITION. THE MULTIPLE ELEMENT LIMITATION MODEL WILL BE USED TO INTEGRATE DATA FROM THE VARIOUS COMPONENTS OF THE STUDY, ADVANCE UNDERSTANDING OF NUTRIENT LIMITATION, AND EXTRAPOLATE THE RESULTS TO IMPROVE MANAGEMENT OF FOREST ECOSYSTEMS IN THE FACE OF INCREASING CO2, CHANGING CLIMATE, AND A LEGACY OF ATMOSPHERIC DEPOSITION.