Aboveground productivity and biodiversity in the longleaf pine ecosystem: Long-term landscape-scale resource manipulations

This long-term study uses resource manipulations (i.e., water and nutrients) across natural soil moisture gradients, to examine mechanisms controlling productivity and species richness in the fire-maintained longleaf pine-wiregrass ecosystem.  We applied a 2 x 2 factorial design of nitrogen addition and irrigation at a landscape scale.  Thirty-two 0.5 ha treatment plots were established, 16 on xeric sites and 16 on mesic sites.  Treatments were randomly assigned to each plot.  Nitrogen was added at a rate of 50 kg ha^-1, soil moisture was maintained at 40% field capacity by irrigation.  All plots were burned with prescribed fire with a 2 year return interval. Annual aboveground net primary productivity was determined by understory clip plots and measurement of overstory tree dbh.  Soil moisture was monitored monthly using soil TDR measurements.  Nitrogen availability was estimated with in situ soil core incubations of the mineral soil.  Species richness was sampled in nested quadrats and species-area curves were developed.  Difference in understory biomass varied among guilds (p< .00001).  Available nitrogen differed among treatments on xeric sites (p=0.0239).  Soil moisture was different on xeric sites only and varied among treatments (p=0.0054). Species richness increased due to the fertilization/irrigation treatment on xeric sites and was spatially dependent (p = 0.0073).