Fire and vegetation effects on productivity and nitrogen cycling across a forest-grassland continuum

Mixed tree-grass vegetation is important globally at ecotones between grass lands and forests. To address uncertainties vis-g-vis productivity and nitr ogen (N) cycling in such systems we studied 20 mature oak savanna stands, r anging from 90% woody dominated to 80% herbaceous dominated, growing on com parable soils in a 32-yr-old fire frequency experiment in Minnesota, USA. F ire frequencies rang ed from almost annual burning to complete fire protect ion. Across all stands, aboveground net primary productivity (ANPP) ranged from 2 to 12 Mg(.)ha(-1.)yr(-1), decreased with fire frequency (r(2) = 0.59 ), increased with woody canopy dominance (r(2) = 0.83), and increased with soil net N mineralization rates (r(2) = 0.79), which varied from 25 to 150 kg(.)ha(-1.)yr(-1). ANPP was positively related to total biomass (r(2) = 0. 95), total canopy leaf N content (r(2) = 0.88), leaf area index (LAI; r(2) = 0.87), annual litterfall N cycling (r(2) = 0.70), foliage N concentration (r(2) = 0.62), and fine root N concentration (r(2) = 0.35), all of which a lso increased with increasing tree canopy cover. ANPP, soil N mineralizatio n, and estimated root turnover rates increased with woody canopy cover even for stands with similar fire frequency. ANPP and N mineralization both dec reased with fire frequency for stands having a comparable percentage of woo dy canopy cover. Fine root standing biomass increased with increasing grass dominance. However, fine root turnover rate estimated with a nitrogen budg et technique decreased proportionally more with increasing grass dominance, and hence fine root productivity decreased along the same gradient. Via several direct and indirect and mutually reinforcing (feedback) effects , the combination of low fire frequency and high tree dominance leads to hi gh 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.