In 1982, Experiment 001 was established by David Tilman to examine the long-term effects of low-level nitrogen addition on undisturbed nitrogen-limited grassland ecosystems. For 20 years plots were sampled annually for above-ground biomass (sorted by species). However, because plot data have shown few changes in recent years, and because treatment responses have converged among the various fields, sampling is now periodic and a new treatment has been imposed in one of the fields. Soil chemistry (NH4, NO3, Ca, Mg, P, and K), belowground biomass, insect abundances, light penetration, and small mammal densities have been sampled intermittently.
The 207 experimental plots are divided among three successional grassland fields (Fields A, B and C - 54 plots each) and a savanna prairie opening (Field D - 45 plots). In each locale, nine treatments are imposed on sets of replicate plots. 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. Starting in 2005, the successional grassland plots have been burned annually. The savanna plots have been burned every two out of three since 1987.
In 1986, aluminum flashing was buried between the individual plots to prevent plants from spreading by vegetative reproduction. In 1982, the plot grids were enclosed by fencing to exclude large mammalian herbivores, however in 2004 this was removed. At the same time, in one of the successional grasslands, half of the replicate plots were individually re-fenced. Gophers are trapped and removed from all experiment plots.
Experiment 001 has provided insights into causes of successional dynamics (e.g., Tilman 1987 , 1988, 1990 ), effects of N deposition on C and N storage (Wedin and Tilman 1996), causes of diversity differences along productivity gradients (Tilman 1990 , 1993 , 1996a), impacts of diversity on ecosystem stability (Tilman and Downing 1994, Tilman 1996a, 1999a, Tilman et al. 1998, Lehman and Tilman, 2000), impacts of climatic variation on biodiversity (Tilman and El Haddi 1992, Tilman 1996a), and long-term dynamics after a major drought (Haddad et al. 2002).
Experiment 001 was the first multi-decadal experiment to examine the impacts of chronic, experimental nitrogen addition as low as 10 kg of N per hectare per year above ambient atmospheric nitrogen deposition (6 kg of N per hectare per year). This total input rate is comparable to terrestrial nitrogen deposition in many industrialized nations. Chronic low-level nitrogen addition rates were found to reduce plant species numbers by 17% relative to controls receiving ambient N deposition (Fig. 1, Clark and Tilman 2008). Moreover, species number were reduced more per unit of added nitrogen at lower addition rates, suggesting that chronic but low-level nitrogen deposition may have a greater impact on diversity than previously thought (Fig. 2, Clark and Tilman 2008).
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Figure 1. Relative species number versus time. The treatment-specific average annual relative species numbers (+/- one s.e.m.) through time averaged over the three fields are shown. Dashed lines correspond to annual standard errors in control plots, and arrows indicate the year of first significant (P<0.01) detection of relative species loss for a particular nitrogen addition treatment rate using MANOVA over three-year intervals (middle year highlighted). For clarity, only three of five nitrogen addition treatments are shown. (Clark and Tilman, 2008) |
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Figure 2. Proportional species loss versus nitrogen input rate for (a) 2002-2004 and (b) 1983-1985. Plot averages for each field over the three year-period fitted to a logarithmic curve excluding controls (95% confidence curves included). P values correspond to the significance of the nitrogen input term (N input = experimental N addition + atmospheric N deposition) in a model of the proportional loss of species regressed on the natural logarithm of the nitrogen input rate, Field, and their interaction. Dashed lines correspond to linear interpolation between the mean effect at the highest nitrogen addition rate and controls. (Clark and Tilman 2008) |
The resistance of total plant biomass to drought was significantly greater at higher diversity in Experiment 001 plots, even after controlling for numerous potentially confounding variables (Fig. 4; Tilman and Downing 1994). For all non-drought years, interannual variation in total plant biomass was greater at lower diversity (Fig. 5; Tilman 1996, 1999). Results of both papers support the diversity-stability hypothesis.
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Figure 4. Relationship between drought resistance of grassland plots and plant species richness (SR86) preceding a severe drought. Mean, standard error and number of plots with a given species richness are shown. Drought resistance was measured as dB/Bdt (per year), that is, as 0.5 (ln[biomass(1988)/biomass(1986)]; left had scale. The right-hand scale shows the proportionate decrease in plant biomass associated with the dB/Bdt values. (Tilman and Downing 1994) |
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Fig 5. The dependence of the coefficient of variation (cv) of total community biomass on plant species diversity, based on the data for Experiment 001. The cv measures the extent of year-to-year variation in total plant biomass within a plot (relative to mean biomass). The lower coefficients of variation of the more diverse plots mean that total community biomass is stabilized by diversity. As shown in Tilman (1996), abundances of individual species are destabilized by diversity. Coefficients of variation of each field were adjusted for differences in intercepts as determined by a GLM regression. (Tilman 1999) |
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Figure 6. The dynamics of total plant biomass before and after drought in Experiment 001. (Haddad et al. 2002) |
Nitrogen addition impacts ecosystem carbon and nitrogen stores in Experiment 001 via effects on species composition and thus on litter C:N (Wedin and Tilman 1996). At higher N addition, diversity is lower, C4 grasses are less abundant, and litter and root C:N ratios are lower (Fig. 7A-D). The nitrogen-dependent shift to low C:N species corresponds with decreased ecosystem retention of added nitrogen (Fig. 8A) and to lower carbon storage (Fig. 8B), likely because of immobilization and decomposition effects (Fig. 8C).
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Figure 7. Ecosystem C and N and vegetation responses to 12 years of N addition in Experiment 001. Points represent treatment means (6 replicates per N addition level, 12 for controls) for each of three fields. (Wedin and Tilman 1996) |
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Figure 8. (A) Nitrogen dynamics after 12 years of N addition. Net N retention after 12 years estimated as the change in total system N (relative to controls) divided by the sum of experimental N additions. (B) Net C storage per unit experimentally added N after 12 years. Because C storage rates (g C/g N) did not differ significantly between Fields B and C (34), overall treatment means for the two C4-dominated fields are presented. (C) The relationship between soil NO3- and the C:N ratio of plant biomass (aboveground dead biomass plus belowground biomass). Vertical line represents a biomass C:N ratio of 32. (Wedin and Tilman 1996) |
Shifts in species abundances in response to nitrogen addition did not support predictions of Hubbell's (2001) neutral theory (Fig. 9; Harpole and Tilman 2006).
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Figure 9. Relationship between R* and abundance changes along an experimental N addition gradient: species ranking switches from dominance of good N competitors (low R* values) to poor N competitors (high R* values) with increasing rates of N addition. Each point is the treatment-level mean correlation of R* indexes with species abundances. Dashed lines indicate the bootstrapped mean (short dash) and 95% confidence region. |
Data from this experiment have also been extensively shared with and used by non-CDR researchers, and been included in a variety of publications (Gough et al. 2000; Gross et al. 2000; Knapp and Smith 2001; Johnson et al. 2003ab, 2005; Pennings et al. 2005; Suding et al. 2005)
Clark, C.M.; Cleland, E.E.; Collins, S.L.; Fargione, J.E.; Gough, L.; Gross, K.L.; Pennings, S.C.; Suding, K.N.; Grace, J.B.; Environmental and plant community determinants of species loss following nitrogen enrichment. ECOLOGY LETTERS 10:596-607. 2007 [Abstract] [Full Text] e001
Gough, L.; Osenberg, C. W.; Gross, K. L.; Collins, S. L.; Fertilization effects on species density and primary productivity in herbaceous plant communities. Oikos 89(3):428-439. 2000 [Full Text] e001
Haddad, N. M.; Haarstad, J.; Tilman, D.; The effects of long-term nitrogen loading on grassland insect communities. Oecologia 124:73-84. 2000 [Full Text] e001
Hobbie, S. E.; Nitrogen Effects on Decomposition: A five-year experiment in eight temperate sites, Ecology, 89(9), 2008, pp. 2633-2644 2008 by the Ecological Society of America 2008 [Abstract] [Full Text] e001
Johnson, N. C.; Tilman, D.; Wedin, D.; Plant and soil controls on mycorrhizal fungal communities. Ecology 73(6):2034-2042. 1992 [Abstract] [Full Text] e001
Kitajima, K.; Tilman, D.; Seed banks and seedling establishment on an experimental productivity gradient. Oikos 76:381-391. 1996 [Abstract] [Full Text] e001
Knops, J. M. H.; Lehman, C. L.; "Modeling range expansions in biological invasions. Review of Shigesada, N. and Kawasaki, K., Biological Invasions: Theory and Practice. Oxford University Press, New York, 205 pp. Ecology 79:2578" 1998 [Full Text] e001
Knops, J. M. H.; Ritchie, M. E.; Tilman, D.; Selective herbivory on a nitrogen fixing legume (Lathyrus venosus) influences productivity and ecosystem nitrogen pools in an oak savanna. Ecoscience 7(2):166-174. 2000 [Full Text] e001
Pennings, S.; Clark, C. M.; Cleland, E. E.; Collins, S. L.; Gough, L.; Gross, K. L.; Milchunas, D. G.; Suding, K. N.; Do individual plant species show predictable responses to nitrogen addition across multiple experiments? Oikos 110:547-555. 2005 [Full Text] e001
Ritchie, M.; Tilman, D.; Knops, J. M. H.; Herbivore effects on plant and nitrogen dynamics in oak savanna. Ecology 79:165-177. 1998 [Abstract] [Full Text] e001
Ritchie, M.; Tilman, D.; Responses of legumes to herbivores and nutrients during succession on a nitrogen-poor soil. Ecology 76(8):2648-2655. 1995 [Abstract] [Full Text] e001
Smith, V.; Tilman, G. D.; Nekola, J. C.; "Eutrophication: impacts of excess nutrient inputs on freshwater, marine, and terrestrial ecosystems. Environmental Pollution 100:179-196." 1999 [Abstract] [Full Text] e001
Spotswood, E.; Bradley, K. L.; Knops, J. M. H.; Effects of herbivory on the reproductive effort of 4 prairie perennials. BMC Ecology 2:2. 2002 [Full Text] e001
Suding, K.; Collins, S. L.; Gough, L.; Clark, C.; Cleland, E. E.; Gross, K. L.; Milchunas, D. G.; Pennings, S.; Functional- and abundance-based mechanisms explain diversity loss due to N fertilization. Proceedings of the National Academy of Sciences 102:4387-4392. 2005 [Full Text] e001
Tilman, D.; "Plant Strategies and the Dynamics and Structure of Plant Communities. Monographs in Population Biology, Princeton University Press. 360 pp. (Includes a 61 page chapter on Cedar Creek LTER, and new theory relevant to Cedar Creek and cross-site comparisons.)" 1988 e001
Tilman, D.; "Carbon dioxide limitation and potential direct effects of its accumulation on plant communities. Pages 333-346 in Kareiva, P. M., J. G. Kingsolver, and R. B. Juey, Eds., Biotic Interactions and Global Change, Sinauer Associates, Sunderland, MA." 1993 [Abstract] [Full Text] e001
Tilman, D.; A consumer-resource approach to community structure. American Zoologist 26:5-22. 1986 [Abstract] [Full Text] e001
Tilman, D.; Biodiversity: Population versus ecosystem stability. Ecology 77(3):350-363. (Highlighted in Science 271:1497 by Anne S. Moffat.) 1996 [Abstract] [Full Text] e001
Tilman, D.; Downing, J. A.; Wedin, D. A.; "Does diversity beget stability? (scientific correspondence, reply to Givnish). Nature 371:113-114" 1994 [Full Text] e001
Tilman, D.; Lehman, C. L.; Bristow, C. E.; Diversity-stability relationships: statistical inevitability or ecological consequence? The American Naturalist 151:277-282. 1998 [Abstract] [Full Text] e001
Tilman, D.; Species richness of experimental productivity gradients: How important is colonization limitation? Ecology 74:2179-2191. 1993 [Abstract] [Full Text] e001
Tilman, D.; The resource-ratio hypothesis of succession. The American Naturalist 125:827-852. 1985 [Abstract] [Full Text] e001
Tilman, D.; Wilson, S. D.; Belcher, J. W.; Wisheu, I.; Keddy, P. A.; Tilman, D.; Morris, E. C.; Grace, J. B.; McGraw, J. B.; Olff, H.; Turkington, R.; Klein, E.; Leung, Y.; Shipley, B.; Hulst, R. van; Johansson, M. E.; Nilsson, C.; Gurevitch, J.; Beisner, B. E.; Plant competition in relation to neighbor biomass: an intercontinental study with Poa pratensis. Ecology 75:1753-1760. 1994 [Abstract] [Full Text] e001
Vitousek, P.; Aber, J. D.; Howarth, R. W.; Likens, G. E.; Matson, P. A.; Schindler, D. W.; Schlesinger, W. H.; Tilman, D. G.; Human alteration of the global nitrogen cycle: Sources and consequences. Ecological Applications 7:737-750. 1997 [Abstract] [Full Text] e001
Wedin, D.; Tilman, D.; "Influence of nitrogen loading and species composition on the carbon balance of grasslands. Science 274:1720-1723. (Highlighted in Science Environment 274:1610-1611 by J. Kaiser, and The New York Times December 10, 1996.)" 1996 [Abstract] [Full Text] e001
