Patterns of colonization by pioneering species and their allelochemic and competitive relations were studied in 4 reclaimed areas in western North Dakota, ranging in age from 1 to 4 years after mining; a site from a native mixed grass prairie was used for comparison. Early changes in floristic composition revealed that despite seeding of agronomic species, initial colonization was by fugitive species. Of the 95 species recorded, Kochia scoparia (Chenopodiaceae) dominated in the first 2 years after mining. Kochia had relatively low densities in the first year (50-80 plants m-2), but was robust and attained heights of 88 cm, with a biomass of about 400 g m-2. In the second year, Kochia densities increased to over 10 000 m-2, but height and biomass were reduced to 15 cm and 90 g m-2, respectively. During the third and fourth years, Kochia density declined, while the planted Agropyron grasses increased. Other pioneers like Amaranthus retroflexus, Chenopodium album, Helianthus annuus, Salsola spp. and Setaria spp. showed a less abrupt, but similar, decline. Chemical analyses of the soils over the same time-period showed decreases in electrical conductivity, and in the concentrations of Ca, Mg, Na, Li, Sr, and S04 (due to leaching), while total N and organic matter increased. Field studies were conducted on the interspecific relationships of the 2 dominant species, Kochia scoparia and Agropyron spp., during early succession. It was found that Kochia acted as a "nurse crop" for several months during the first year of Agropyron spp. establishment, but then began to shade heavily by late July, which reduced grass tillering. Autotoxicity in Kochia appears to be a major factor responsible for the decline in its growth. Results from a field experiment indicated that thinning dense second-year Kochia stands to the density of first-year stands did not alter the growth of Kochia, providing strong evidence for autotoxicity. Similarly, several growth-chamber experiments showed that small amounts of decaying Kochia leaves and (especially) roots were toxic to Kochia, but not to Melilotus officinalis or Agropyron caninum. Chemical analyses of the soils and plant tissues indicated that nutritional imbalances, as shown by P/Mn and P/Zn ratios, may be responsible for autotoxicity. A bioassay experiment indicated that allelochemic influences may be important for several colonizing species besides Kochia. Although our results indicate that later-stage species have greater toxicities than first-year colonizers, autotoxicity in the very initial stages seems to be accentuated by harsh habitat conditions.