Ecological studies of natural and established ecosystems on energy related disturbances in Colorado

Abstract

During this research period studies have concentrated on soil, plant, and microbial interactions to gain a better understanding of plant community changes over time on perturbed systems. These studies have shown that disturbance and revegetation practices influence vegetation structure and succession primarily in two ways: (1) by modifying chemical, physical, and biological properties of the soil and (2) by influencing the initial floristic composition of the plant community. Both the intensity and the type of disturbance, through their effect on soil chemical and physical properties have been shown to influence aboveground vegetation structure and succession. These studies show that types of disturbances which create highly productive soil conditions result in low plant diversity, while disturbances which create less productive soil conditions result in high diversity. In addition, very intense disturbances which increase the rockiness of the surface soil have been shown to not only alter the rate of succession but also the direction of succession. Similarly, the nature of the disturbance can have major effects on soil biological properties. Disturbed and revegetated soils continue to have markedly higher microbial activity and organic matter contents than undisturbed native soils. Where topsoil has been stockpiled, however, microbial activity is generally reduced. Stockpiling affects various microbial populations differently, depending on the length of the stockpiling period and whether or not the stockpile is vegetated. When the stockpile is vegetated, there is a relative increase in bacteria 1 and fungal populations while when a stockpile is not vegetated actinomycetes show greater relative abundance. When topsoil is stored for a period of four years, significant and predictable declines in Mycorrhiza Inoculum Potential (MIP) of the soil occur. However, the MI P of the upper level s (< 90 cm) of topsoil can be preserved and enhanced by seeding with plant species that host YAM fungi. Certain reclamation practices may temporarily influence soil chemical and physical characteristics and thus affect biological structure and succession in the above- and below ground compartments. Fertilization, for example, continues to have a positive influence on plant production in some studies. Its influence on plant species diversity, however, has been negative. The effect of fertilizer on the belowground compartment is most apparent with fungal populations. In general, fertilization causes reduced fungal hyphal lengths and a reduction in MIP values. When disturbance results in a material such as retorted oil shale being used as a growth medium, the chemical, physical, and biological properties are drastically altered. Few plant species have been shown to perform we 11 on this material and as a result, plant communities established on Paraho retorted shale are low in diversity and canopy cover. In the belowground compartment, retorted shale has a negative effect on phosphatase activity and nitrogen fixation and seems to prevent mycorrhizal formation. Mixing retorted shale with topsoil ameliorates these effects somewhat. Mycorrhizal formation is not inhibited until the amount of added shale exceeds 50%. The negative effects of retorted shale are primarily due to its high salt content and its high pH which results in a high availability of toxic elements and poor nutrient availability. The chemical equilibria involved in producing the high pH in oi 1 shale have been studied. During the processing of oil shale at high temperatures, carbonate minerals are often destroyed and silicate minerals such as CaSiO3 (pseudowallastonite) and MgSiO3 (clinoenstatite) are formed. These minerals buffer the pH of spent shale near 12. When processed Lurgi shale is recarbonated by bubbling CO2 through suspensions of spent shale, the pH is decreased from 11.6 to 7.9. The result is a disappearance of the silicate minerals and formation of CaCO3 (calcite) and MgCO3 (magnesite). In addition to modifying soil properties, these studies show that the second major way that disturbances and revegetation practices affect vegetation structure and succession is by influencing the initial floristic composition of the plant community. By initially seeding grasses and forbs alone, shrubs can be prevented from invading the stand in spite of the fact that shrubs are well adapted to the site and there is a ready seed source. Including adapted shrub species in the initial seed mixture on the same sites, however, can result in greater total biomass without significant reductions in grass and forb biomass. Since results of another study on competition among woody plants offer no support for the hypothesis that intensity of competition between shrubs is correlated with the abiotic environment, planting densities for the shrubs studied may be selected without consideration of shrub competition. Once the initial floristic composition of the community is determined, changes in species composition may occur due to competitive interactions. The competitive interactions among four native grass species occurring on the site have been studied. Competitive relationships among species are discussed in terms of the effect of fertilizer, soil depth, and phenologic stage on biomass and gross energy content of competing pairs. Identification of adapted species for use in revegetation is often difficult since ecotypes of the same species can be quite variable. Therefore, the ecogenic variation within eight native species (five shrubs, one forb, and two grasses) has been studied and adaptive advantages of the genetic differences are discussed.