Established at the same time in 1982, Experiment 002 is a nitrogen addition experiment whose initial design was almost identical to Experiment 001. Like Experiment 001, and often in connection with it, Experiment 002 has been used to examine the effects of low-level nitrogen addition on nitrogen-limited grassland ecosystems. Unlike Experiment 001, however, the native vegetation in Experiment 002 was disturbed via thorough agricultural disking prior to the start of treatments. Also, the Experiment 002 plots were established in only three locations, all of which were successional grassland fields (54 plots each).
In each field, nine treatments are imposed on sets of replicate plots (54 plots per field). Two of the treatments are controls, neither of which receives ammonium nitrate fertilization, but one of which receives other nutrients (P, K, Ca, Mg and trace metals). The seven remaining treatments add ammonium nitrate at varying rates as well as the other nutrients. Plots have been sampled for above ground biomass (sorted by species) annually through 2004, and once subsequently in 2008. Plots are sampled intermittently for soil chemistry (NH4, NO3, Ca, Mg, P and K), belowground biomass, light penetration, and small mammal densities.
In 1986, aluminum flashing was buried between the individual plots to prevent plants from spreading by vegetative reproduction. Initially, the plot grids were enclosed by fencing to exclude large mammalian herbivores, however in 2004 this was removed. Gophers are trapped and removed.
In 1992, two modifications were made to the Experiment 002 treatment regime. In two of the three fields, cessation treatments were initiated with all nutrient addition being discontinued for half of the replicates. In the third field, fertilization treatments are ongoing, but half of the replicates are burned annually.
Within a decade after establishment and after undergoing rapid successional changes, plots that shared the same treatment in Experiment 001 and Experiment 002 had converged in composition, plant abundances (Fig 1; Inouye and Tilman 1995). While Experiment 002 plots were initially dominated by annual species, often not of native origin, these were quickly replaced by native perennials.
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Figure 1 Graphs of Percent Similarity (PS) vs. time for convergence within fields. Each set of eight graphs presents data comparing disturbed and undisturbed grids in one field, separated by nitrogen treatment (fertilization rates increase alphabetically). Small circular points represent individual comparisons of two plots of the same treatment, one plot on the undisturbed grid and one plot on the disturbed grid. Each graph contains36 such points for each year (six disturbed plots x six undisturbed plots). Stars indicate average PS for each year. For regression analyses, transormation were done on average PS (arcsine square-root) and (year - 1981).
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Figure. 2 Recovery of relative species number after cessation of nitrogen addition. Relative species number of all plots that continued to receive nitrogen (+N) and of those plots for which nitrogen addition ceased from 1991 and on (-N) is shown as the average across all nitrogen addition levels each year. There were no significant interactions between the rate of nitrogen addition and either year or the cessation treatment. (Clark and Tilman 2008) |
Inouye, R. S.; Tilman, D.; Convergence and divergence of old-field vegetation after 11 yr of nitrogen addition. Ecology 76:1872-1887. 1995 [Abstract] [Full Text] e002
Tilman, D.; Plant succession and gopher disturbance along an experimental gradient. Oecologia (Berlin) 60:285-292. 1983 [Abstract] [Full Text] e002
