Gross, K.L. and L. Huberty
Presented at the All Scientist Meeting (1996-07-16 to 1996-07-17 )
We have been following the dynamics of old-field succession in Treatment 7 on the KBS LTER since the site was established in 1989. We have examined how colonization and turnover of species varies in undisturbed successional fields in control and nutrient enriched plots to which ammonium-nitrate is added each year. An additional “macro plot” area in this treatment is plowed in early spring each year to create an annually disturbed community that we can compare to the agronomic treatments (1-4) and the undisturbed successional plots. The large spatial scale and replicated design of this study allows us to examine the generality of patterns observed and provides some information on how landscape position can influence successional dynamics and diversity in old-fields.Nitrogen addition increased above-ground biomass (Figure 1) had no effect on plant species richness, diversity or the rate of turnover in life form composition in the successional site (Figure 2, Figure 3). Both sites were initially dominated by annual forbs and grasses which are replaced by perennials within four years after abandonment. Species richness increased at the same rate in the control and Nitrogen addition treatments and had leveled off in both treatments to an average of about 13/m2 within 4 years of abandonment (Figure 3). Perennial forbs currently dominate both the control and fertilized plots and though these treatments have similar dominant species, the composition of the subordinate community varies and the overlap in species is about 35%. Landscape position appears to influence the composition of these communities, as there is considerable variation among replicates in species composition; though the patterns are similar in control and fertilized plots (Figure 4)In the annually-tilled area, nitrogen addition significantly increased above-ground biomass, and had a significant effect on plant species richness. Mean species richness does not vary over years, and averages 7-8 species/m2 (Figure 3). Though the plots are always dominated by species with an annual life history, the relative dominance by grasses vs. forbs also shows strong temporal variation (Figure 2).Species composition across the replicates is more similar than in the successional plots, suggesting that the species pool which can colonize these sites is less spatially variable than in the successional plots (Figure 4).These studies have shown that nitrogen addition can have little effect on the composition and dynamics of succession on sites with moderately-high fertility and a diverse species pool. These results contrast with other studies conducted on lower-fertility soils where nitrogen addition appears to strongly reduce diversity (e..g the Cedar Creek LTER site). The reasons for this disparity in results are continuing to be explored. In addition, how these differences in fertility and production may influence the composition and diversity of soil microbial communities is the subject of current studies by several KBS and off-site investigators (e.g. Nancy Collins-Johnson and Edith Allen).
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