Community and Population Level Responses to Fertilization in an Old‐Field Ecosystem

Abstract

The effects of nutrient enrichment by 2 applications of N—P—K pebble fertilizer (November and April) on the plant community of an 8—year old—field successional ecosystem were studied in 2 adjacent 1—acre [=.4047 hectares] experimental plots in Georgia. Net primary production, standing crop biomass, turnover and dominance increased, while all components of diversity decreased as a result of the treatment. These responses generally support Odum's (1969) model of ecosystem development, if the assumption is that eutrophication should involve a "set back" in development to earlier (younger) stages of autogenic succession. However, species composition was not "set back" since species characteristic of earlier successional stages (i.e., Ambrosia) exhibited reduced importance in the treated areas, while dominants more characteristic of the 8—year stage increased in importance. Results of this study suggest that fertilization has its major effect through competition—altered changes in the organization of realized niches of primary producers. A few opportunistic species, already established in the community, expand their niches by preempting some niche space of subordinates, thus reducing overall diversity. At the population level, productivity and niche width were increased for the dominant, Solidago, but greatly reduced for Aster which was codominant in control plots. Some 21 species were stressed by the nutrient enrichment (in that their net production was reduced), but a positive response by 16 species produced an overall subsidy effect for the vegetation as a whole.