Date of Award

5-1-1981

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

Abstract

Four reclaimed mined areas in western North Dakota ranging in age from one to four years since mining, along with a site from a native mixed grass prairie for comparison, were studied to quantify the early successional changes in floristic composition, soil chemistry and plant chemical composition. The mined sites had been contoured, topsoiled, fertilized, and seeded. However, analysis of the seed bank in topsoil showed that the most dominant colonizers immigrate after topsoiling. Of the 95 species encountered, Kochia scoparia was the most dominant in the first two years following mining. Kochia had relatively low densities 2 in the first year (50-80/m ), but were robust and attained heights 2 of 88 cm with a biomass of about 400 g/m . In the second year, the 2 plant densities increases to over 10,000/m but the height and 2 biomass were reduced to 15 cm and 90 g/m . However, during the third and fourth years Kochia density declined and was practically nonexistent by the fourth year, while the planted Agropyron grasses had concomitantly increased. Other pioneers like Amaranthus retroflexus, Chenopodium album, Helianthus annuus, Salsola spp., and Setaria spp. showed a less abrupt but similar decline. Chemical analysis of the soils over the same time period showed decreases in electrical conductivity (E.C.) and in the concentrations of Ca, Mg, Na, Li, Sr, and S04 (due to leaching), while organic matter (O.M.) increased.

Field studies were conducted on the interspecific relations of the dominant species during early succession - Kochia, Salsola collina and Agropyron spp. It was found that Kochia and Salsola acted as "nurse" crops for several months during the first year of Agropyron spp. establishment, but then began to shade heavily by late July thus reducing grass tillering. Field and growth chamber experiments on the intraspecific relations of Kochia demonstrated that it conformed to the laws of reciprocal yield, self-thinning, and constant final yield.

Autotoxicity in Kochia appears to be the main factor causing the decline in its density. 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 demonstrated that small amounts of decaying Kochia leaves and especially roots were toxic to Kochia growth, but not Melilotus officinalis or Agropyron caninum. Chemical analysis of the soils and tissues indicated nutritional imbalances as shown by P/Zn and P/Mn ratios may be responsible for the autotoxicity. In addition to Kochia, a bioassay experiment showed allelochemics to be important in several other colonizing species. Results indicated that later stage species generally have greater toxicities than first year colonizers.

An analysis of the seeds present in topsoils (a grazed area, an ungrazed area, and two stockpiles) indicated that seeds of the most prevalent colonizers were not present in the topsoil upon respreading but rather appeared by immigration from the surrounding areas. The grazed site had a seed density of over 7,700 seeds/m^ (of which 43% were from weedy species) and the ungrazed site had 3,900 seeds/m (of which 1/ were weeds); the stockpiles of topsoil had very low seed densities.

Mowing of first year Kochia and Salsola just prior to seed set benefited the establishment of grasses, since Agropyron spp. produced over eight times greater biomass after mowing compared with Kochia which produced only one-fourth the biomass. Burning dead Kochia stems and seedlings in second year helped other weedy species; grass growth was impaired. Burning of a six year old reclaimed site resulted in significant reductions in soil water, standing dead biomass, and litter, and some differences in biomass of the species components; however, biomass of species of Agropyron increased whereas those of Stipa decreased by burning. Overall, no significant differences in total live biomass were recorded within two years after the burn.

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