Interactive effects of fertilization and disturbance on community structure and resource availability in an old-field plant community

The interactive effects of fertilization and disturbance on plant community structure and resource availability were studied by supplying four levels of nitrogen and applying four intensities of tilling to a 30 year old field in a factorial design for 2 year. Live above-ground biomass, root biomass, and litter generally increased with nitrogen supply and decreased with disturbance. Species composition varied significantly, with annuals increasing with both nitrogen and disturbance, but with perennials unaffected by nitrogen and decreased by disturbance. Species diversity decreased with disturbance, but decreased with nitrogen only in undisturbed vegetation. Root: shoot ratios decreased with added nitrogen, leaf allocation decreased with disturbance, and flowering allocation increased. Surprisingly, stem allocation was unaffected by disturbance. This result reflected a shift from vertical stems to horizontal stems as disturbance increased. Resource measurements suggested that the vegetation responded to interactions between the treatments as well as to direct treatment effects. Variation in light penetration was reduced by fertilization in undisturbed vegetation but not in tilled plots; variability was not directly affected by disturbance. The availability of nitrogen, the limiting soil nutrient, increased with fertilization but was not significantly affected by disturbance. In contrast, the ratio of ammonium to nitrate was significantly reduced by disturbance but unaffected by supply rates, suggesting that nitrogen may have had different effects under different disturbance regimes, even though its total availability was constant. While many community responses to fertilization and disturbance conformed to those reported earlier, resource and allocation measurements indicated that their interactions are not always predictable from their separate effects.