Influence of shrub encroachment on aboveground net primary productivity and carbon and nitrogen pools in a mesic grassland

The clonal shrub Cornus drummondii C.A. Mey. is rapidly increasing in cover and displacing mesic grassland species in the central USA as a consequence of fire suppression. We assessed the impact of C. drummondii on carbon (C) and nitrogen (N) pools and C fluxes in a tallgrass prairie in eastern Kansas, USA, through a comparison of both burned and unburned C. drummondii islands with open grassland areas. Allometric equations relating C. drummondii foliage and wood biomass to basal stem diameter were developed to estimate aboveground biomass and net primary productivity (ANPP) of C. drummondii. Within C. drummondii islands, ANPP was 496 ± 45 g C·m–2·year–1, nearly three times that within open grassland (167 ± 13 g C·m–2·year–1). As a result of greater aboveground biomass, aboveground C and N storage within shrub islands (3270 ± 466 g C·m–2, 37.9 ± 5.3 g N·m–2) was substantially greater than that within open grassland (241 ± 33 g C·m–2, 6.1 ± 0.8 g N·m–2). No change in soil organic C or total N to 10-cm depth was evident; however, soil CO2 flux was significantly reduced in C. drummondii islands relative to the open grassland. The storage of C in aboveground biomass of C. drummondii represents a significant short-term increase in C storage relative to open grassland. However, potential alterations in belowground processes must be quantified before the long-term net effect of shrub encroachment on C and N pools within this mesic grassland can be determined.