Mixed tree-grass vegetation is important globally at ecotones between grass- lands and forests. To address uncertainties vis-a`-vis productivity and nitrogen (N) cycling in such systems we studied 20 mature oak savanna stands, ranging from 90% woody dominated to 80% herbaceous dominated, growing on comparable soils in a 32-yr-old fire frequency experiment in Minnesota, USA. Fire frequencies ranged from almost annual burning to complete fire protection. Across all stands, aboveground net primary productivity (ANPP) ranged from 2 to 12 Mg·ha 21 ·yr 21 , decreased with fire frequency (r 2 5 0.59), increased with woody canopy dominance (r 2 5 0.83), and increased with soil net N min- eralization rates (r 2 5 0.79), which varied from 25 to 150 kg·ha 21 ·yr 21 . ANPP was positively related to total biomass (r 2 5 0.95), total canopy leaf N content (r 2 5 0.88), leaf area index (LAI; r 2 5 0.87), annual litterfall N cycling ( r 2 5 0.70), foliage N concentration (r 2 5 0.62), and fine root N concentration (r 2 5 0.35), all of which also increased with increasing tree canopy cover. ANPP, soil N mineralization, and estimated root turnover rates increased with woody canopy cover even for stands with similar fire frequency. ANPP and N min- eralization both decreased with fire frequency for stands having a comparable percentage of woody canopy cover. Fine root standing biomass increased with increasing grass dom- inance. However, fine root turnover rate estimated with a nitrogen budget technique de- creased proportionally more with increasing grass dominance, and hence fine root produc- tivity decreased along the same gradient. Via several direct and indirect and mutually reinforcing (feedback) effects, the com- bination of low fire frequency and high tree dominance leads to high rates of N cycling, LAI, and productivity; while the opposite, high fire frequency and high grass dominance, leads to low rates of N cycling, LAI, and productivity. Carbon and N cycling were tightly coupled across the fire frequency and vegetation type gradients.